Dynamically allocated two dimensional (double) array in C

Question

I'm trying to dynamically allocate two dimensional array N+1xN with all elements double. Then I want to put value (1.6) for each element of an array. The last step is to print this array. My function

void inic(double **t)
{
    for(int y=0;y<N;y++)
    {
        for(int x=0;x<N+1;x++)
        {
            t[x][y]=1.5;
        }
    }
}

isn't working as it should and I don't know why.

For example the (for n=3) result is:

[0][0][0][0] 
[0][0][0][0] 
[0][0][0][0]

And I expected:

[1.6][1.6][1.6][1.6] 
[1.6][1.6][1.6][1.6] 
[1.6][1.6][1.6][1.6] 
[1.6][1.6][1.6][1.6]

Here is my code:

#include <stdio.h>
#include <stdlib.h>
int N;

void inic(double **t)
{
    for(int y=0;y<N;y++)
    {
        for(int x=0;x<N+1;x++)
        {

            t[x][y]=1.5;

        }
    }
}

void print(double **t)
{
    for(int y=0;y<N;y++)
    {
        for(int x=0;x<N+1;x++)
        {
            printf("%i ",t[x][y]);
        }
        printf("\n");
    }
}




int main()
{
    double **t;
    N=3;
    t=malloc((N+1)*sizeof(double*));
    for(int i=0;i<N+1;i++)
    {
        t[i]=malloc(N*sizeof(double));
    }

    inic(t);
    print(t);
    printf("\n");
}

double ** is not a 2D array and cannot point to one. You can allocate a 2D array dynamically without problem, why not use it? And "my code is not working" is not a specific problem description. — too honest for this site
– too honest for this site, Commented Apr 17, 2017 at 17:25
In the first place, your array has (N+1).N elements, not N+1. — user1196549
– user1196549, Commented Apr 17, 2017 at 17:26
How do you expect it to have 1.6 if you put everywhere 1.5???? :) — Alex Lop.
– Alex Lop., Commented Apr 17, 2017 at 17:27

Vlad from Moscow · Accepted Answer · 2017-04-17 17:40:31Z

1

You are using incorrect conversion specifier. Try the following

printf("%lf ",t[x][y]);

Also it would be more natural to use another order of the indices that is when this statement

t=malloc((N)*sizeof(double*));

allocates N "rows" and then the N + 1 "columns" are allocated in the loop

for(int i=0;i<N;i++)
{
    t[i]=malloc( ( N + 1 )*sizeof(double));
}

In this case other functions will look like

void inic(double **t)
{
    for(int x = 0; x < N; x++)
    {
        for( int y = 0;  y < N + 1; y++ )
        {

            t[x][y]=1.5;

        }
    }
}

void print(double **t)
{
    for( int x = 0; x < N; x++ )
    {
        for( int y = 0; y < N + 1; y++)
        {
            printf("%lf ",t[x][y]);
        }
        printf("\n");
    }
}

Also you should free all the allocated memory at the end of the program.

edited Apr 17, 2017 at 17:40

answered Apr 17, 2017 at 17:31

Vlad from Moscow

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1 Comment

Nowak Grzegorz Over a year ago

It worked however I had to change: "printf("%lf ",t[x][y]);" for "printf("%f ",t[x][y]);". :D

devalone · Accepted Answer · 2017-04-17 17:28:06Z

1

try to replace t[x][y] to t[y][x]

answered Apr 17, 2017 at 17:28

devalone

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1 Comment

devalone Over a year ago

also replace printf("%i ",t[y][x]); to printf("%f ",t[y][x]);

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