How to generate array of random float numbers using functions in C

Question

I already asked a question regarding a program. I got an answer but I have a new issue. The program I wrote is not working and I don't know why.

The function is working fine (I guess) but I want to be sure by printing the values of each element of the array and see if they are correct (I will need this for other purposes).

I tried with a simple for instruction for a printf, but the array seems to be empty. The problem may be related to the memory address of the value I want to print.

Please have a look, any advice will be welcome!

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

int * rand_gen(int N, float fl_value); // Function declaration

int main()  // Main function
{

int N;  // Declaration: Number of trials
printf("Number of trials: ");
scanf("%d", &N);    // Asks for Number of trials

float alpha = 0.3;
float beta  = 0.4;
float gamma = 0.5;
float delta = 0.6;

int i;  // Index
int seed = time(NULL); // Random number generator seed (based on current time)
srand(seed);

// Populate arrays
float *bin_array_alpha[] = { rand_gen( N, alpha ) };
float *bin_array_beta[]  = { rand_gen( N, beta )  };
float *bin_array_gamma[] = { rand_gen( N, gamma ) };
float *bin_array_delta[] = { rand_gen( N, delta ) };



// Here I would like to print the elements of the arrays, something like:

for ( i=0 ; i<N ; i++)
{
    printf("\nbin_array_alpha[%d] = %f",i,bin_array_alpha[i]);
    printf("\nbin_array_beta[%d]  = %f",i,bin_array_beta[i]);
    printf("\nbin_array_gamma[%d] = %f",i,bin_array_gamma[i]);
    printf("\nbin_array_delta[%d] = %f",i,bin_array_delta[i]);
    printf("\n");
}


// Free the memory
static const size_t m = sizeof(bin_array_alpha)/sizeof(bin_array_alpha[0]);

for ( size_t i = 0 ; i < m ; ++i )
{
    free(bin_array_alpha[i]);
    free(bin_array_beta[i]);
    free(bin_array_gamma[i]);
    free(bin_array_delta[i]);

    bin_array_alpha[i] = NULL;
    bin_array_beta[i]  = NULL;
    bin_array_gamma[i] = NULL;
    bin_array_delta[i] = NULL;
}

printf("\n");

return(0);
}


// Function: generate an array populated by: array[j] = rand()*fl_value
int * rand_gen(int N, float fl_value)
{
    int *array;
    array = (int *)malloc(sizeof(float)*N);
    if(array == NULL)
    {
        printf("\nRun out of memory!\n");
        exit(1);
    }

    int j;
    float x;
    for( j = 0 ; j < N ; j++ )
    {
        x = rand(); // Generates a random number
        x = x/RAND_MAX; // 0 < x < 1
        x = x * fl_value;
        array[j] = x; // array[j] = x * fl_value
    }

    return array;
}

The result is always the same (for example N=3):

bin_array_alpha[0] = 0.000000
bin_array_beta[0]  = 0.000000
bin_array_gamma[0] = 0.000000
bin_array_delta[0] = 0.000000

bin_array_alpha[1] = 0.000000
bin_array_beta[1]  = 0.000000
bin_array_gamma[1] = 0.000000
bin_array_delta[1] = 0.000000

bin_array_alpha[2] = 0.000000
bin_array_beta[2]  = 0.000000
bin_array_gamma[2] = 0.000000
bin_array_delta[2] = 0.000000

Thank you so much again!

You have array of pointer-to-float, but you're printing as if you have an array of float. And you're initializing the array with one element from a function that returns a pointer-to-int. — user3386109
– user3386109, Commented Nov 9, 2015 at 19:48
@user3386109 so how should I print the pointer to float? And how do I change the initialisation pointer to float rather than pointer to int? Thank you by the way ;) — Pullo86
– Pullo86, Commented Nov 9, 2015 at 20:01
Remove the [] after each of the 4 declarations. The function returns a pointer, although, its int* pointer is incompatible with a float* pointer.. — Weather Vane
– Weather Vane, Commented Nov 9, 2015 at 20:07

gaetanoM · Accepted Answer · 2015-11-10 17:52:23Z

2

your problem is in the allocation of the arrays of float that you not simply confused with int but the technique of allocation is wrong:

float **rand_gen(int N, float fl_value)
{
    float **array;
    array = (float **)malloc(sizeof(float *) * N + 1);
    if (array == NULL)
    {
        printf("\nRun out of memory!\n");
        exit(1);
    }
    int i;
    for (i = 0; i < N; i++)
    {
        array[i] = (int *)malloc(sizeof(float));
        array[i][0] = 0;
    }
    array[N] = NULL;

    int j;
    float x;
    for (j = 0; j < N; j++)
    {
        x = rand(); // Generates a random number
        x = x / RAND_MAX; // 0 < x < 1
        x = x * fl_value;
        *(array[j]) = x; // array[j] = x * fl_value
    }

    return array;
}

An array of float is a pointer to a memory area where each element is a pointer to a float.

This means you have to correct the declaration:

float **rand_gen(int N, float fl_value); // Function declaration

and the usage in the main:

float **bin_array_alpha = rand_gen(N, alpha);
float **bin_array_beta = rand_gen(N, beta);
float **bin_array_gamma = rand_gen(N, gamma);
float **bin_array_delta = rand_gen(N, delta);

The same happens for the printf:

printf("\nbin_array_alpha[%d] = %f", i, *bin_array_alpha[i]);

Instead to free the memory you need to add the free not only of each float but also of the memory base:

free(bin_array_alpha

edited Nov 10, 2015 at 17:52

answered Nov 9, 2015 at 20:40

gaetanoM

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4 Comments

Pullo86 Over a year ago

Worked! Thank you so much! I used to program in C but never had this kind of things to do (arrays from functions etc.).

phoxis Over a year ago

Why the float ** you could have taken a float * and why typecast the malloc return as int ** ?? It should be float ** or just omit it.

gaetanoM Over a year ago

Why I used the double pointer? To make clear the meaning of memory allocation with pointers of float/char/int/... You are right, indeed the function can be reduced to float *rand_gen(int N, float fl_value) but my intention is to make visible the needs to allocate before the pointer to the array and after the memory required for each element. I could make an example with a design to clarify the memory allocation in c. I apologize for that. I hope now it's clear my intention. Your comments are of a great value for me. Thanks so much

phoxis Over a year ago

That's great, but as the question is on generating random numbers, the pointer stuffs in addition is not on the context and adds additional complexity which is unrelated.

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