CODEWITHSUNDEEP
c
MikhaelM
MikhaelM

Reputation: 173

Read and Display matrix

I'm having a small issue with this problem and I could really use another set of eyes with this. I am basically trying to read a matrix that I input and then display the said matrix.

The program always returns a null matrix (0 on all positions). The size of the matrix (columns/rows) is good. What am I doing wrong?

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

#define MAXN 10

void display_matrix(int n, int m, double a[MAXN][MAXN], char ch)
{
    int i, j;

    printf("\n MATRIX %c\n", ch);
    for(i = 0; i < n; i++)
    {
        for(j = 0; j < m; j++) printf("%8.2lf ",a[i][j]);
        printf("\n");
    }
}

void read_matrix(int *n, int *m, double a[MAXN][MAXN])
{
     int i,j;
     printf("\nInput of size and elements of a matrix\n");
     printf("\n\tNumber of rows, n=");
     scanf("%d", n);
     printf("\n\tNumber of columns, m=");
     scanf("%d", m);
     printf("\n\tThe elements of the matrix\n");
     for (i = 0; i < *n; i++)
     {
         for(j = 0; j < *m; j++)
         {
            printf("a[%d,%d]=", i, j);
            scanf("%lf",&a[i][j]);
         }
     }
     printf("\n");
}

void main()
{
    int n, m;
    double a[MAXN][MAXN];
    read_matrix(&n, &m, a);
    display_matrix(n, m, a, 'X');
    return 0;
}

Upvotes: 0

Views: 149

Answers (1)

Nisse Engstr&#246;m
Nisse Engstr&#246;m

Reputation: 4752

Your code seems mostly fine (main() should have a return type of int, and you should always check the return value of scanf()), except possibly for the following line:

printf("%8.2lf ",a[i][j]);

Prior to the C99 standard, the l modifier was not defined in conjunction with the f conversion specifier. Using the two in combination resulted in undefined behaviour. I don't know what the odds are, but it is not inconceivable that you have a pre-C99 C library that either misbehaves or implements %lf to match something entirely different, perhaps long double.

To print a double, you should use the %f conversion specifier:

printf("%8.2f ",a[i][j]);

This is just clutching at straws though. It might not change anything at all for you.

Background

The scanf() function has different conversion specifiers for float (%f) and double (%lf), because it need to know the exact type that the corresponding pointer argument is pointing to.

But when you pass a float to a variadic function, such as printf(), the default argument promotions convert the float to a double. Hence, the printf() function does not need a conversion specifier for float, because there is no way to pass a float to it. The C99 standard added the l modifier just for symmetry with the scanf() function.

References

Upvotes: 2

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