Generate a random array of integers in a loop in C

Question

I was trying to generate a random array of integers of length 16 inside a loop. The elements of the array will lie inside [0, 255]. I thought assigning random integers from [0, 255] would suffice; but it does not work (one random array is created which remains unchanged over iterations). Then I tried shuffling, but the situation does not improve (I also notice that the probability of 0 in the 'random' array is significantly larger).

Here is my code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
//#define Shuffle

/* Generate a random integer array of length `size` with elements from [0, 255] */
int* random_sample(int size) 
{
int i, j, k;
int *elements = malloc(size*sizeof(int));

/* Assign random integers */
for (i = size - 1; i > 0; --i)
{   
    elements[i] = rand() % 256;
}

/* Shuffle */
#ifdef Shuffle
for (i = size - 1; i > 0; --i) {
    j = rand() % size;

    k = elements[i];
    elements[i] = elements[j];
    elements[j] = k;
}
#endif

return elements;
}

int main(int argc, char const *argv[])
{
    int LENGTH = 16, i, iteration;
    int *random_array = random_sample(LENGTH);

    srand(time(NULL));

    for (iteration = 0; iteration < 10; ++iteration)
    {
        for (i = 0; i < LENGTH; ++i) 
            printf("%d, ", random_array[i]); 

        puts("");
    }

    return 0;
}

A typical output looks like:

0, 227, 251, 242, 171, 186, 205, 41, 236, 74, 255, 81, 115, 105, 198, 103, 
0, 227, 251, 242, 171, 186, 205, 41, 236, 74, 255, 81, 115, 105, 198, 103, 
0, 227, 251, 242, 171, 186, 205, 41, 236, 74, 255, 81, 115, 105, 198, 103, 
0, 227, 251, 242, 171, 186, 205, 41, 236, 74, 255, 81, 115, 105, 198, 103, 
0, 227, 251, 242, 171, 186, 205, 41, 236, 74, 255, 81, 115, 105, 198, 103, 
0, 227, 251, 242, 171, 186, 205, 41, 236, 74, 255, 81, 115, 105, 198, 103, 
0, 227, 251, 242, 171, 186, 205, 41, 236, 74, 255, 81, 115, 105, 198, 103, 
0, 227, 251, 242, 171, 186, 205, 41, 236, 74, 255, 81, 115, 105, 198, 103, 
0, 227, 251, 242, 171, 186, 205, 41, 236, 74, 255, 81, 115, 105, 198, 103, 
0, 227, 251, 242, 171, 186, 205, 41, 236, 74, 255, 81, 115, 105, 198, 103,

I tried several variations of the above code, but none of them works. Need help!

EDIT

I was trying variations of the code, and mistakenly printed the same unchanged array (as pointed out by some others - thanks to them); originally I wrote

int main(int argc, char const *argv[])
{
    int LENGTH = 16, i, iteration;
    int *random_array;

    srand(time(NULL));

    for (iteration = 0; iteration < 10; ++iteration)
    {
        random_array = random_sample(LENGTH);
        for (i = 0; i < LENGTH; ++i) 
            printf("%d, ", random_array[i]); 

        puts("");
    }

    return 0;
}

which print much more 0s.

EDIT (2)

Thanks to @pmg, I found the problem. Inside random_sample function, changing the first
for (i = size - 1; i > 0; --i) to for (i = size - 1; i >= 0; --i) works fine!

Thank you all.

You also seed AFTER generating the random numbers. Too late. — Fred Larson
– Fred Larson, Commented Apr 17, 2014 at 15:46
"(one random array is created which remains unchanged over iterations)" - that's exactly what you code is supposed to do. why did you expect something else? — Karoly Horvath
– Karoly Horvath, Commented Apr 17, 2014 at 15:47
You call random_sample() once. Inside the function you never assign to elements[0]. Use the standard for loop construct: for (i = 0; i < LENGTH, i++) /* ... */;; you don't need to deviate from the norm. — pmg
– pmg, Commented Apr 17, 2014 at 15:47

simurg · Accepted Answer · 2014-04-17 15:50:09Z

3

Try the code below. For the array to contain different (random) values at every iteration, you need to put different values in it :)

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
//#define Shuffle

/* Generate a random integer array of length `size` with elements from [0, 255] */
int* random_sample(int size) 
{
        int i, j, k;
        int *elements = malloc(size*sizeof(int));

        /* Assign random integers */
        for (i = size - 1; i > 0; --i)
        {   
                elements[i] = rand() % 256;
        }

        /* Shuffle */
#ifdef Shuffle
        for (i = size - 1; i > 0; --i) {
                j = rand() % size;

                k = elements[i];
                elements[i] = elements[j];
                elements[j] = k;
        }
#endif

        return elements;
}

int main(int argc, char const *argv[])
{
        int LENGTH = 16, i, iteration;
        int *random_array;

        srand(time(NULL));

        for (iteration = 0; iteration < 10; ++iteration)
        {
                random_array = random_sample(LENGTH);
                for (i = 0; i < LENGTH; ++i) 
                        printf("%d, ", random_array[i]); 

                puts("");
                free(random_array);
        }

        return 0;
}

answered Apr 17, 2014 at 15:50

simurg

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2 Comments

pmg Over a year ago

Inside the function you never assign to elements[0]. Use the standard for loop construct: for (i = 0; i < LENGTH, i++) /* ... */;; you don't need to deviate from the norm.

simurg Over a year ago

I just fixed the relevant portion of OPs code but a good catch none the less.

abelenky · Accepted Answer · 2014-04-17 15:54:23Z

2

Here's some comments on your code to clarify the problem.

int main(int argc, char const *argv[])
{
    int LENGTH = 16, i, iteration;
    int *random_array = random_sample(LENGTH);   // ARRAY is created and filled in here.

    srand(time(NULL));  // Random generator is initialized here.
                        // WAIT WHAT? You already filled in the array above!
                        // It is too late now to initialize the Generator!

answered Apr 17, 2014 at 15:54

abelenky

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1 Comment

pmg Over a year ago

+1 but if the PRNG is not seeded, it works normally as if seeded with srand(1);. This is not his immediate issue.

Safayet Ahmed · Accepted Answer · 2014-04-17 16:08:57Z

In addition to what others have said above, I think its important to note the following on the rand function as written in the man page:

Note: The versions of rand() and srand() in the Linux C Library use the same random number generator as random(3) and srandom(3), so the lower-order bits should be as random as the higher-order bits. However, on older rand() implementations, and on current implementations on different systems, the lower-order bits are much less random than the higher-order bits. Do not use this function in applications intended to be portable when good randomness is needed. (Use random(3) instead.)

There are two possible alternatives:

1) Use the random function as recommended by the note.

2) Use the higher-order bits of the random number as follows:

elements[i] = (rand() * 256) / RANDMAX;

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