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++?

Answer 1

 void gen_random(char *s, const int len) {
     for (int i = 0; i < len; ++i) {
         int randomChar = rand()%(26+26+10);
         if (randomChar < 26)
             s[i] = 'a' + randomChar;
         else if (randomChar < 26+26)
             s[i] = 'A' + randomChar - 26;
         else
             s[i] = '0' + randomChar - 26 - 26;
     }
     s[len] = 0;
 }

Answer 2

Could you generate GUID's from the built in function(s) and then just trim off the length you need?

Answer 3

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:

void gen_random(char *s, const int len) {
    static const char alphanum[] =
        "0123456789"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz";

    for (int i = 0; i < len; ++i) {
        s[i] = alphanum[rand() % (sizeof(alphanum) - 1)];
    }

    s[len] = 0;
}

Answer 4

I tend to always use the structured C++ ways for this kind of initialization. Notice that fundamentally, it's no different than Altan's solution. To a C++ programmer, it just expresses the intent a tad better and might be easier portable to other data types. In this instance, the C++ function generate_n expresses exactly what you want:

struct rnd_gen {
    rnd_gen(char const* range = "abcdefghijklmnopqrstuvwxyz0123456789")
        : range(range), len(std::strlen(range)) { }

    char operator ()() const {
        return range[static_cast<std::size_t>(std::rand() * (1.0 / (RAND_MAX + 1.0 )) * len)];
    }
private:
    char const* range;
    std::size_t len;
};

std::generate_n(s, len, rnd_gen());
s[len] = '\0';

By the way, read Julienne’s essay on why this calculation of the index is preferred over simpler methods (like taking the modulus).

Answer 5

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 <cstdlib>   // for rand()
#include <cctype>    // for isalnum()   
#include <algorithm> // for back_inserter
#include <string>

char 
rand_alnum()
{
    char c;
    while (!std::isalnum(c = static_cast<char>(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;
}

Answer 6

To do in C# go to this post

http://www.dotnetfunda.com/forums/thread1357-how-do-generate-unique-alpha-numeric-random-number-in-aspnet.aspx

Thanks