How can you get the first digit in an int C

Question

In C   what s the best way to get the 1st digit in an int  The method I came up with is to turn the int into a string  find the 1st char of the string  then turn it back to an int   int start   Convert ToInt32 curr ToString   Substring 0  1      While this does the job  it feels like there is probably a good  simple  math-based solution to such a problem  String manipulation feels clunky   Edit  irrespective of speed differences  mystring 0  instead of Substring   is still just string manipulation

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variation on Anton s answer       cut down the number of divisions  assuming i is positive  amp  32 bits  if  i  gt   100000000  i    100000000  if  i  gt   10000  i    10000  if  i  gt   100  i    100  if  i  gt   10  i    10

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Had the same idea as Lennaert   int start   number    0   0   number    int  Math Pow 10 Math Floor Math Log10 Math Abs number        This also works with negative numbers

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int myNumber   8383  char firstDigit   myNumber ToString   0      char    8

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Just to give you an alternative  you could repeatedly divide the integer by 10  and then rollback one value once you reach zero  Since string operations are generally slow  this may be faster than string manipulation  but is by no means elegant   Something like this   while curr gt  10       curr    10

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int start   curr  while  start  gt   10    start    10    This is more efficient than a ToString   approach which internally must implement a similar loop and has to construct  and parse  a string object on the way

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Here is a simpler way that does not involve looping  int number   1234 int firstDigit   Math Floor number  Math Pow 10  number ToString   length - 1     That would give us 1234 Math Pow 10  4 - 1    1234 1000   1

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Benchmarks  Firstly  you must decide on what you mean by  best  solution  of course that takes into account the efficiency of the algorithm  its readability maintainability  and the likelihood of bugs creeping up in the future   Careful unit tests can generally avoid those problems  however   I ran each of these examples 10 million times  and the results value is the number of ElapsedTicks that have passed   Without further ado  from slowest to quickest  the algorithms are   Converting to a string  take first character  int firstDigit    int  Value ToString   0   - 48    Results   12 552 893 ticks   Using a logarithm  int firstDigit    int  Value   Math Pow 10   int Math Floor Math Log10 Value        Results   9 165 089 ticks   Looping  while  number  gt   10      number    10    Results   6 001 570 ticks   Conditionals  int firstdigit  if  Value  lt  10       firstdigit   Value  else if  Value  lt  100       firstdigit   Value   10  else if  Value  lt  1000       firstdigit   Value   100  else if  Value  lt  10000       firstdigit   Value   1000  else if  Value  lt  100000       firstdigit   Value   10000  else if  Value  lt  1000000       firstdigit   Value   100000  else if  Value  lt  10000000       firstdigit   Value   1000000  else if  Value  lt  100000000       firstdigit   Value   10000000  else if  Value  lt  1000000000       firstdigit   Value   100000000  else      firstdigit   Value   1000000000    Results   1 421 659 ticks   Unrolled  amp  optimized loop  if  i  gt   100000000  i    100000000  if  i  gt   10000  i    10000  if  i  gt   100  i    100  if  i  gt   10  i    10    Results   1 399 788 ticks   Note   each test calls Random Next   to get the next int

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Using all the examples below to get this code   using System  using System Collections Generic  using System Linq  using System Text  using System Diagnostics   namespace Benfords       class Program               static int FirstDigit1 int value                        return Convert ToInt32 value ToString   Substring 0  1                       static int FirstDigit2 int value                        while  value  gt   10  value    10              return value                      static int FirstDigit3 int value                        return  int  value ToString   0   - 48                     static int FirstDigit4 int value                        return  int  value   Math Pow 10   int Math Floor Math Log10 value                         static int FirstDigit5 int value                        if  value  lt  10  return value              if  value  lt  100  return value   10              if  value  lt  1000  return value   100              if  value  lt  10000  return value   1000              if  value  lt  100000  return value   10000              if  value  lt  1000000  return value   100000              if  value  lt  10000000  return value   1000000              if  value  lt  100000000  return value   10000000              if  value  lt  1000000000  return value   100000000              return value   1000000000                     static int FirstDigit6 int value                        if  value  gt   100000000  value    100000000              if  value  gt   10000  value    10000              if  value  gt   100  value    100              if  value  gt   10  value    10              return value                     const int mcTests   1000000           static void Main string   args                        Stopwatch lswWatch   new Stopwatch                Random lrRandom   new Random                 int liCounter               lswWatch Start                for  liCounter   0  liCounter  lt  mcTests  liCounter                    FirstDigit1 lrRandom Next                 lswWatch Stop                Console WriteLine  Test  0     1  ticks   1  lswWatch ElapsedTicks                lswWatch Reset                lswWatch Start                for  liCounter   0  liCounter  lt  mcTests  liCounter                    FirstDigit2 lrRandom Next                 lswWatch Stop                Console WriteLine  Test  0     1  ticks   2  lswWatch ElapsedTicks                lswWatch Reset                lswWatch Start                for  liCounter   0  liCounter  lt  mcTests  liCounter                    FirstDigit3 lrRandom Next                 lswWatch Stop                Console WriteLine  Test  0     1  ticks   3  lswWatch ElapsedTicks                lswWatch Reset                lswWatch Start                for  liCounter   0  liCounter  lt  mcTests  liCounter                    FirstDigit4 lrRandom Next                 lswWatch Stop                Console WriteLine  Test  0     1  ticks   4  lswWatch ElapsedTicks                lswWatch Reset                lswWatch Start                for  liCounter   0  liCounter  lt  mcTests  liCounter                    FirstDigit5 lrRandom Next                 lswWatch Stop                Console WriteLine  Test  0     1  ticks   5  lswWatch ElapsedTicks                lswWatch Reset                lswWatch Start                for  liCounter   0  liCounter  lt  mcTests  liCounter                    FirstDigit6 lrRandom Next                 lswWatch Stop                Console WriteLine  Test  0     1  ticks   6  lswWatch ElapsedTicks                Console ReadLine                        I get these results on an AMD Ahtlon 64 X2 Dual Core 4200   2 2 GHz    Test 1   2352048 ticks Test 2   614550 ticks Test 3   1354784 ticks Test 4   844519 ticks Test 5   150021 ticks Test 6   192303 ticks   But get these on a AMD FX 8350 Eight Core  4 00 GHz   Test 1   3917354 ticks Test 2   811727 ticks Test 3   2187388 ticks Test 4   1790292 ticks Test 5   241150 ticks Test 6   227738 ticks   So whether or not method 5 or 6 is faster depends on the CPU  I can only surmise this is because the branch prediction in the command processor of the CPU is smarter on the new processor  but I m not really sure   I dont have any Intel CPUs  maybe someone could test it for us

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This is what I usually do  please refer my function  below    This function can extract first number occurance from any string you can modify and use this function according to your usage     public static int GetFirstNumber this string strInsput                int number   0          string strNumber               bool bIsContNo   true          bool bNoOccued   false           try                       var arry   strInsput ToCharArray 0  strInsput Length - 1                foreach  char item in arry                                if  char IsNumber item                                         strNumber   strNumber   item ToString                         bIsContNo   true                       bNoOccued   true                                    else                                       bIsContNo   false                                     if  bNoOccued  amp  amp   bIsContNo                                        break                                                 number   Convert ToInt32 strNumber                      catch  Exception ex                         return 0                     return number

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Try this  public int GetFirstDigit int number      if   number  lt  10         return number        return GetFirstDigit    number -  number   10     10       EDIT   Several people have requested the loop version   public static int GetFirstDigitLoop int number        while  number  gt   10                number    number -  number   10     10            return number

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int i   4567789  int digit1   int Parse i ToString   0  ToString

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Very simple  and probably quite fast because it only involves comparisons and one division    if i lt 10     firstdigit   i  else if  i lt 100     firstdigit   i 10  else if  i lt 1000     firstdigit   i 100  else if  i lt 10000     firstdigit   i 1000  else if  i lt 100000     firstdigit   i 10000  else  etc    all the way up to 1000000000

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If you think Keltex s answer is ugly  try this one  it s REALLY ugly  and even faster  It does unrolled binary search to determine the length        leading code along the same lines    i lt 10000    if  i  gt   100     if  i  gt   1000       return i 1000        else    i lt 1000         return i 100        else    i lt 100       if  i  gt   10       return i 10        else    i lt 10         return i          P S  MartinStettner had the same idea

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Did some tests with one of my co-workers here  and found out most of the solutions don t work for numbers under 0     public int GetFirstDigit int number                number   Math Abs number    lt - makes sure you really get the digit           if  number  lt  10                        return number                    return GetFirstDigit  number -  number   10     10

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Here s how  int i   Math Abs 386792   while i  gt   10      i    10    and i will contain what you need

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int temp   i  while  temp  gt   10        temp    10      Result in temp

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while  i  gt  10       i    Int32 Math Floor  Decimal i   10        i is now the first int

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I just stumbled upon this old question and felt inclined to propose another suggestion since none of the other answers so far returns the correct result for all possible input values and it can still be made faster   public static int GetFirstDigit  int i         if  i  lt  0  amp  amp    i   -i    lt  0   return 2      return   i  lt  100       i  lt  1     0     i  lt  10                 i   i   10     i  lt  1000000       i  lt  10000                   i  lt  1000     i   100   i   1000     i  lt  100000                 i   10000   i   100000     i  lt  100000000                   i  lt  10000000     i   1000000   i   10000000                 i  lt  1000000000     i   100000000   i   1000000000      This works for all signed integer values inclusive -2147483648 which is the smallest signed integer and doesn t have a positive counterpart  Math Abs  -2147483648   triggers a System OverflowException and - -2147483648 computes to -2147483648   The implementation can be seen as a combination of the advantages of the two fastest implementations so far  It uses a binary search and avoids superfluous divisions  A quick benchmark with the index of a loop with 100 000 000 iterations shows that it is twice as fast as the currently fastest implementation   It finishes after 2 829 581 ticks   For comparison I also measured a corrected variant of the currently fastest implementation which took 5 664 627 ticks   public static int GetFirstDigitX  int i         if  i  lt  0  amp  amp    i   -i    lt  0   return 2      if  i  gt   100000000   i    100000000      if  i  gt   10000   i    10000      if  i  gt   100   i    100      if  i  gt   10   i    10      return i      The accepted answer with the same correction needed 16 561 929 ticks for this test on my computer   public static int GetFirstDigitY  int i         if  i  lt  0  amp  amp    i   -i    lt  0   return 2      while  i  gt   10           i    10      return i      Simple functions like these can easily be proven for correctness since iterating all possible integer values takes not much more than a few seconds on current hardware  This means that it is less important to implement them in a exceptionally readable fashion as there simply won t ever be the need to fix a bug inside them later on

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The best I can come up with is  int numberOfDigits   Convert ToInt32 Math Floor  Math Log10  value         int firstDigit   value   Math Pow  10  numberOfDigits

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I know it s not C   but it s surprising curious that in python the  get the first char of the string representation of the number  is the faster   EDIT  no  I made a mistake  I forgot to construct again the int  sorry  The unrolled version it s the fastest     cat first digit py def loop n       while n  gt   10          n    10     return n  def unrolled n       while n  gt   100000000    yea    unlimited size int supported            n    100000000     if n  gt   10000          n    10000     if n  gt   100          n    100     if n  gt   10          n    10     return n  def string n       return int str n  0     python -mtimeit -s  from first digit import loop as test         for n in xrange 0  100000000  1000   test n   10 loops  best of 3  275 msec per loop   python -mtimeit -s  from first digit import unrolled as test         for n in xrange 0  100000000  1000   test n   10 loops  best of 3  149 msec per loop   python -mtimeit -s  from first digit import string as test         for n in xrange 0  100000000  1000   test n   10 loops  best of 3  284 msec per loop

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An obvious  but slow  mathematical approach is   int firstDigit    int  i   Math Pow 10   int Math Log10 i

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start   getFirstDigit start      public int getFirstDigit final int start       int number   Math abs start       while number  gt  10           number    10            return number      or  public int getFirstDigit final int start     return getFirstDigit Math abs start   true     private int getFirstDigit final int start  final boolean recurse     if start  lt  10       return start        return getFirstDigit start   10  recurse

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Non iterative formula   public static int GetHighestDigit int num        if  num  lt   0         return 0        return  int   double num   Math Pow 10f  Math Floor Math Log10 num

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Very easy method to get the Last digit   int myInt   1821   int lastDigit   myInt -   myInt 10  10      1821 - 1820   1

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Check this one too    int get1digit Int64 myVal        string q12   myVal ToString   0  ToString        int i   int Parse q12       return i      Also good if you want multiple numbers   int get3digit Int64 myVal    Int64 or whatever numerical data you have       char mg1   myVal ToString   0       char mg2   myVal ToString   1       char mg3   myVal ToString   2       char   chars     mg1  mg2  mg3        string q12  new string chars       int i   int Parse q12       return i

[c#] How can you get the first digit in an int (C#)?

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