Drawing matrix with characters in C

Question

My task is to draw matrix with asteriks, but the problem is that user enters the number of points and their coordinates and asteriks should only be drawn in their position. The total size of matrix is 20x20. And the coordinates are in range [0,19]. I just don't know how to save the input numbers. I am beginner, and I hope you could help.

    #include <stdio.h>
    int main()
    {
        int n, i, j, a, b;
        char m[20][20] = {{0}}, x = '*';
        printf("Enter the number of points: ");
        scanf("%d", &n);
        for (i = 0; i < n; i++)
        {
            printf("Enter the point %d", i + 1);
            scanf("%d %d", &a, &b);
            m[a][b] = x;
        }
        for (i = 0; i < 20; i++)
        {
            for (j = 0; j < 20; j++)
            {
                if (m[a][b])
                    printf("%c", m[a][b]);
                else
                    printf(" ");
            }
            printf("\n");
        }
    }

Answer 1

When you declare a variable (or an array of variables) you have to initialize it to a certain value. Otherwise the value saved in that memory cell remains the same as before the allocation (often nonsense numbers that do not correspond to ascii characters). You can initialize them manually or using library functions like memset or bzero.

Initialize the array manually:

for(i = 0; i < 20; i++)
    for(j = 0; j < 20; j++)
        m[i][j] = 0;

Initialize with memset:

memset(&m, 0, sizeof(char[20][20]));

Fixed code using memset:

#include <stdio.h>
#include <string.h>

int main()
{
    int n, i, j, a, b;
    char m[20][20], x = '*';
    memset(&m, 0, sizeof(char[20][20]));

    printf("Enter the number of points: \n");
    scanf("%d", &n);
    for (i = 0; i < n; i++)
    {
        printf("Enter the point %d\n", i + 1);
        scanf("%d %d", &a, &b);
        m[a][b] = x;
    }
    for (i = 0; i < 20; i++)
    {
        for (j = 0; j < 20; j++)
        {
            if (m[i][j])
                printf("%c", m[i][j]);
            else
                printf(" ");
        }
        printf("\n");
    }
}

Fixed code using a for loop:

#include <stdio.h>
#include <string.h>

int main()
{
    int n, i, j, a, b;
    char m[20][20], x = '*';

    for (i = 0; i < 20; i++)
        for (j = 0; j < 20; j++)
            m[i][j] = 0;

    printf("Enter the number of points: \n");
    scanf("%d", &n);
    for (i = 0; i < n; i++)
    {
        printf("Enter the point %d\n", i + 1);
        scanf("%d %d", &a, &b);
        m[a][b] = x;
    }
    for (i = 0; i < 20; i++)
    {
        for (j = 0; j < 20; j++)
        {
            if (m[i][j])
                printf("%c", m[i][j]);
            else
                printf(" ");
        }
        printf("\n");
    }
}

Output:

****                
*  *                
*  *                
*  *                
*  *                
****                
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    

Answer 2

You should

• Initialize the array m.
• Use i and j instead of a and b to decide what character to print.
• Fix the indentation.

Fixed code:

#include <stdio.h>

int main() {
    int n,i,j,a,b;
    char m[20][20]={{0}},x='*'; /* add ={{0}} to initialize the array */
    printf("Enter the number of points: ");
    scanf("%d", &n);
    for(i=0;i<n;i++){
        printf("Enter the point %d",i+1);
        scanf("%d %d",&a,&b);
        m[a][b]=x;
    }
    for(i=0;i<20;i++){
        for(j=0;j<20;j++){
            if (m[i][j])
                printf("%c", m[i][j]);
            else printf(" ");
        }
        printf("\n");
    }
}