Generate random letters in C - c

Question

here is my code. I am trying generate random alphabet but i see same letters.
example: (YHTGDHFBSHXCHFYFUXZWDYKLXI) How can i fix it? just i need mixed alphabet not same letters. Thank you so much.
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
void random_string(char * string, unsigned length)
{
/* Seed number for rand() */
srand((unsigned int) time(0));
int i;
for (i = 0; i < length; ++i)
{
string[i] = rand() % 26 + 'A';
}
string[i] = '\0';
}
int main(void)
{
char s[26];
random_string(s, 26);
printf("%s\n", s);
return 0;
}

Answer 1

The operation you are looking for is called a shuffle or a permutation. It is not sufficient to call a random-letter function 26 times, since, as you see, you can generate duplicates.
Instead, start with the string "ABCDEFGHIJKLMNOPQRSTUVWXYZ" and perform a shuffle operation. If you want to learn by doing such things from scratch, I recommend reading about the Fisher-Yates Shuffle then crafting an implementation on your own.

Answer 2

You can do this by having a pool of available characters, and taking one from the pool. Please note that your target string was too short to accomodate the string terminator. Similar to Fisher Yates shuffle.
Edit: changed the types to size_t.
#include <time.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define LENGTH 26 // the cipher key length
void random_string(char * string, size_t length)
{
char pool[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
size_t poolsize = strlen(pool);
size_t index;
size_t i;
srand((unsigned)time(NULL));
for(i = 0; i < length && poolsize > 0; ++i)
{
index = rand() % poolsize; // a random index into the pool
string[i] = pool[index]; // take that character
pool[index] = pool[--poolsize]; // replace it with the last pool ...
} // ... element and shorten the pool
string[i] = '\0';
}
int main(void)
{
char s[LENGTH + 1]; // adequate length
random_string(s, LENGTH);
printf("%s\n", s);
return 0;
}
Program output:
QYMUSFALIZCXGONBJRETHPVKDW

Answer 3

// why program get errors this line? (unused variable cipher_text)
char *cipher_text, msg[255];
You declare cipher_text but you only use msg.
You could declare only:
char msg[255];

Answer 4

Copy paste from cplusplus.com/reference
The pseudo-random number generator is initialized using the argument
passed as seed.
For every different seed value used in a call to srand, the
pseudo-random number generator can be expected to generate a different
succession of results in the subsequent calls to rand.
Two different initializations with the same seed will generate the
same succession of results in subsequent calls to rand.
If seed is set to 1, the generator is reinitialized to its initial
value and produces the same values as before any call to rand or
srand.
In order to generate random-like numbers, srand is usually initialized
to some distinctive runtime value, like the value returned by function
time (declared in header ). This is distinctive enough for most
trivial randomization needs.
/* srand example */
#include <stdio.h> /* printf, NULL */
#include <stdlib.h> /* srand, rand */
#include <time.h> /* time */
int main ()
{
printf ("First number: %d\n", rand()%100);
srand (time(NULL));
printf ("Random number: %d\n", rand()%100);
srand (1);
printf ("Again the first number: %d\n", rand()%100);
return 0;
}