Why does this code using random strings print hello world

Question

The following print statement would print  hello world   Could anyone explain this   System out println randomString -229985452          randomString -147909649      And randomString   looks like this   public static String randomString int i        Random ran   new Random i       StringBuilder sb   new StringBuilder        while  true                int k   ran nextInt 27           if  k    0              break           sb append  char        k               return sb toString

User · Answer

It s all about the input seed  Same seed give the same results all   the time  Even you re-run  your program again and again it s the same output    public static void main String   args         randomString -229985452       System out println  ------------        randomString -229985452       private static void randomString int i        Random ran   new Random i       System out println ran nextInt         System out println ran nextInt         System out println ran nextInt         System out println ran nextInt         System out println ran nextInt          Output  -755142161 -1073255141 -369383326 1592674620 -1524828502 ------------ -755142161 -1073255141 -369383326 1592674620 -1524828502

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The other answers explain why  but here is how   Given an instance of Random   Random r   new Random -229985452    The first 6 numbers that r nextInt 27  generates are   8 5 12 12 15 0   and the first 6 numbers that r nextInt 27  generates given Random r   new Random -147909649  are   23 15 18 12 4 0   Then just add those numbers to the integer representation of the character    which is 96    8    96   104 -- gt  h 5    96   101 -- gt  e 12   96   108 -- gt  l 12   96   108 -- gt  l 15   96   111 -- gt  o  23   96   119 -- gt  w 15   96   111 -- gt  o 18   96   114 -- gt  r 12   96   108 -- gt  l 4    96   100 -- gt  d

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I was intrigued by this  I ran this random  word generator on a dictionary word list  Range  Integer MIN VALUE to Integer MAX VALUE  I got 15131 hits   int   arrInt    -2146926310  -1885533740  -274140519                   -2145247212  -1845077092  -2143584283                  -2147483454  -2138225126  -2147375969    for int seed   arrInt       System out print randomString seed              Prints  the quick browny fox jumps over a lazy dog

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From the Java docs  this is an intentional feature when specifying a seed value for the Random class      If two instances of Random are created with the same seed  and the   same sequence of method calls is made for each  they will generate and   return identical sequences of numbers  In order to guarantee this   property  particular algorithms are specified for the class Random    Java implementations must use all the algorithms shown here for the   class Random  for the sake of absolute portability of Java code    http   docs oracle com javase 1 4 2 docs api java util Random html  Odd though  you would think there are implicit security issues in having predictable  random  numbers

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Here is a minor improvement for Denis Tulskiy answer  It cuts the time by half  public static long   generateSeed String goal  long start  long finish        char   input   goal toCharArray         int   dif   new int input length - 1       for  int i   1  i  lt  input length  i              dif i - 1    input i  - input i - 1              mainLoop      for  long seed   start  seed  lt  finish  seed              Random random   new Random seed           int lastChar   random nextInt 27           int base   input 0  - lastChar          for  int d   dif                int nextChar   random nextInt 27               if  nextChar - lastChar    d                    continue mainLoop                            lastChar   nextChar                    if random nextInt 27     0               return new long   seed  base                        throw new NoSuchElementException  Sorry

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Derived from Denis Tulskiy s answer  this method generates the seed   public static long generateSeed String goal  long start  long finish        char   input   goal toCharArray        char   pool   new char input length       label          for  long seed   start  seed  lt  finish  seed                  Random random   new Random seed                for  int i   0  i  lt  input length  i                    pool i     char   random nextInt 27                     if  random nextInt 27     0                    for  int i   0  i  lt  input length  i                          if  input i     pool i                           continue label                                    return seed                                throw new NoSuchElementException  Sorry

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As multi-threading is very easy with Java  here is a variant that searches for a seed using all cores available  http   ideone com ROhmTA  import java util ArrayList  import java util Random  import java util concurrent Callable  import java util concurrent ExecutorService  import java util concurrent Executors  import java util concurrent ThreadFactory   public class SeedFinder      static class SearchTask implements Callable lt Long gt         private final char   goal      private final long start  step       public SearchTask final String goal  final long offset  final long step          final char   goalAsArray   goal toCharArray          this goal   new char goalAsArray length   1         System arraycopy goalAsArray  0  this goal  0  goalAsArray length         this start   Long MIN VALUE   offset        this step   step              Override     public Long call   throws Exception         final long LIMIT   Long MAX VALUE - this step        final Random random   new Random          int position  rnd        long seed   this start         while   Thread interrupted      false   amp  amp   seed  lt  LIMIT             random setSeed seed           position   0          rnd   random nextInt 27           while    rnd    0   amp  amp   this goal position     0                        char         rnd     this goal position                  position            if  position    this goal length                return seed                        rnd   random nextInt 27                     seed    this step                 throw new Exception  No match found                 public static void main String   args        final String GOAL    hello  toLowerCase        final int NUM CORES   Runtime getRuntime   availableProcessors         final ArrayList lt SearchTask gt  tasks   new ArrayList lt  gt  NUM CORES       for  int i   0  i  lt  NUM CORES    i          tasks add new SearchTask GOAL  i  NUM CORES               final ExecutorService executor   Executors newFixedThreadPool NUM CORES  new ThreadFactory             Override       public Thread newThread Runnable r            final Thread result   new Thread r           result setPriority Thread MIN PRIORITY      make sure we do not block more important tasks         result setDaemon false           return result                      try         final Long result   executor invokeAny tasks         System out println  Seed for       GOAL       found      result         catch  Exception ex          System err println  Calculation failed      ex         finally         executor shutdownNow

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Random always return the same sequence  It s used for shuffling arrays and other operations as permutations   To get different sequences  it s necessary initialize the sequence in some position  called  seed    The randomSting get the random number in the i position  seed   -229985452  of the  random  sequence  Then uses the ASCII code for the next 27 character in the sequence after the seed position until this value are equal to 0  This return the  hello   The same operation is done for  world     I think that the code did not work for any other words  The guy that programmed that knows the random sequence very well   It s very great geek code

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I ll just leave it here  Whoever has a lot of  CPU  time to spare  feel free to experiment    Also  if you have mastered some fork-join-fu to make this thing burn all CPU cores  just threads are boring  right    please share your code  I would greatly appreciate it   public static void main String   args        long time   System currentTimeMillis        generate  stack        generate  over        generate  flow        generate  rulez         System out println  Took      System currentTimeMillis   - time      ms       private static void generate String goal        long   seed   generateSeed goal  Long MIN VALUE  Long MAX VALUE       System out println seed 0        System out println randomString seed 0    char  seed 1        public static long   generateSeed String goal  long start  long finish        char   input   goal toCharArray        char   pool   new char input length       label      for  long seed   start  seed  lt  finish  seed              Random random   new Random seed            for  int i   0  i  lt  input length  i                pool i     char  random nextInt 27            if  random nextInt 27     0                int base   input 0  - pool 0               for  int i   1  i  lt  input length  i                      if  input i  - pool i     base                      continue label                            return new long   seed  base                         throw new NoSuchElementException  Sorry          public static String randomString long i  char base        System out println  Using base       base             Random ran   new Random i       StringBuilder sb   new StringBuilder        for  int n   0    n              int k   ran nextInt 27           if  k    0              break           sb append  char   base   k               return sb toString        Output   -9223372036808280701 Using base   Z  stack -9223372036853943469 Using base   b  over -9223372036852834412 Using base   e  flow -9223372036838149518 Using base   d  rulez Took 7087 ms

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I wrote a quick program to find these seeds   import java lang    import java util    import java io     public class RandomWords       public static void main  String   args            Set lt String gt  wordSet   new HashSet lt String gt             String fileName    args length  gt  0   args 0      usr share dict words            readWordMap wordSet  fileName           System err println wordSet size       words read             findRandomWords wordSet              private static void readWordMap  Set lt String gt  wordSet  String fileName            try               BufferedReader reader   new BufferedReader new FileReader fileName                String line              while   line   reader readLine       null                    line   line trim   toLowerCase                    if  isLowerAlpha line   wordSet add line                                   catch  IOException e                System err println  Error reading from     fileName          e                        private static boolean isLowerAlpha  String word            char   c   word toCharArray            for  int i   0  i  lt  c length  i                  if  c i   lt   a     c i   gt   z   return false                    return true             private static void findRandomWords  Set lt String gt  wordSet            char   c   new char 256           Random r   new Random            for  long seed0   0  seed0  gt   0  seed0                  for  int sign   -1  sign  lt   1  sign    2                    long seed   seed0   sign                  r setSeed seed                   int i                  for  i   0  i  lt  c length  i                          int n   r nextInt 27                       if  n    0  break                      c i     char   int  a    n - 1                                     String s   new String c  0  i                   if  wordSet contains s                         System out println s          seed                       wordSet remove s                                                       I have it running in the background now  but it s already found enough words for a classic pangram   import java lang    import java util     public class RandomWordsTest       public static void main  String   args            long   a    -73  -157512326  -112386651  71425  -104434815                      -128911  -88019  -7691161  1115727           for  int i   0  i  lt  a length  i                  Random r   new Random a i                StringBuilder sb   new StringBuilder                int n              while   n   r nextInt 27    gt  0  sb append  char        n                System out println sb                        Demo on ideone    Ps  -727295876  -128911  -1611659  -235516779

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Everyone here did a great job of explaining how the code works and showing how you can construct your own examples  but here s an information theoretical answer showing why we can reasonably expect a solution to exist that the brute force search will eventually find  The 26 different lower-case letters form our alphabet S  To allow generating words of different lengths  we further add a terminator symbol   to yield an extended alphabet S     S        Let a be a symbol and X a uniformly distributed random variable over S   The probability of obtaining that symbol  P X   a   and its information content  I a   are given by   P X   a    1  S     1 27 I a    -log2 P X   a     -log2 1 27    log2 27   For a word     S  and its  -terminated counterpart                 S     we have  I       I              log2 27           1    log2 27   Since the Pseudorandom Number Generator  PRNG  is initialized with a 32-bit seed  we can expect most words of length up to      floor 32 log2 27   - 1   5  to be generated by at least one seed  Even if we were to search for a 6-character word  we would still be successful about 41 06  of the time  Not too shabby  For 7 letters we re looking at closer to 1 52   but I hadn t realized that before giving it a try   include  lt iostream gt   include  lt random gt    int main         std  mt19937 rng 631647094       std  uniform int distribution lt char gt  dist  a    z    1         char alpha      while   alpha   dist rng       z    1                std  cout  lt  lt  alpha           See the output  http   ideone com JRGb3l

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The principal is the Random Class constructed with the same seed will generate the same pattern of numbers every time

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When an instance of java util Random is constructed with a specific seed parameter  in this case -229985452 or -147909649   it follows the random number generation algorithm beginning with that seed value   Every Random constructed with the same seed will generate the same pattern of numbers every time

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Most random number generators are  in fact   pseudo random   They are Linear Congruential Generators  or LCGs  http   en wikipedia org wiki Linear congruential generator   LCGs are quite predictable given a fixed seed  Basically  use a seed that gives you your first letter  then write an app that continues to generate the next int  char  until you hit the next letter in your target string and write down how many times you had to invoke the LCG  Continue until you ve generated each and every letter

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It is about  seed   Same seeds give the same result

[java] Why does this code using random strings print "hello world"?

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