How do I create a random alpha-numeric string in C

Question

I d like to create a random string  consisting of alpha-numeric characters  I want to be able to be specify the length of the string   How do I do this in C

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C   Simple Code  include  lt bits stdc   h gt  using namespace std  int main     vector lt char gt  alphanum         0   1   2   3   4    5   6   7   8   9    A   B   C   D   E   F    G   H   I   J   K    L   M   N   O   P    Q   R   S   T   U    V   W   X   Y   Z    a   b   c   d   e   f    g   h   i   j   k    l   m   n   o   p    q   r   s   t   u    v   w   x   y   z     string s  quot  quot   int len 5  srand time 0     for  int i   0  i  lt len  i          int t alphanum size  -1      int idx rand   t      s   alphanum idx     cout lt  lt s lt  lt  quot   quot   return 0

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Something even simpler and more basic in case you re happy for your string to contain any printable characters    include  lt time h gt       we ll use time for the seed  include  lt string h gt     this is for strcpy  void randomString int size  char  output     pass the destination size and the destination itself       srand time NULL       seed with time      char src size       size   rand     size     this randomises the size  optional       src size      0      start with the end of the string               and work your way backwards     while --size  gt  -1          src size     rand     94    32     generate a string ranging from the space character to    tilde       strcpy output  src      store the random string

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My 2p solution    include  lt random gt   include  lt string gt   std  string random string std  string  size type length        static auto amp  chrs    0123456789           abcdefghijklmnopqrstuvwxyz           ABCDEFGHIJKLMNOPQRSTUVWXYZ        thread local static std  mt19937 rg std  random device           thread local static std  uniform int distribution lt std  string  size type gt  pick 0  sizeof chrs  - 2        std  string s       s reserve length        while length--          s    chrs pick rg         return s

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Mehrdad Afshari s answer would do the trick  but I found it a bit too verbose for this simple task  Look-up tables can sometimes do wonders   include  lt iostream gt   include  lt ctime gt   include  lt unistd h gt   using namespace std   string gen random const int len             string tmp s      static const char alphanum              quot 0123456789 quot           quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot           quot abcdefghijklmnopqrstuvwxyz quot            srand   unsigned  time NULL    getpid          tmp s reserve len        for  int i   0  i  lt  len    i           tmp s    alphanum rand      sizeof alphanum  - 1                  return tmp s          int main int argc  char  argv               cout  lt  lt  gen random 12   lt  lt  endl           return 0

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Mehrdad Afshari s answer would do the trick  but I found it a bit too verbose for this simple task  Look-up tables can sometimes do wonders   include  lt iostream gt   include  lt ctime gt   include  lt unistd h gt   using namespace std   string gen random const int len             string tmp s      static const char alphanum              quot 0123456789 quot           quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot           quot abcdefghijklmnopqrstuvwxyz quot            srand   unsigned  time NULL    getpid          tmp s reserve len        for  int i   0  i  lt  len    i           tmp s    alphanum rand      sizeof alphanum  - 1                  return tmp s          int main int argc  char  argv               cout  lt  lt  gen random 12   lt  lt  endl           return 0

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I just tested this  it works sweet and doesn t require a lookup table  rand alnum   sort of forces out alphanumerics but because it selects 62 out of a possible 256 chars it isn t a big deal    include  lt cstdlib gt       for rand    include  lt cctype gt        for isalnum       include  lt algorithm gt     for back inserter  include  lt string gt   char  rand alnum         char c      while   std  isalnum c   static cast lt char gt  std  rand                    return c      std  string  rand alnum str  std  string  size type sz        std  string s      s reserve   sz       generate n  std  back inserter s   sz  rand alnum       return s

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Rather than manually looping  prefer using the appropriate C   algorithm  in this case std  generate n  with a proper random number generator  auto generate random alphanumeric string std  size t len  - gt  std  string       static constexpr auto chars            quot 0123456789 quot           quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot           quot abcdefghijklmnopqrstuvwxyz quot       thread local auto rng   random generator lt  gt         auto dist   std  uniform int distribution     std  strlen chars  - 1       auto result   std  string len    0        std  generate n begin result   len    amp       return chars dist rng            return result     This is close to something I would call the    canonical    solution for this problem  Unfortunately  correctly seeding a generic C   random number generator  e g  MT19937  is really hard  The above code therefore uses a helper function template  random generator  template  lt typename T   std  mt19937 gt  auto random generator   - gt  T       auto constexpr seed bytes   sizeof typename T  result type    T  state size      auto constexpr seed len   seed bytes   sizeof std  seed seq  result type       auto seed   std  array lt std  seed seq  result type  seed len gt         auto dev   std  random device        std  generate n begin seed   seed len  std  ref dev        auto seed seq   std  seed seq begin seed   end seed        return T seed seq      This is complex and relatively inefficient  Luckily it   s used to initialise a thread local variable and is therefore only invoked once per thread  Finally  the necessary includes for the above are   include  lt algorithm gt   include  lt array gt   include  lt cstring gt   include  lt functional gt   include  lt random gt   include  lt string gt   The above code uses class template argument deduction and thus requires C  17  It can be trivially adapted for earlier versions by adding the required template arguments

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I hope this helps someone   Tested at https   www codechef com ide with C   4 9 2   include  lt iostream gt   include  lt string gt   include  lt stdlib h gt         srand  rand     using namespace std   string RandomString int len       string str    0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz      string newstr     int pos     while newstr size      len        pos     rand      str size   - 1         newstr    str substr pos 1           return newstr     int main        srand time 0       string random str   RandomString 100      cout  lt  lt   random str      lt  lt  random str  lt  lt  endl      Output  random str   DNAT1LAmbJYO0GvVo4LGqYpNcyK3eZ6t0IN3dYpHtRfwheSYipoZOf04gK7OwFIwXg2BHsSBMB84rceaTTCtBC0uZ8JWPdVxKXBd

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void gen random char  s  size t len         for  size t i   0  i  lt  len    i             int randomChar   rand    26 26 10            if  randomChar  lt  26               s i     a    randomChar           else if  randomChar  lt  26 26               s i     A    randomChar - 26           else              s i     0    randomChar - 26 - 26              s len    0

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Yet another adaptation because non of the answers would suffice my needs  First of all if rand   is used to generate random numbers you will get the same output at each run  The seed for random number generator has to be some sort of random  With C  11 you can include  random  library and you can initialize the seed with random device and mt19937  This seed will be supplied by the OS and it will be random enough for us for ex  clock   You can give a range boundaries are included  0 25  in my case  And last but not least I only needed random string of lowercase letters so I utilized char addition  With a pool of characters approach did not work out for me    include  lt random gt      void gen random char  s  const int len       static std  random device rd      static std  mt19937 mt rd         static std  uniform int distribution lt int gt  dist 0  25       for  int i   0  i  lt  len    i            s i     a    dist mt             s len    0

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Rather than manually looping  prefer using the appropriate C   algorithm  in this case std  generate n  with a proper random number generator  auto generate random alphanumeric string std  size t len  - gt  std  string       static constexpr auto chars            quot 0123456789 quot           quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot           quot abcdefghijklmnopqrstuvwxyz quot       thread local auto rng   random generator lt  gt         auto dist   std  uniform int distribution     std  strlen chars  - 1       auto result   std  string len    0        std  generate n begin result   len    amp       return chars dist rng            return result     This is close to something I would call the    canonical    solution for this problem  Unfortunately  correctly seeding a generic C   random number generator  e g  MT19937  is really hard  The above code therefore uses a helper function template  random generator  template  lt typename T   std  mt19937 gt  auto random generator   - gt  T       auto constexpr seed bytes   sizeof typename T  result type    T  state size      auto constexpr seed len   seed bytes   sizeof std  seed seq  result type       auto seed   std  array lt std  seed seq  result type  seed len gt         auto dev   std  random device        std  generate n begin seed   seed len  std  ref dev        auto seed seq   std  seed seq begin seed   end seed        return T seed seq      This is complex and relatively inefficient  Luckily it   s used to initialise a thread local variable and is therefore only invoked once per thread  Finally  the necessary includes for the above are   include  lt algorithm gt   include  lt array gt   include  lt cstring gt   include  lt functional gt   include  lt random gt   include  lt string gt   The above code uses class template argument deduction and thus requires C  17  It can be trivially adapted for earlier versions by adding the required template arguments

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My 2p solution    include  lt random gt   include  lt string gt   std  string random string std  string  size type length        static auto amp  chrs    0123456789           abcdefghijklmnopqrstuvwxyz           ABCDEFGHIJKLMNOPQRSTUVWXYZ        thread local static std  mt19937 rg std  random device           thread local static std  uniform int distribution lt std  string  size type gt  pick 0  sizeof chrs  - 2        std  string s       s reserve length        while length--          s    chrs pick rg         return s

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C   Simple Code  include  lt bits stdc   h gt  using namespace std  int main     vector lt char gt  alphanum         0   1   2   3   4    5   6   7   8   9    A   B   C   D   E   F    G   H   I   J   K    L   M   N   O   P    Q   R   S   T   U    V   W   X   Y   Z    a   b   c   d   e   f    g   h   i   j   k    l   m   n   o   p    q   r   s   t   u    v   w   x   y   z     string s  quot  quot   int len 5  srand time 0     for  int i   0  i  lt len  i          int t alphanum size  -1      int idx rand   t      s   alphanum idx     cout lt  lt s lt  lt  quot   quot   return 0

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void strGetRandomAlphaNum char  sStr  unsigned int iLen      char Syms      0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ     unsigned int Ind   0    srand time NULL    rand       while Ind  lt  iLen          sStr Ind      Syms rand   62         sStr iLen      0

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Mehrdad Afshari s answer would do the trick  but I found it a bit too verbose for this simple task  Look-up tables can sometimes do wonders   include  lt iostream gt   include  lt ctime gt   include  lt unistd h gt   using namespace std   string gen random const int len             string tmp s      static const char alphanum              quot 0123456789 quot           quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot           quot abcdefghijklmnopqrstuvwxyz quot            srand   unsigned  time NULL    getpid          tmp s reserve len        for  int i   0  i  lt  len    i           tmp s    alphanum rand      sizeof alphanum  - 1                  return tmp s          int main int argc  char  argv               cout  lt  lt  gen random 12   lt  lt  endl           return 0

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Be ware when calling the function   string gen random const int len    static const char alphanum      0123456789           ABCDEFGHIJKLMNOPQRSTUVWXYZ    stringstream ss   for  int i   0  i  lt  len    i        ss  lt  lt  alphanum rand      sizeof alphanum  - 1      return ss str         adapted of  Ates Goral  it will result in the same character sequence every time  Use  srand time NULL      before calling the function  although the rand   function is always seeded with 1  kjfletch   For Example   void SerialNumberGenerator          srand time NULL        for  int i   0  i  lt  5  i              cout  lt  lt  gen random 10   lt  lt  endl

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Yet another adaptation because non of the answers would suffice my needs  First of all if rand   is used to generate random numbers you will get the same output at each run  The seed for random number generator has to be some sort of random  With C  11 you can include  random  library and you can initialize the seed with random device and mt19937  This seed will be supplied by the OS and it will be random enough for us for ex  clock   You can give a range boundaries are included  0 25  in my case  And last but not least I only needed random string of lowercase letters so I utilized char addition  With a pool of characters approach did not work out for me    include  lt random gt      void gen random char  s  const int len       static std  random device rd      static std  mt19937 mt rd         static std  uniform int distribution lt int gt  dist 0  25       for  int i   0  i  lt  len    i            s i     a    dist mt             s len    0

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Let s make random convenient again    I made up a nice C  11 header only solution  You could easily add one header file to your project and then add your tests or use random strings for another purposes   That s a quick description  but you can follow the link to check full code  The main part of solution is in class Randomer   class Randomer          random seed by default     std  mt19937 gen       std  uniform int distribution lt size t gt  dist    public             some convenience ctors             Randomer size t min  size t max  unsigned int seed   std  random device                gen  seed   dist  min  max                  if you want predictable numbers     void SetSeed unsigned int seed            gen  seed seed              size t operator               return dist  gen               Randomer incapsulates all random stuff and you can add your own functionality to it easily  After we have Randomer  it s very easy to generate strings   std  string GenerateString size t len        std  string str      auto rand char         return alphabet randomer            std  generate n std  back inserter str   len  rand char       return str      Write your suggestions for improvement below   https   gist github com VjGusev e6da2cb4d4b0b531c1d009cd1f8904ad

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void gen random char  s  size t len         for  size t i   0  i  lt  len    i             int randomChar   rand    26 26 10            if  randomChar  lt  26               s i     a    randomChar           else if  randomChar  lt  26 26               s i     A    randomChar - 26           else              s i     0    randomChar - 26 - 26              s len    0

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include  lt iostream gt   include  lt string gt   include  lt random gt   std  string generateRandomId size t length   0        static const std  string allowed chars   123456789BCDFGHJKLMNPQRSTVWXZbcdfghjklmnpqrstvwxz         static thread local std  default random engine randomEngine std  random device           static thread local std  uniform int distribution lt int gt  randomDistribution 0  allowed chars size   - 1        std  string id length   length   32    0         for  std  string  value type amp  c   id            c   allowed chars randomDistribution randomEngine               return id     int main         std  cout  lt  lt  generateRandomId    lt  lt  std  endl

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I hope this helps someone   Tested at https   www codechef com ide with C   4 9 2   include  lt iostream gt   include  lt string gt   include  lt stdlib h gt         srand  rand     using namespace std   string RandomString int len       string str    0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz      string newstr     int pos     while newstr size      len        pos     rand      str size   - 1         newstr    str substr pos 1           return newstr     int main        srand time 0       string random str   RandomString 100      cout  lt  lt   random str      lt  lt  random str  lt  lt  endl      Output  random str   DNAT1LAmbJYO0GvVo4LGqYpNcyK3eZ6t0IN3dYpHtRfwheSYipoZOf04gK7OwFIwXg2BHsSBMB84rceaTTCtBC0uZ8JWPdVxKXBd

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void gen random char  s  size t len         for  size t i   0  i  lt  len    i             int randomChar   rand    26 26 10            if  randomChar  lt  26               s i     a    randomChar           else if  randomChar  lt  26 26               s i     A    randomChar - 26           else              s i     0    randomChar - 26 - 26              s len    0

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Example for Qt use    QString random string int length 32  QString allow symbols QString  abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789          QString result      qsrand QTime  currentTime   msec         for  int i   0  i  lt  length    i                        result append allow symbols at qrand      allow symbols length                 return result

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Here s a funny one-liner  Needs ASCII   void gen random char  s  int l        for  int c  c rand   62   s      c  07    c 16  26    l-- gt 0

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I just tested this  it works sweet and doesn t require a lookup table  rand alnum   sort of forces out alphanumerics but because it selects 62 out of a possible 256 chars it isn t a big deal    include  lt cstdlib gt       for rand    include  lt cctype gt        for isalnum       include  lt algorithm gt     for back inserter  include  lt string gt   char  rand alnum         char c      while   std  isalnum c   static cast lt char gt  std  rand                    return c      std  string  rand alnum str  std  string  size type sz        std  string s      s reserve   sz       generate n  std  back inserter s   sz  rand alnum       return s

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Random string  every run file   different string          auto randchar        - gt  char               const char charset                  ABCDEFGHIJKLMNOPQRSTUVWXYZ               abcdefghijklmnopqrstuvwxyz            const size t max index    sizeof charset  - 1            return charset randomGenerator 0  max index                       std  string custom string              size t LENGTH NAME   6    length of name     generate n custom string begin    LENGTH NAME  randchar

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Let s make random convenient again    I made up a nice C  11 header only solution  You could easily add one header file to your project and then add your tests or use random strings for another purposes   That s a quick description  but you can follow the link to check full code  The main part of solution is in class Randomer   class Randomer          random seed by default     std  mt19937 gen       std  uniform int distribution lt size t gt  dist    public             some convenience ctors             Randomer size t min  size t max  unsigned int seed   std  random device                gen  seed   dist  min  max                  if you want predictable numbers     void SetSeed unsigned int seed            gen  seed seed              size t operator               return dist  gen               Randomer incapsulates all random stuff and you can add your own functionality to it easily  After we have Randomer  it s very easy to generate strings   std  string GenerateString size t len        std  string str      auto rand char         return alphabet randomer            std  generate n std  back inserter str   len  rand char       return str      Write your suggestions for improvement below   https   gist github com VjGusev e6da2cb4d4b0b531c1d009cd1f8904ad

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include  lt iostream gt   include  lt string gt   include  lt random gt   std  string generateRandomId size t length   0        static const std  string allowed chars   123456789BCDFGHJKLMNPQRSTVWXZbcdfghjklmnpqrstvwxz         static thread local std  default random engine randomEngine std  random device           static thread local std  uniform int distribution lt int gt  randomDistribution 0  allowed chars size   - 1        std  string id length   length   32    0         for  std  string  value type amp  c   id            c   allowed chars randomDistribution randomEngine               return id     int main         std  cout  lt  lt  generateRandomId    lt  lt  std  endl

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void gen random char  s  size t len         for  size t i   0  i  lt  len    i             int randomChar   rand    26 26 10            if  randomChar  lt  26               s i     a    randomChar           else if  randomChar  lt  26 26               s i     A    randomChar - 26           else              s i     0    randomChar - 26 - 26              s len    0

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Here s my adaptation of Ates Goral s answer using C  11  I ve added the lambda in here  but the principle is that you could pass it in and thereby control what characters your string contains   std  string random string  size t length         auto randchar        - gt  char               const char charset              0123456789           ABCDEFGHIJKLMNOPQRSTUVWXYZ           abcdefghijklmnopqrstuvwxyz           const size t max index    sizeof charset  - 1           return charset  rand     max index               std  string str length 0       std  generate n  str begin    length  randchar        return str      Here is an example of passing in a lambda to the random string function  http   ideone com Ya8EKf  Why would you use C  11     Because you can produce strings that follow a certain probability distribution  or distribution combination  for the character set you re interested in  Because it has built-in support for non-deterministic random numbers Because it supports unicode  so you could change this to an internationalized version    For example    include  lt iostream gt   include  lt vector gt   include  lt random gt   include  lt functional gt    for std  function  include  lt algorithm gt     for std  generate n  typedef std  vector lt char gt  char array   char array charset           Change this to suit     return char array         0   1   2   3   4        5   6   7   8   9        A   B   C   D   E   F        G   H   I   J   K        L   M   N   O   P        Q   R   S   T   U        V   W   X   Y   Z        a   b   c   d   e   f        g   h   i   j   k        l   m   n   o   p        q   r   s   t   u        v   w   x   y   z                     given a function that generates a random character     return a string of the requested length std  string random string  size t length  std  function lt char void  gt  rand char         std  string str length 0       std  generate n  str begin    length  rand char        return str     int main           0  create the character set           yes  you can use an array here            but a function is cleaner and more flexible     const auto ch set   charset           1  create a non-deterministic random number generator           std  default random engine rng std  random device              2  create a random number  shaper  that will give          us uniformly distributed indices into the character set     std  uniform int distribution lt  gt  dist 0  ch set size  -1          3  create a function that ties them together  to get           a non-deterministic uniform distribution from the           character set of your choice      auto randchar     ch set  amp dist  amp rng     return ch set  dist rng              4  set the length of the string you want and profit              auto length   5      std  cout lt  lt random string length randchar  lt  lt std  endl      return 0      Sample output

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Random string  every run file   different string          auto randchar        - gt  char               const char charset                  ABCDEFGHIJKLMNOPQRSTUVWXYZ               abcdefghijklmnopqrstuvwxyz            const size t max index    sizeof charset  - 1            return charset randomGenerator 0  max index                       std  string custom string              size t LENGTH NAME   6    length of name     generate n custom string begin    LENGTH NAME  randchar

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Rather than manually looping  prefer using the appropriate C   algorithm  in this case std  generate n  with a proper random number generator  auto generate random alphanumeric string std  size t len  - gt  std  string       static constexpr auto chars            quot 0123456789 quot           quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot           quot abcdefghijklmnopqrstuvwxyz quot       thread local auto rng   random generator lt  gt         auto dist   std  uniform int distribution     std  strlen chars  - 1       auto result   std  string len    0        std  generate n begin result   len    amp       return chars dist rng            return result     This is close to something I would call the    canonical    solution for this problem  Unfortunately  correctly seeding a generic C   random number generator  e g  MT19937  is really hard  The above code therefore uses a helper function template  random generator  template  lt typename T   std  mt19937 gt  auto random generator   - gt  T       auto constexpr seed bytes   sizeof typename T  result type    T  state size      auto constexpr seed len   seed bytes   sizeof std  seed seq  result type       auto seed   std  array lt std  seed seq  result type  seed len gt         auto dev   std  random device        std  generate n begin seed   seed len  std  ref dev        auto seed seq   std  seed seq begin seed   end seed        return T seed seq      This is complex and relatively inefficient  Luckily it   s used to initialise a thread local variable and is therefore only invoked once per thread  Finally  the necessary includes for the above are   include  lt algorithm gt   include  lt array gt   include  lt cstring gt   include  lt functional gt   include  lt random gt   include  lt string gt   The above code uses class template argument deduction and thus requires C  17  It can be trivially adapted for earlier versions by adding the required template arguments

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Example for Qt use    QString random string int length 32  QString allow symbols QString  abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789          QString result      qsrand QTime  currentTime   msec         for  int i   0  i  lt  length    i                        result append allow symbols at qrand      allow symbols length                 return result

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include  lt iostream gt   include  lt string gt   include  lt stdlib h gt  int main         int size      std  cout  lt  lt   Enter size          std  cin  gt  gt  size      std  string str      for  int i   0  i  lt  size  i                  auto d   rand     26    a           str push back d             for  int i   0  i  lt  size  i                  std  cout  lt  lt  str i   lt  lt    t              return 0

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I just tested this  it works sweet and doesn t require a lookup table  rand alnum   sort of forces out alphanumerics but because it selects 62 out of a possible 256 chars it isn t a big deal    include  lt cstdlib gt       for rand    include  lt cctype gt        for isalnum       include  lt algorithm gt     for back inserter  include  lt string gt   char  rand alnum         char c      while   std  isalnum c   static cast lt char gt  std  rand                    return c      std  string  rand alnum str  std  string  size type sz        std  string s      s reserve   sz       generate n  std  back inserter s   sz  rand alnum       return s

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Mehrdad Afshari s answer would do the trick  but I found it a bit too verbose for this simple task  Look-up tables can sometimes do wonders   include  lt iostream gt   include  lt ctime gt   include  lt unistd h gt   using namespace std   string gen random const int len             string tmp s      static const char alphanum              quot 0123456789 quot           quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot           quot abcdefghijklmnopqrstuvwxyz quot            srand   unsigned  time NULL    getpid          tmp s reserve len        for  int i   0  i  lt  len    i           tmp s    alphanum rand      sizeof alphanum  - 1                  return tmp s          int main int argc  char  argv               cout  lt  lt  gen random 12   lt  lt  endl           return 0

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void strGetRandomAlphaNum char  sStr  unsigned int iLen      char Syms      0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ     unsigned int Ind   0    srand time NULL    rand       while Ind  lt  iLen          sStr Ind      Syms rand   62         sStr iLen      0

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Here s a funny one-liner  Needs ASCII   void gen random char  s  int l        for  int c  c rand   62   s      c  07    c 16  26    l-- gt 0

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Rather than manually looping  prefer using the appropriate C   algorithm  in this case std  generate n  with a proper random number generator  auto generate random alphanumeric string std  size t len  - gt  std  string       static constexpr auto chars            quot 0123456789 quot           quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot           quot abcdefghijklmnopqrstuvwxyz quot       thread local auto rng   random generator lt  gt         auto dist   std  uniform int distribution     std  strlen chars  - 1       auto result   std  string len    0        std  generate n begin result   len    amp       return chars dist rng            return result     This is close to something I would call the    canonical    solution for this problem  Unfortunately  correctly seeding a generic C   random number generator  e g  MT19937  is really hard  The above code therefore uses a helper function template  random generator  template  lt typename T   std  mt19937 gt  auto random generator   - gt  T       auto constexpr seed bytes   sizeof typename T  result type    T  state size      auto constexpr seed len   seed bytes   sizeof std  seed seq  result type       auto seed   std  array lt std  seed seq  result type  seed len gt         auto dev   std  random device        std  generate n begin seed   seed len  std  ref dev        auto seed seq   std  seed seq begin seed   end seed        return T seed seq      This is complex and relatively inefficient  Luckily it   s used to initialise a thread local variable and is therefore only invoked once per thread  Finally  the necessary includes for the above are   include  lt algorithm gt   include  lt array gt   include  lt cstring gt   include  lt functional gt   include  lt random gt   include  lt string gt   The above code uses class template argument deduction and thus requires C  17  It can be trivially adapted for earlier versions by adding the required template arguments

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Something even simpler and more basic in case you re happy for your string to contain any printable characters    include  lt time h gt       we ll use time for the seed  include  lt string h gt     this is for strcpy  void randomString int size  char  output     pass the destination size and the destination itself       srand time NULL       seed with time      char src size       size   rand     size     this randomises the size  optional       src size      0      start with the end of the string               and work your way backwards     while --size  gt  -1          src size     rand     94    32     generate a string ranging from the space character to    tilde       strcpy output  src      store the random string

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I just tested this  it works sweet and doesn t require a lookup table  rand alnum   sort of forces out alphanumerics but because it selects 62 out of a possible 256 chars it isn t a big deal    include  lt cstdlib gt       for rand    include  lt cctype gt        for isalnum       include  lt algorithm gt     for back inserter  include  lt string gt   char  rand alnum         char c      while   std  isalnum c   static cast lt char gt  std  rand                    return c      std  string  rand alnum str  std  string  size type sz        std  string s      s reserve   sz       generate n  std  back inserter s   sz  rand alnum       return s

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include  lt iostream gt   include  lt string gt   include  lt stdlib h gt  int main         int size      std  cout  lt  lt   Enter size          std  cin  gt  gt  size      std  string str      for  int i   0  i  lt  size  i                  auto d   rand     26    a           str push back d             for  int i   0  i  lt  size  i                  std  cout  lt  lt  str i   lt  lt    t              return 0

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Be ware when calling the function   string gen random const int len    static const char alphanum      0123456789           ABCDEFGHIJKLMNOPQRSTUVWXYZ    stringstream ss   for  int i   0  i  lt  len    i        ss  lt  lt  alphanum rand      sizeof alphanum  - 1      return ss str         adapted of  Ates Goral  it will result in the same character sequence every time  Use  srand time NULL      before calling the function  although the rand   function is always seeded with 1  kjfletch   For Example   void SerialNumberGenerator          srand time NULL        for  int i   0  i  lt  5  i              cout  lt  lt  gen random 10   lt  lt  endl

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Here s my adaptation of Ates Goral s answer using C  11  I ve added the lambda in here  but the principle is that you could pass it in and thereby control what characters your string contains   std  string random string  size t length         auto randchar        - gt  char               const char charset              0123456789           ABCDEFGHIJKLMNOPQRSTUVWXYZ           abcdefghijklmnopqrstuvwxyz           const size t max index    sizeof charset  - 1           return charset  rand     max index               std  string str length 0       std  generate n  str begin    length  randchar        return str      Here is an example of passing in a lambda to the random string function  http   ideone com Ya8EKf  Why would you use C  11     Because you can produce strings that follow a certain probability distribution  or distribution combination  for the character set you re interested in  Because it has built-in support for non-deterministic random numbers Because it supports unicode  so you could change this to an internationalized version    For example    include  lt iostream gt   include  lt vector gt   include  lt random gt   include  lt functional gt    for std  function  include  lt algorithm gt     for std  generate n  typedef std  vector lt char gt  char array   char array charset           Change this to suit     return char array         0   1   2   3   4        5   6   7   8   9        A   B   C   D   E   F        G   H   I   J   K        L   M   N   O   P        Q   R   S   T   U        V   W   X   Y   Z        a   b   c   d   e   f        g   h   i   j   k        l   m   n   o   p        q   r   s   t   u        v   w   x   y   z                     given a function that generates a random character     return a string of the requested length std  string random string  size t length  std  function lt char void  gt  rand char         std  string str length 0       std  generate n  str begin    length  rand char        return str     int main           0  create the character set           yes  you can use an array here            but a function is cleaner and more flexible     const auto ch set   charset           1  create a non-deterministic random number generator           std  default random engine rng std  random device              2  create a random number  shaper  that will give          us uniformly distributed indices into the character set     std  uniform int distribution lt  gt  dist 0  ch set size  -1          3  create a function that ties them together  to get           a non-deterministic uniform distribution from the           character set of your choice      auto randchar     ch set  amp dist  amp rng     return ch set  dist rng              4  set the length of the string you want and profit              auto length   5      std  cout lt  lt random string length randchar  lt  lt std  endl      return 0      Sample output

[c++] How do I create a random alpha-numeric string in C++?

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