Random Hexadecimal in C

Question

I am making a bus reservation program. I want to give random hex numbers to the buses. How can i hold it in an array? in C

#include <stdlib.h>
#include <stdio.h>
#include <time.h>

char rand_hex_string()
{
  //hexadecimal characters
  char hex_characters[]={'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'};
  return (char) hex_characters[rand()%16];
}


int main(int argc, char *argv[]) {
    srand(time(NULL));
    char busNo[6] = { '3', '7' , rand_hex_string() , rand_hex_string(), rand_hex_string(), rand_hex_string()};

printf("%s", busNo);

return 0;
}

I can print them all side by side but I cannot assign them all to an array.

For example, a number called 37604F was created. I want the first element of an array to be 37604F.

I want to assign a randomly generated 6-digit hexadecimal number starting with 37 to an array as a string. i dont know C very well

Answer 1

UPDATE

The code in my old answer isn't very efficient, but was very easy to understand and to implement. If you need to run the code on non-unix systems, or you can't access /dev/urandom, and need a more performant code this is probably the best answer.

Please note that rand_buf() I tried to code was worse then this function most of the times, so I decided to just copy-paste it.

#include <stdlib.h>
#include <stdio.h>
#include <time.h>

#if RAND_MAX == 0x7FFF
#define RAND_MAX_BITS 15
#elif RAND_MAX == 0x7FFFFFFF
#define RAND_MAX_BITS 31
#else
#error TBD code
#endif

#define byte unsigned char

#define MAX_BUS 32

//https://stackoverflow.com/a/50820616/9373031
//Queues leftover bits to optimize rand() 15 bits usage
void rand_buf(byte *dest, size_t size) {

    int r;
    int r_queue = 0;
    int r_bit_count = 0;

    for (size_t i = 0; i < size; i++) {
        r = 0;
        //printf("%3zu %2d %8x\n", i, r_bit_count, r_queue);
        if (r_bit_count < 8) {
            int need = 8 - r_bit_count;
            r = r_queue << need;
            r_queue = rand();
            r ^= r_queue;  // OK to flip bits already saved in `r`
            r_queue >>= need;
            r_bit_count = RAND_MAX_BITS - need;
        } else {
            r = r_queue;
            r_queue >>= 8;
            r_bit_count -= 8;
        }
        dest[i] = r;
    }

}

int main(int argc, char *argv[]) {

    srand(time(NULL));

    byte * rbuf = malloc(MAX_BUS*2);
    char buses[MAX_BUS][7];

    rand_buf(rbuf, MAX_BUS*2);

    for (int i = 0; i < MAX_BUS; i++) {

        sprintf(buses[i], "37%02X%02X", rbuf[i*2], rbuf[i*2 + 1]);
        //printf("%d = %s\n", i, buses[i]);

    }

    return 0;
}

rand_buf() simply works by storing 7 leftover bits (rand() returns at least 15 bits) and using "recycle" them later on.

OLD ANSWER

I would suggest using a better name for your function. Also you might want to end the arrays with \0 to create proper C strings. If you don't do it printf("%s") will read the whole array

After that you have to choose a size for a new c-string array (char[MAX_BUS][6], where MAX_BUS is the number of buses you will need to store).

#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <string.h>

#define MAX_BUS 32

char rand_hex_char() {     //returns a char not a char* (aka c-string)
  //hexadecimal characters
  char hex_characters[]={'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'};
  return hex_characters[rand()%16];
}

int main(int argc, char *argv[]) {
    srand(time(NULL));

    char buses[MAX_BUS][7];

    for (int i = 0; i < MAX_BUS; i++) {
        buses[i][0] = '3';
        buses[i][1] = '7';
        for (int j = 0; j < 4 /*rand chars*/; j++)
            buses[i][j+2] = rand_hex_char();
        buses[i][6] = 0;
        printf("%d = %s\n", i, buses[i]);
    }

    printf("%s\n", buses[0]);   //access the first bus

    return 0;
}