Converting Integers to Roman Numerals - Java

Question

This is a homework assignment I am having trouble with   I need to make an integer to Roman Numeral converter using a method  Later  I must then use the program to write out 1 to 3999 in Roman numerals  so hardcoding is out  My code below is very bare-bones  it is a basic I O loop with a way to exit while using a package for getIntegerFromUser we made in class   Is there a way to assign values to Strings and then add them together when I call the method   Update  I got some pseudo code from my professor to help me  and while I understand what he is trying to say  I am having some trouble with the ifs  Will I be needing many  many if statements so that my converter will correctly handle the Roman numeral formatting or is there a manner in which I can do this with more efficiency  I ve updated my code to reflect my placeholder method   Update  Oct 28 2012   I got it working  Here s what I ended up using    public static String IntegerToRomanNumeral int input        if  input  lt  1    input  gt  3999          return  Invalid Roman Number Value       String s           while  input  gt   1000            s     M           input -  1000               while  input  gt   900            s     CM           input -  900            while  input  gt   500            s     D           input -  500            while  input  gt   400            s     CD           input -  400            while  input  gt   100            s     C           input -  100            while  input  gt   90            s     XC           input -  90            while  input  gt   50            s     L           input -  50            while  input  gt   40            s     XL           input -  40            while  input  gt   10            s     X           input -  10            while  input  gt   9            s     IX           input -  9            while  input  gt   5            s     V           input -  5            while  input  gt   4            s     IV           input -  4            while  input  gt   1            s     I           input -  1                return s

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My solution is in function getRoman   public  String getRoman int number         String riman       M   XM   CM   D   XD   CD   C   XC   L   XL   X   IX   V   IV   I        int arab      1000  990  900  500  490  400  100  90  50  40  10  9  5  4  1       StringBuilder result   new StringBuilder        int i   0      while  number  gt  0    arab length     i - 1             while   number - arab i    gt   0                number -  arab i               result append riman i                      i              return result toString

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I think my solution is one of the more concise ones   private static String convertToRoman int mInt        String   rnChars      M     CM    D    C     XC    L     X    IX    V    I         int   rnVals      1000  900  500  100  90  50  10  9  5  1        String retVal            for  int i   0  i  lt  rnVals length  i              int numberInPlace   mInt   rnVals i           if  numberInPlace    0  continue          retVal    numberInPlace    4  amp  amp  i  gt  0  rnChars i    rnChars i - 1               new String new char numberInPlace   replace   0  rnChars i            mInt   mInt   rnVals i             return retVal

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I did this before three years  may be it helps you    public class ToRoman        public static String toRoman int number                StringBuilder br   new StringBuilder              while number  0                        while number gt  1000                                br append  M                    number- 1000                               while number gt  900                                br append  CM                    number- 900                                while number gt  500                                br append  D                    number- 500                                while number gt  400                                br append  CD                    number- 400                                while number gt  100                                br append  C                    number- 100                                while number gt  90                                br append  XC                    number- 90                             while number gt  50                                br append  L                    number- 50                             while number gt  40                                br append  XL                    number- 40                             while number gt  10                                br append  X                    number- 10                             while number gt  9                                br append  IX                    number- 9                              while number gt  5                                br append  V                    number- 5                              while number gt  4                                br append  IV                    number- 4                              while number gt  1                                br append  I                    number- 1                                    return br toString               public static void main String    args                System out println toRoman 2000

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I like using a Chain of Responsiblity pattern myself  I think it makes a lot of sense for this scenario   public abstract class NumberChainOfResponsibility       protected NumberChainOfResponsibility next      protected int decimalValue      protected String romanNumeralValue       public NumberChainOfResponsibility                public String convert int decimal            int remainder   decimal          StringBuilder numerals   new StringBuilder            while  remainder    0                if  remainder  gt   this decimalValue                    numerals append this romanNumeralValue                   remainder -  this decimalValue                else                   numerals append next convert remainder                    remainder   0                                  return numerals toString              Then I create a class extending this one for every roman numeral  1 5 10 50 100 500 1000 as well as 4 9 40 90 400 900    1000  public class Cor1000 extends NumberChainOfResponsibility       public Cor1000             super            this decimalValue   1000          this romanNumeralValue    M           this next   new Cor900              1  public class Cor1 extends NumberChainOfResponsibility       public Cor1             super            this decimalValue   1          this romanNumeralValue    I           this next   null            A class serving as an  interface  to the converter  exposing a method to convert a specific number   public class Converter       private static int MAX VALUE   5000      private static int MIN VALUE   0      private static String ERROR TOO BIG    Value is too big        private static String ERROR TOO SMALL    Value is too small         public String convertThisIntToRomanNumerals int decimal            Cor1000 startingCor   new Cor1000            if  decimal  gt   MAX VALUE              return ERROR TOO BIG          if  decimal  lt   MIN VALUE              return ERROR TOO SMALL           String numeralsWithoutConversion   startingCor convert decimal           return numeralsWithoutConversion            And the client code  in my case a JUnit test     Test public void assertConversionWorks         Assert assertEquals  MMMMCMXCIX   converter convertThisIntToRomanNumerals 4999        Assert assertEquals  CMXCIX   converter convertThisIntToRomanNumerals 999        Assert assertEquals  CMLXXXIX   converter convertThisIntToRomanNumerals 989        Assert assertEquals  DCXXVI   converter convertThisIntToRomanNumerals 626        Assert assertEquals  DCXXIV   converter convertThisIntToRomanNumerals 624        Assert assertEquals  CDXCVIII   converter convertThisIntToRomanNumerals 498        Assert assertEquals  CXXIII   converter convertThisIntToRomanNumerals 123        Assert assertEquals  XCIX   converter convertThisIntToRomanNumerals 99        Assert assertEquals  LI   converter convertThisIntToRomanNumerals 51        Assert assertEquals  XLIX   converter convertThisIntToRomanNumerals 49        See the whole example on my Github account

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Despite the fact that a lot of solutions have been already proposed   I guess that the following one will be short and clear   public class IntegerToRoman       public static String intToRoman int number            String   thousands         M    MM    MMM            String   hundreds         C    CC    CCC    CD    D    DC    DCC    DCCC    CM            String   tens         X    XX    XXX    XL    L    LX    LXX    LXXX    XC            String   units         I    II    III    IV    V    VI    VII    VIII    IX             return thousands number   1000                    hundreds  number   1000    100                    tens  number   100    10                    units number   10              public static void main String   args            int   numbers    1  2  3  5  10  14  17  20  25  38  49  63  72  81  97  98  99  100  101  248  253  799  1325  1900  2000  2456  1715            final Instant startTimeIter   Instant now            for  int number   numbers                System out printf   4d - gt   8s n   number  intToRoman number                      final Instant endTimeIter   Instant now            System out printf  Elapsed time   d ms n n   Duration between startTimeIter  endTimeIter  toMillis               Output      1 - gt         I    2 - gt        II    3 - gt       III        2456 - gt   MMCDLVI 1715 - gt    MDCCXV Elapsed time  66 ms   Logic is quite simple     we just go through integer number from left to right and taking for each number appropriate array value  thousands  hundrets     this method could convert till 3000 number

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We can avoid the loop and if with this solution   public class RomanNumber        private final static Supplier lt TreeMap lt Integer  String gt  gt  romanNumerals      - gt            final TreeMap lt Integer  String gt  map   new TreeMap lt  gt             map put 1000   M            map put 900   CM            map put 500   D            map put 400   CD            map put 100   C            map put 90   XC            map put 50   L            map put 40   XL            map put 10   X            map put 9   IX            map put 5   V            map put 4   IV            map put 1   I            return map              private static Function lt TreeMap lt Integer  String gt   Function lt Integer  String gt  gt  numeralConverter   map - gt  number - gt  Optional              ofNullable map floorKey number                filter number  equals               map map  get               or    - gt  Optional                      ofNullable map floorKey number                        map num - gt  map get num    RomanNumber numeralConverter apply map  apply number - num                 orElse  NaN         public static Function lt Integer  String gt  toRoman   numeralConverter apply romanNumerals get

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There is actually another way of looking at this problem  not as a number problem  but a Unary problem  starting with the base character of Roman numbers   I   So we represent the number with just I  and then we replace the characters in ascending value of the roman characters      public String getRomanNumber int number        return join     nCopies number   I                 replace  IIIII    V                replace  IIII    IV                replace  VV    X                replace  VIV    IX                replace  XXXXX    L                replace  XXXX    XL                replace  LL    C                replace  LXL    XC                replace  CCCCC    D                replace  CCCC    CD                replace  DD    M                replace  DCD    CM        I especially like this method of solving this problem rather than using a lot of ifs and while loops  or table lookups  It is also actually a quit intuitive solution when you thinking of the problem not as a number problem

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Simplest solution   public class RomanNumerals        private static int    arabic    50  40  10  9  5  4  1        private static String    roman     L    XL    X    IX    V    IV    I         public static String convert int arabicNumber             StringBuilder romanNumerals   new StringBuilder            int remainder   arabicNumber           for  int i 0 i lt arabic length i                   while  remainder  gt   arabic i                     romanNumerals append roman i                    remainder -  arabic i                                    return romanNumerals toString

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This may help   using System   using System Text   public class Test    public static string ToRoman int number        StringBuilder br new StringBuilder          while number  0                if number gt  1000                        br Append  M                number- 1000                       if number gt  900                        br Append  CM                number- 900                        if number gt  500                        br Append  D                number- 500                        if number gt  400                        br Append  CD                number- 400                        if number gt  100                        br Append  C                number- 100                        if number gt  90                        br Append  XC                number- 90                     if number gt  50                        br Append  L                number- 50                     if number gt  40                        br Append  XL                number- 40                     if number gt  10                        br Append  X                number- 10                     if number gt  9                        br Append  IX                number- 9                      if number gt  5                        br Append  V                number- 5                      if number gt  4                        br Append  IV                number- 4                      if number gt  1                        br Append  I                number- 1                        return br ToString      public static void Main         Console WriteLine ToRoman int Parse Console ReadLine

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I think if you study the theory of roman numerals carefully you don t require mappings for numbers 4 9 40 etc because the theory tells us if the roman numeral is IV   5-1   4  hence when the prefix is smaller than the succeeding number in that case you have to subtract the former number from the succeeding number to get the actual value and this is what I have incorporated into my code for the problem  take a look and point out any mistakes if necessary  I followed this table to devise my logic - http   literacy kent edu Minigrants Cinci romanchart htm  import java util Set  import java io File  import java util HashMap  import java util HashSet  import java io FileReader  import java io IOException  import java io BufferedReader   public class RomanStringToIntegerConversion       public static void main String   args  throws IOException          BufferedReader br   new BufferedReader new InputStreamReader System in             String   romanString   br readLine   split               HashMap lt String  Integer gt  romanToIntegerMap   new HashMap lt String  Integer gt             romanToIntegerMap put  I   1           romanToIntegerMap put  V   5           romanToIntegerMap put  X   10           romanToIntegerMap put  L   50           romanToIntegerMap put  C   100           romanToIntegerMap put  D   500           romanToIntegerMap put  M   1000            int numLength   romanString length          Set lt Integer gt  lessIndices   new HashSet lt Integer gt              for int i   0  i  lt  numLength    i               if i 1  lt  numLength                   if romanToIntegerMap get romanString i    lt  romanToIntegerMap get romanString i 1                        lessIndices add i                                    int num   0          for int i   0  i  lt  numLength                if  lessIndices contains i                    num   num   romanToIntegerMap get romanString i                      i                            else                  num   num   romanToIntegerMap get romanString i 1   - romanToIntegerMap get romanString i                    i  2                                  System out println  The integer representation of the roman numeral is       num

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First break the number into its decimal factors like 995   900   90   5  then convert each factor recursively  public class IntegerToRoman     private Map lt Integer  String gt  romanChars   new HashMap lt  gt        public IntegerToRoman         romanChars put 1   I        romanChars put 5   V        romanChars put 10   X        romanChars put 50   L        romanChars put 100   C        romanChars put 500   D        romanChars put 1000   M        romanChars put 5000   V          public String intToRoman int num        if  num    0            return               int decimalFact   0      StringBuilder result   new StringBuilder        for  int i    int Math log10 num   i  gt   0  i--            int divisor    int  Math pow 10  i           decimalFact   num - num   divisor          result append convertDecimalFact decimalFact            num   num   divisor            return result toString       private String convertDecimalFact int decimalFact     if decimalFact    0  return        int   keyArray   romanChars keySet   stream   mapToInt key - gt  key           sorted   toArray        for int i  0   i 1 lt keyArray length   i           if  keyArray i   lt   decimalFact  amp  amp  decimalFact lt   keyArray i 1               int bigger1stDgt   getLeftMostNum keyArray i 1             int decimalFact1stDgt   getLeftMostNum decimalFact            return decimalFact1stDgt  gt   bigger1stDgt-1                    intToRoman keyArray i 1 -decimalFact  romanChars get keyArray i 1                     romanChars get keyArray i   intToRoman decimalFact - keyArray i                        return        private int getLeftMostNum int number        int oneDgt   Integer valueOf Integer valueOf number  toString                     substring 0  0  1        if number lt 10           return oneDgt                   int twoDgts   Integer valueOf Integer valueOf number  toString                      substring 0  0  2        return twoDgts  10   twoDgts   oneDgt     public static void main String   args         IntegerToRoman solution   new IntegerToRoman        System out format   Decimal 3 - gt  Roman  s  n    solution intToRoman 3        System out format  Decimal 4 - gt  Roman  s  n    solution intToRoman 4        System out format  Decimal 8 - gt  Roman  s  n    solution intToRoman 8        System out format  Decimal 58 - gt  Roman  s  n    solution intToRoman 58        System out format  Decimal 344 - gt  Roman  s  n    solution intToRoman 344        System out format  Decimal 995 - gt  Roman  s  n    solution intToRoman 995        System out format  Decimal 1994 - gt  Roman  s  n    solution intToRoman 1994           Output is like   Decimal 3 -  Roman III   Decimal 4 -  Roman IV  Decimal 8 -  Roman VIII   Decimal 58 -  Roman LVIII   Decimal 344 -  Roman CCCXLIV   Decimal 995 -  Roman CMXCV   Decimal 1994 -  Roman MCMXCIV

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After doing some research and analysing answers above  I ended up with this   package roman   public class RomanNumbers     public static final int   decimal    1  4  5  9  10  40  50  90  100  400  500  900  1000   public static final String   letters     I    IV    V    IX    X    XL    L    XC    C    CD    D    CM    M     public static String stringToRoman int num        String roman            if  num  lt  1    num  gt  3999            System out println  Invalid roman number value                while  num  gt  0            int maxFound   0          for  int i 0  i  lt  decimal length  i                  if  num  gt   decimal i                     maxFound   i                                  roman    letters maxFound           num -  decimal maxFound              return roman                 Unit tests also passed   package roman   import static org junit Assert     import org junit Test   public class RomanNumbersTest     Test public void testReturn1         String actual   RomanNumbers stringToRoman 1       String expected    I       assertEquals expected  actual       Test public void testReturn5         String actual   RomanNumbers stringToRoman 5       String expected    V       assertEquals expected  actual       Test public void testReturn2         String actual   RomanNumbers stringToRoman 2       String expected    II       assertEquals expected  actual       Test public void testReturn4         String actual   RomanNumbers stringToRoman 4       String expected    IV       assertEquals expected  actual       Test public void testReturn399         String actual   RomanNumbers stringToRoman 399       String expected    CCCXCIX       assertEquals expected  actual       Test public void testReturn3992         String actual   RomanNumbers stringToRoman 3992       String expected    MMMCMXCII       assertEquals expected  actual

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I have written a very simple solution  All we have to do is divide and find how many times a particular letter or letter combination occurs  and append that to the StringBuilder object sb  We also should keep track of the remaining number  num    public static String intToRoman int num        StringBuilder sb   new StringBuilder        int times   0      String   romans   new String      I    IV    V    IX    X    XL    L                XC    C    CD    D    CM    M         int   ints   new int     1  4  5  9  10  40  50  90  100  400  500              900  1000        for  int i   ints length - 1  i  gt   0  i--            times   num   ints i           num    ints i           while  times  gt  0                sb append romans i                times--                      return sb toString

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Alternative solution based on the OP s own solution by utilizing an enum  Additionally  a parser and round-trip tests are included   public class RomanNumber       public enum Digit           M 1000  3           CM 900  1           D 500  1           CD 400  1           C 100  3           XC 90  1           L 50  1           XL 40  1           X 10  3           IX 9  1           V 5  1           IV 4  1           I 1  3            public final int value          public final String symbol   name            public final int maxArity           private Digit int value  int maxArity                this value   value              this maxArity   maxArity                       private static final Digit   DIGITS   Digit values         public static String of int number            if  number  lt  1    3999  lt  number                throw new IllegalArgumentException String format                       Roman numbers are only defined for numbers between 1 and 3999   d was given                        number                                    StringBuilder sb   new StringBuilder            for  Digit digit   DIGITS                int value   digit value              String symbol   digit symbol               while  number  gt   value                    sb append symbol                   number -  value                                   return sb toString               public static int parse String roman            if  roman isEmpty                  throw new NumberFormatException  The empty string does not comprise a valid Roman number                       int number   0          int offset   0          for  Digit digit   DIGITS                int value   digit value              int maxArity   digit maxArity              String symbol   digit symbol               for  int i   0  i  lt  maxArity  amp  amp  roman startsWith symbol  offset   i                      number    value                  offset    symbol length                                    if  offset    roman length                  throw new NumberFormatException String format                       The string   s  does not comprise a valid Roman number                       roman                                   return number                 TESTS        public static void main String   args                Demonstrating round-trip for all possible inputs              for  int number   1  number  lt   3999  number                  String roman   of number               int parsed   parse roman               if  parsed    number                    System err format                           ERROR  number   d  roman   s  parsed   d n                           number                          roman                          parsed                                                        Some illegal inputs              int   illegalNumbers     -1  0  4000  4001            for  int illegalNumber   illegalNumbers                try                   of illegalNumber                   System err format                           ERROR  Expected failure on number  d n                           illegalNumber                                  catch  IllegalArgumentException e                       Failed as expected                                   String   illegalRomans      MMMM    CDCD    IM    T        VV    DM             for  String illegalRoman   illegalRomans                try                   parse illegalRoman                   System err format                           ERROR  Expected failure on roman  s n                           illegalRoman                                  catch  NumberFormatException e                       Failed as expected

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String convert int i        String ones           String tens           String hundreds           String thousands           String result        boolean error   false       Vector v   new Vector           assign passed integer to temporary value temp     int temp i         flags an error if number is greater than 3999     if  temp  gt  4000           error   true               loops while temp can no more be divided by 10          Lets say i   3254  then temp is also 3254 at line 14                              3254            3254 10   25          3254 10   4                                      now temp   25       325     4  - here 4 is added to the vector v s 0th index                                  now temp   32     32   5  - here 5 is added to the vector v s 1st index                                now temp   3    3    2  - here 2 is added to the vector v s 2nd index  and loop exits                               since temp 10   0                   0   3  - here 3 is not added to the vector v s 3rd index as loop exits when                             temp 10   0               while  temp 10    0            if  temp   10    0  amp  amp  temp  lt 4000                v add temp 10               temp   temp   10           else                    break                         therefore you have to add temp one last time to the vector     v add temp          as in the example now you have 4 5 2 3 respectively in v s 0 1 2 3 indices        for  int j   0  j  lt  v size    j                 you see that v s 0th index has number of ones  So make them roman ones here          if  j  0                switch  v get 0  toString                     case  0    ones       break                  case  1    ones    I   break                  case  2    ones    II   break                  case  3    ones    III   break                  case  4    ones    IV   break                  case  5    ones    V   break                  case  6    ones    VI   break                  case  7    ones    VII   break                  case  8    ones    VIII   break                  case  9    ones    IX   break                                in the second iteration of the loop  when j  1                 index 1 of v is checked  Now you understand that v s 1st index               has the tens           else if  j    1                switch  v get 1  toString                     case  0    tens       break                  case  1    tens    X   break                  case  2    tens    XX   break                  case  3    tens    XXX   break                  case  4    tens    XL   break                  case  5    tens    L   break                  case  6    tens    LX   break                  case  7    tens    LXX   break                  case  8    tens    LXXX   break                  case  9    tens    XC   break                          else if j    2      and hundreds             switch  v get 2  toString                     case  0    hundreds       break                  case  1    hundreds    C   break                  case  2    hundreds    CC   break                  case  3    hundreds    CCC   break                  case  4    hundreds    CD   break                  case  5    hundreds    D   break                  case  6    hundreds    DC   break                  case  7    hundreds    DCC   break                  case  8    hundreds    DCCC   break                  case  9    hundreds    CM   break                            else if j    3     and finally thousands              switch  v get 3  toString                                case  0    thousands       break                  case  1    thousands    M   break                  case  2    thousands    MM   break                  case  3    thousands    MMM   break                                         if  error           result    Error         else          result   thousands   hundreds   tens   ones             return result

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After seeing some of the answers here I had to post this  I think that my algorithm is far the most easiest to understand and the bit of performance lost is not important even on a relatively large scale  I m also obeying the standardized coding conventions as opposed to some of the users here   Average conversion time  0 05ms  based on converting all numbers 1-3999 and dividing by 3999   public static String getRomanNumeral int arabicNumber         if  arabicNumber  gt  0  amp  amp  arabicNumber  lt  4000             final LinkedHashMap lt Integer  String gt  numberLimits                new LinkedHashMap lt  gt              numberLimits put 1   I            numberLimits put 4   IV            numberLimits put 5   V            numberLimits put 9   IX            numberLimits put 10   X            numberLimits put 40   XL            numberLimits put 50   L            numberLimits put 90   XC            numberLimits put 100   C            numberLimits put 400   CD            numberLimits put 500   D            numberLimits put 900   CM            numberLimits put 1000   M             String romanNumeral                while  arabicNumber  gt  0                int highestFound   0              for  Map Entry lt Integer  String gt  current   numberLimits entrySet                     if  current getKey    lt   arabicNumber                        highestFound   current getKey                                                romanNumeral    numberLimits get highestFound               arabicNumber -  highestFound                     return romanNumeral         else           throw new UnsupportedOperationException arabicNumber                  is not a valid Roman numeral               First you have to take into account that Roman numerals are only in the interval of  lt 1-4000   but that can be solved by a simple if and a thrown exception  Then you can try to find the largest set roman numeral in a given integer and if found subtract it from the original number and add it to the result  Repeat with the newly acquired number until you hit zero

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I noticed that it s quite easy to translate from integer to Roman Numeral  because there s always sort of 1  5 and 10 for every digit  i e  I  V and X for 1-10  X  L and C for 10-100 etc   That s why I made an array of Roman Numerals to get the right letter from   In my example  I go through the whole number one digit per time  using modulo operator to get the last digit each time  Then I form the Roman Numeral from current digit inside a switch-statement  adding it in the beginning of asRomanNumerals String  After the digit has been translated  it gets removed from the number and index used to look for letter in array gets increased with two  IVX -  XLC    public static void main String   args            number is the one to be translated into Roman Numerals     int number   2345      number   Math min 3999  Math max 1  number       wraps number between 1-3999     String asRomanNumerals               Array including numerals in ascending order     String   RN     I    V    X    L    C    D    M         int i   0     Index used to keep track which digit we are translating     while  number  gt  0            switch number   10            case 1  asRomanNumerals   RN i    asRomanNumerals  break          case 2  asRomanNumerals   RN i    RN i    asRomanNumerals  break          case 3  asRomanNumerals   RN i    RN i    RN i    asRomanNumerals  break          case 4  asRomanNumerals   RN i    RN i   1    asRomanNumerals  break          case 5  asRomanNumerals   RN i   1    asRomanNumerals  break          case 6  asRomanNumerals   RN i   1    RN i    asRomanNumerals  break          case 7  asRomanNumerals   RN i   1    RN i    RN i    asRomanNumerals  break          case 8  asRomanNumerals   RN i   1    RN i    RN i    RN i   asRomanNumerals  break          case 9  asRomanNumerals   RN i    RN i   2    asRomanNumerals  break                    number    int  number   10          i    2            System out println asRomanNumerals

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I am curious how this is going to end up  I d start looking into the mapping 1 2 3 5 6 7 8 9 10 to I II III IV V VI VII VII IX X     then you might look into the rule for roman numbers   I II III are created by concatentation V  X  L  C  D and M are symbols for 5  10  50  100  500 and 1000 The romans thought that they could save space in writing numbers by instead of writing e g  IIII for 4  use IV  meaning  5 minus 1      You might want to look into those rules e g  at http   en wikipedia org wiki Roman numerals and capture them in code e g  in a class  RomanNumbers  If you would like to cheat you might want to follow the link http   www moxlotus alternatifs eu programmation-converter html

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private static String toRoman int n        String   romanNumerals      M     CM    D    CD    C    XC    L     X    IX    V    I         int   romanNumeralNums      1000  900  500   400   100   90   50    10     9    5    1        String finalRomanNum            for  int i   0  i  lt  romanNumeralNums length  i                   int currentNum   n  romanNumeralNums i               if  currentNum  0                    continue                             for  int j   0  j  lt  currentNum  j                      finalRomanNum   romanNumerals i                              n   n romanNumeralNums i             return finalRomanNum

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From Java Notes 6 0 website                      An object of type RomanNumeral is an integer between 1 and 3999   It can          be constructed either from an integer or from a string that represents          a Roman numeral in this range   The function toString   will return a          standardized Roman numeral representation of the number   The function          toInt   will return the number as a value of type int                  public class RomanNumeral             private final int num       The number represented by this Roman numeral               The following arrays are used by the toString   function to construct             the standard Roman numeral representation of the number   For each i              the number numbers i  is represented by the corresponding string  letters i                         private static int      numbers     1000   900   500   400   100    90                                                   50    40    10     9     5     4     1              private static String   letters      M     CM     D     CD    C     XC                                                 L     XL     X     IX    V     IV    I                               Constructor   Creates the Roman number with the int value specified             by the parameter   Throws a NumberFormatException if arabic is             not in the range 1 to 3999 inclusive                        public RomanNumeral int arabic                if  arabic  lt  1                 throw new NumberFormatException  Value of RomanNumeral must be positive                 if  arabic  gt  3999                 throw new NumberFormatException  Value of RomanNumeral must be 3999 or less                 num   arabic                                       Constructor   Creates the Roman number with the given representation              For example  RomanNumeral  xvii   is 17   If the parameter is not a             legal Roman numeral  a NumberFormatException is thrown   Both upper and             lower case letters are allowed                        public RomanNumeral String roman                 if  roman length      0                 throw new NumberFormatException  An empty string does not define a Roman numeral                  roman   roman toUpperCase        Convert to upper case letters               int i   0           A position in the string  roman              int arabic   0      Arabic numeral equivalent of the part of the string that has                                    been converted so far               while  i  lt  roman length                      char letter   roman charAt i             Letter at current position in string                 int number   letterToNumber letter       Numerical equivalent of letter                  i        Move on to next position in the string                 if  i    roman length                              There is no letter in the string following the one we have just processed                          So just add the number corresponding to the single letter to arabic                    arabic    number                                  else                           Look at the next letter in the string   If it has a larger Roman numeral                         equivalent than number  then the two letters are counted together as                         a Roman numeral with value  nextNumber - number                     int nextNumber   letterToNumber roman charAt i                      if  nextNumber  gt  number                              Combine the two letters to get one value  and move on to next position in string                       arabic     nextNumber - number                        i                                          else                             Don t combine the letters   Just add the value of the one letter onto the number                       arabic    number                                                          end while              if  arabic  gt  3999                 throw new NumberFormatException  Roman numeral must have value 3999 or less                  num   arabic                 end constructor                            Find the integer value of letter considered as a Roman numeral   Throws             NumberFormatException if letter is not a legal Roman numeral   The letter              must be upper case                        private int letterToNumber char letter                switch  letter                   case  I    return 1                 case  V    return 5                 case  X    return 10                 case  L    return 50                 case  C    return 100                 case  D    return 500                 case  M    return 1000                 default    throw new NumberFormatException                               Illegal character       letter       in Roman numeral                                                        Return the standard representation of this Roman numeral                        public String toString                 String roman           The roman numeral              int N   num            N represents the part of num that still has                                      to be converted to Roman numeral representation              for  int i   0  i  lt  numbers length  i                     while  N  gt   numbers i                       roman    letters i                     N -  numbers i                                              return roman                                        Return the value of this Roman numeral as an int                        public int toInt                 return num

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private String convertToRoman int num      String result         while num  gt  0       if num  gt   1000           result     M           num -  1000       else if num  gt   900           result     CM           num -  900            else if num  gt   500           result     D           num -  500       else if num  gt   400           result     CD           num -  400       else if num  gt   100           result     C           num -  100       else if num  gt   90           result     XC           num -  90       else if num  gt   50           result     L           num -  50       else if num  gt   40           result     XL           num -  40            else if num  gt   10           result     X           num -  10       else if num  gt   9           result     IX           num -  9            else if num  gt   5           result     V           num -  5       else if num  gt   4           result     IV           num -  4       else if num  gt   1           result     I           num -  1             else        break               return result

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A compact implementation using Java TreeMap and recursion   import java util TreeMap   public class RomanNumber        private final static TreeMap lt Integer  String gt  map   new TreeMap lt Integer  String gt          static            map put 1000   M            map put 900   CM            map put 500   D            map put 400   CD            map put 100   C            map put 90   XC            map put 50   L            map put 40   XL            map put 10   X            map put 9   IX            map put 5   V            map put 4   IV            map put 1   I                public final static String toRoman int number            int l    map floorKey number           if   number    l                 return map get number                     return map get l    toRoman number-l              Testing   public void testRomanConversion          for  int i   1  i lt   100  i              System out println i   t   t   RomanNumber toRoman i

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enum Numeral           I 1   IV 4   V 5   IX 9   X 10   XL 40   L 50   XC 90   C 100   CD 400   D 500   CM 900   M 1000           int weight           Numeral int weight                this weight   weight                        public static String roman long n             if  n  lt   0                throw new IllegalArgumentException                       StringBuilder buf   new StringBuilder             final Numeral   values   Numeral values            for  int i   values length - 1  i  gt   0  i--                while  n  gt   values i  weight                    buf append values i                    n -  values i  weight                                  return buf toString               public static void test long n            System out println n           roman n               public static void main String   args            test 1999           test 25           test 944           test 0

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I think instead of taking 2 separate arrays  it can be done using a single array  Also i recommend array over Map  Array guarantee O 1  complexity for getting any element in any case if we know the position  I have used StringBuilder instead of String because of frequent append operations  class Solution            public String intToRoman int num            StringBuilder str   new StringBuilder            RomanDataStore arr   new RomanDataStore 13           arr 0  new RomanDataStore 1000  quot M quot            arr 1  new RomanDataStore 900   quot CM quot            arr 2  new RomanDataStore 500   quot D quot            arr 3  new RomanDataStore 400   quot CD quot            arr 4  new RomanDataStore 100   quot C quot            arr 5  new RomanDataStore 90   quot XC quot            arr 6  new RomanDataStore 50   quot L quot            arr 7  new RomanDataStore 40   quot XL quot            arr 8  new RomanDataStore 10   quot X quot            arr 9  new RomanDataStore 9   quot IX quot            arr 10  new RomanDataStore 5   quot V quot            arr 11  new RomanDataStore 4   quot IV quot            arr 12  new RomanDataStore 1   quot I quot            int itr 0          RomanDataStore temp null          while num  0               temp arr itr               if num gt  temp val                   for int i 0 i lt num temp val i                      str append temp s                                        num num temp val                            itr                               return str toString                            private class RomanDataStore          private int val          private String s          RomanDataStore int val  String s               this val val              this s s

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Use these libraries   import java util LinkedHashMap  import java util Map    The code     public static String RomanNumerals int Int        LinkedHashMap lt String  Integer gt  roman numerals   new LinkedHashMap lt String  Integer gt         roman numerals put  M   1000       roman numerals put  CM   900       roman numerals put  D   500       roman numerals put  CD   400       roman numerals put  C   100       roman numerals put  XC   90       roman numerals put  L   50       roman numerals put  XL   40       roman numerals put  X   10       roman numerals put  IX   9       roman numerals put  V   5       roman numerals put  IV   4       roman numerals put  I   1       String res           for Map Entry lt String  Integer gt  entry   roman numerals entrySet           int matches   Int entry getValue          res    repeat entry getKey    matches         Int   Int   entry getValue              return res        public static String repeat String s  int n        if s    null            return null            final StringBuilder sb   new StringBuilder        for int i   0  i  lt  n  i              sb append s             return sb toString          Testing the code     for  int i   1 i lt 256 i          System out println  i   i   - gt    RomanNumerals i          The output     i 1 - gt  I   i 2 - gt  II   i 3 - gt  III   i 4 - gt  IV   i 5 - gt  V   i 6 - gt  VI   i 7 - gt  VII   i 8 - gt  VIII   i 9 - gt  IX   i 10 - gt  X   i 11 - gt  XI   i 12 - gt  XII   i 13 - gt  XIII   i 14 - gt  XIV   i 15 - gt  XV   i 16 - gt  XVI   i 17 - gt  XVII   i 18 - gt  XVIII   i 19 - gt  XIX   i 20 - gt  XX   i 21 - gt  XXI   i 22 - gt  XXII   i 23 - gt  XXIII   i 24 - gt  XXIV   i 25 - gt  XXV   i 26 - gt  XXVI   i 27 - gt  XXVII   i 28 - gt  XXVIII   i 29 - gt  XXIX   i 30 - gt  XXX   i 31 - gt  XXXI   i 32 - gt  XXXII   i 33 - gt  XXXIII   i 34 - gt  XXXIV   i 35 - gt  XXXV   i 36 - gt  XXXVI   i 37 - gt  XXXVII   i 38 - gt  XXXVIII   i 39 - gt  XXXIX   i 40 - gt  XL   i 41 - gt  XLI   i 42 - gt  XLII   i 43 - gt  XLIII   i 44 - gt  XLIV   i 45 - gt  XLV   i 46 - gt  XLVI   i 47 - gt  XLVII   i 48 - gt  XLVIII   i 49 - gt  XLIX   i 50 - gt  L   i 51 - gt  LI   i 52 - gt  LII   i 53 - gt  LIII   i 54 - gt  LIV   i 55 - gt  LV   i 56 - gt  LVI   i 57 - gt  LVII   i 58 - gt  LVIII   i 59 - gt  LIX   i 60 - gt  LX   i 61 - gt  LXI   i 62 - gt  LXII   i 63 - gt  LXIII   i 64 - gt  LXIV   i 65 - gt  LXV   i 66 - gt  LXVI   i 67 - gt  LXVII   i 68 - gt  LXVIII   i 69 - gt  LXIX   i 70 - gt  LXX   i 71 - gt  LXXI   i 72 - gt  LXXII   i 73 - gt  LXXIII   i 74 - gt  LXXIV   i 75 - gt  LXXV   i 76 - gt  LXXVI   i 77 - gt  LXXVII   i 78 - gt  LXXVIII   i 79 - gt  LXXIX   i 80 - gt  LXXX   i 81 - gt  LXXXI   i 82 - gt  LXXXII   i 83 - gt  LXXXIII   i 84 - gt  LXXXIV   i 85 - gt  LXXXV   i 86 - gt  LXXXVI   i 87 - gt  LXXXVII   i 88 - gt  LXXXVIII   i 89 - gt  LXXXIX   i 90 - gt  XC   i 91 - gt  XCI   i 92 - gt  XCII   i 93 - gt  XCIII   i 94 - gt  XCIV   i 95 - gt  XCV   i 96 - gt  XCVI   i 97 - gt  XCVII   i 98 - gt  XCVIII   i 99 - gt  XCIX   i 100 - gt  C   i 101 - gt  CI   i 102 - gt  CII   i 103 - gt  CIII   i 104 - gt  CIV   i 105 - gt  CV   i 106 - gt  CVI   i 107 - gt  CVII   i 108 - gt  CVIII   i 109 - gt  CIX   i 110 - gt  CX   i 111 - gt  CXI   i 112 - gt  CXII   i 113 - gt  CXIII   i 114 - gt  CXIV   i 115 - gt  CXV   i 116 - gt  CXVI   i 117 - gt  CXVII   i 118 - gt  CXVIII   i 119 - gt  CXIX   i 120 - gt  CXX   i 121 - gt  CXXI   i 122 - gt  CXXII   i 123 - gt  CXXIII   i 124 - gt  CXXIV   i 125 - gt  CXXV   i 126 - gt  CXXVI   i 127 - gt  CXXVII   i 128 - gt  CXXVIII   i 129 - gt  CXXIX   i 130 - gt  CXXX   i 131 - gt  CXXXI   i 132 - gt  CXXXII   i 133 - gt  CXXXIII   i 134 - gt  CXXXIV   i 135 - gt  CXXXV   i 136 - gt  CXXXVI   i 137 - gt  CXXXVII   i 138 - gt  CXXXVIII   i 139 - gt  CXXXIX   i 140 - gt  CXL   i 141 - gt  CXLI   i 142 - gt  CXLII   i 143 - gt  CXLIII   i 144 - gt  CXLIV   i 145 - gt  CXLV   i 146 - gt  CXLVI   i 147 - gt  CXLVII   i 148 - gt  CXLVIII   i 149 - gt  CXLIX   i 150 - gt  CL   i 151 - gt  CLI   i 152 - gt  CLII   i 153 - gt  CLIII   i 154 - gt  CLIV   i 155 - gt  CLV   i 156 - gt  CLVI   i 157 - gt  CLVII   i 158 - gt  CLVIII   i 159 - gt  CLIX   i 160 - gt  CLX   i 161 - gt  CLXI   i 162 - gt  CLXII   i 163 - gt  CLXIII   i 164 - gt  CLXIV   i 165 - gt  CLXV   i 166 - gt  CLXVI   i 167 - gt  CLXVII   i 168 - gt  CLXVIII   i 169 - gt  CLXIX   i 170 - gt  CLXX   i 171 - gt  CLXXI   i 172 - gt  CLXXII   i 173 - gt  CLXXIII   i 174 - gt  CLXXIV   i 175 - gt  CLXXV   i 176 - gt  CLXXVI   i 177 - gt  CLXXVII   i 178 - gt  CLXXVIII   i 179 - gt  CLXXIX   i 180 - gt  CLXXX   i 181 - gt  CLXXXI   i 182 - gt  CLXXXII   i 183 - gt  CLXXXIII   i 184 - gt  CLXXXIV   i 185 - gt  CLXXXV   i 186 - gt  CLXXXVI   i 187 - gt  CLXXXVII   i 188 - gt  CLXXXVIII   i 189 - gt  CLXXXIX   i 190 - gt  CXC   i 191 - gt  CXCI   i 192 - gt  CXCII   i 193 - gt  CXCIII   i 194 - gt  CXCIV   i 195 - gt  CXCV   i 196 - gt  CXCVI   i 197 - gt  CXCVII   i 198 - gt  CXCVIII   i 199 - gt  CXCIX   i 200 - gt  CC   i 201 - gt  CCI   i 202 - gt  CCII   i 203 - gt  CCIII   i 204 - gt  CCIV   i 205 - gt  CCV   i 206 - gt  CCVI   i 207 - gt  CCVII   i 208 - gt  CCVIII   i 209 - gt  CCIX   i 210 - gt  CCX   i 211 - gt  CCXI   i 212 - gt  CCXII   i 213 - gt  CCXIII   i 214 - gt  CCXIV   i 215 - gt  CCXV   i 216 - gt  CCXVI   i 217 - gt  CCXVII   i 218 - gt  CCXVIII   i 219 - gt  CCXIX   i 220 - gt  CCXX   i 221 - gt  CCXXI   i 222 - gt  CCXXII   i 223 - gt  CCXXIII   i 224 - gt  CCXXIV   i 225 - gt  CCXXV   i 226 - gt  CCXXVI   i 227 - gt  CCXXVII   i 228 - gt  CCXXVIII   i 229 - gt  CCXXIX   i 230 - gt  CCXXX   i 231 - gt  CCXXXI   i 232 - gt  CCXXXII   i 233 - gt  CCXXXIII   i 234 - gt  CCXXXIV   i 235 - gt  CCXXXV   i 236 - gt  CCXXXVI   i 237 - gt  CCXXXVII   i 238 - gt  CCXXXVIII   i 239 - gt  CCXXXIX   i 240 - gt  CCXL   i 241 - gt  CCXLI   i 242 - gt  CCXLII   i 243 - gt  CCXLIII   i 244 - gt  CCXLIV   i 245 - gt  CCXLV   i 246 - gt  CCXLVI   i 247 - gt  CCXLVII   i 248 - gt  CCXLVIII   i 249 - gt  CCXLIX   i 250 - gt  CCL   i 251 - gt  CCLI   i 252 - gt  CCLII   i 253 - gt  CCLIII   i 254 - gt  CCLIV   i 255 - gt  CCLV

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Here s the results of MY homework  It doesn t ensure the input is in the right range and I probably should be using StringBuilder  time I looked it up   and isn t one single method  But if anyone is reading this far I d appreciate both positive and negative feedback on it   import java util Scanner                Main   allows user input and tests 1-3999       toRoman   breaks the number down into digits and passes them to romanLogic         romanLogic   converts each digit into a the numerals that represent it              public class RomanNumerals               public static void main String args                 Scanner in   new Scanner System in               System out print  give us an integer  lt  4000                          System out println  the roman numeral version is      toRoman in nextInt                  for  int i   1  i lt  3999  i                     System out println i           toRoman i                                    public static String toRoman int i               String output                   int digits   i 10              int tens    i 100  10              int hundreds    i 1000  100              int thousands    i 10000  1000              return  romanLogic thousands   M                              romanLogic hundreds  C   D   M                        romanLogic tens  X   L   C                        romanLogic digits  I   V   X                       public static String romanLogic int i  String ones  String fives  String tens               String result                   if  i    0                   return result                else                   if   i gt  4  amp  amp  i lt  8                                        result    fives                                    if  i  9                       result    tens                                    if i 5  lt  4                       while i 5  gt  0                           result    ones                          i--                                                          if i 5    4                       result   ones   result                                              return result

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I like Andr   Kramer Orten s answer  very elegant  I particularly like how it avoids loops  I thought of another way to handle it also avoiding loops   It uses  integer division and modulo on the input to select the correct index from a hard-coded set of string arrays for each unit type   The nice thing here is you can specify exact conversions depending on if you want the additive or subtractive numeral form i e  IIII vs IV  Here I use the   the  subtractive form  for all numbers in the form 5x-1  4 9 14 19 40 90 etc   It would also be trivial to extend this to allow for larger numbers by simply extending the thousands array with further additive or subtractive forms  i e   IV    V  or  MMMM    MMMMM   For bonus points I actually make sure the number parameter is within the given  range for the problem   public class RomanNumeralGenerator       static final int MIN VALUE   1      static final int MAX VALUE   3999      static final String   RN M         M    MM    MMM        static final String   RN C         C    CC    CCC    CD    D    DC    DCC    DCCC    CM        static final String   RN X         X    XX    XXX    XL    L    LX    LXX    LXXX    XC        static final String   RN I         I    II    III    IV    V    VI    VII    VIII    IX         public String generate int number            if  number  lt  MIN VALUE    number  gt  MAX VALUE                throw new IllegalArgumentException                      String format                               The number must be in the range   d   d                                MIN VALUE                              MAX VALUE                                                         return new StringBuilder                    append RN M number   1000                    append RN C number   1000   100                    append RN X number   100   10                    append RN I number   10                    toString

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Just to keep up with the technology  here s a Java 8 version using streams and a custom Collector  eliminating the need for loops or if statements   import java util Arrays  import java util Collections  import java util Set  import java util function BiConsumer  import java util function BinaryOperator  import java util function Function  import java util function Supplier  import java util stream Collector  import java util stream IntStream   public class RomanNumeral        public static void main String arg              IntStream range 1  4000  forEach value - gt  System out println  Arrays stream Mark values    collect new MarkCollector lt Mark gt  value   toString                 enum Mark           M 1000   CM 900   D 500   CD 400   C 100   XC 90   L 50   XL 40   X 10   IX 9   V 5   IV 4   I 1            private final int value           private Mark int value    this value   value             public int value     return value               static class MarkCollector lt T extends Mark gt  implements Collector lt T  StringBuilder  StringBuilder gt             private final int   valueholder   new int 1            MarkCollector int value    valueholder 0    value              Override         public Supplier lt StringBuilder gt  supplier     return    - gt  StringBuilder  new              Override         public BiConsumer lt StringBuilder  T gt  accumulator                 return  builder  mark  - gt                    builder append String join     Collections nCopies valueholder 0    mark value    mark name                       valueholder 0    valueholder 0    mark value                                       Override         public BinaryOperator lt StringBuilder gt  combiner     return null              Override         public Function lt StringBuilder  StringBuilder gt  finisher     return Function identity                Override         public Set lt Characteristics gt  characteristics     return Collections singleton Characteristics IDENTITY FINISH

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import java util Scanner  import java io      then  try        Scanner input   new Scanner new File  lettered in          int dataCollect   input nextInt         int sum   0       String lineInput            for  int i   0  i  lt   dataCollect  i               while  input hasNext                   lineInput   input next                char lineArray     lineInput toCharArray                 for  int j   0  j  lt  lineArray length  j                       switch lineArray j                     case  A                         sum    1                      continue                                    case  B                         sum    10                      continue                                    case  C                         sum    100                                    case  D                         sum    1000                      continue                                    case  E                         sum    10000                      continue                                    case  F                         sum    100000                      continue                                    case   G                         sum    1000000                      continue                                    case  X                         System out println sum                       sum   0                      continue                                    default                                                       catch  FileNotFoundException fileNotFound        System out println  File has not been found

[java] Converting Integers to Roman Numerals - Java

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