CODEWITHSUNDEEP
carrays
xsun
xsun

Reputation: 11

2D Array elements do not save values in C

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


int main(void) {
    double** F = (double**) malloc(4097 * sizeof(double*));;
    int flops = 0;
    int i, j, k;
    double r;

    for (i=0; i<4097; i++) {
        F[i] = (double*) malloc(4097*sizeof(double));
    }

    // insert code to initialize array elements to random values between 1.0 and 2.0
    for (i=0; i<4097; i++) {
        for (j=0; j<4097; j++) {
            r = (double)rand()/(double)(RAND_MAX/2.0);
            F[i][j] = r;
            printf("%f %f\n", r, F[i,j]);
        }
    }

}

I am trying to generate random numbers and store them into a 4097 x 4097 array but when the above code is run the array is not updated.

Upvotes: 1

Views: 100

Answers (2)

alfalfasprout
alfalfasprout

Reputation: 271

Why are you using the comma operator F[i,j]? The array populates correctly... just replace this with F[i][j] to verify.

Also, free your memory.

Upvotes: 0

P.P
P.P

Reputation: 121397

This line 

        printf("%f %f\n", r, F[i,j]);

should be

        printf("%f %f\n", r, F[i][j]);

However, the reason it's a valid syntax is because of the comma operator. Comma operator evaluates its operands and yields only the last operand's value. So, 

        printf("%f %f\n", r, F[i,j]);

is equivalent to

        printf("%f %f\n", r, F[j]);

However, this has a problem. F[j] is of type double * whereas %f expects a double. GCC warns:

    warning: format ‘%f’ expects argument of type ‘double’, but argument 3  
has type ‘double *’ [-Wformat=]

Increasing your compiler warning levels would help catch these.

Upvotes: 1

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