Printing a multi dimensional array

Question

I've written a little thing which asks the user for some input (rows and cols), which should then set everything in an array to a dot (".") and print it out, but this crashes my application.

void main()
{


    int i,j, m, n;

          printf("The number of lines (m): ");
          scanf("%d", m );
          printf("\nThe number of columns (n): ");
          scanf("%d", n);


    //create my array

    char mineGrid[n][m];

    //set all fields in to safe (.)

    for (j = 0; j <= n; j++)
    {
       for (i = 0; i <= m; i++)
          {
             mineGrid[j][i] = ".";
          }
    }
   //print a grid of dots

    for (j = 0; j <= n; j++)
    {
       for (i = 0; i <= m; i++)
          {
             printf("%s", mineGrid[j][i]);
          }
    }
}

Any idea why this is crashing?

dmckee --- ex-moderator kitten · Accepted Answer · 2013-03-30 03:30:46Z

2

On cause of major trouble here is that you have a lot of loop that look like

for (j = 0; j <= n; j++) 
/*             ^      */
/*             |      */
/*           Look!    */

which will run j from 0 to n, but you have declared your array as

char mineGrid[n][m];

which means that space has been allocated for rows numbered 0 to n-1.

All you index loops are wrong in that way. The idomatic way to write those loops is

for (j = 0; j < n; ++j)

where I have fixed the range and also changed the increment from post- to pre- which is an old micro-optimization that generally does not make any difference in c these days (because compilers are smart enough to fix it), but can if you switch to c++ and use a non-trivial class in that way. So I keep it in my list of little things to "fix".

answered Mar 30, 2013 at 3:30

dmckee --- ex-moderator kitten

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1 Comment

user968127 Over a year ago

Also cheers!, I was going a bit mad there for a second! Was just about to plot it with pen & paper.

shaish · Accepted Answer · 2013-03-30 03:17:00Z

0

That's because you're putting a string in the array instead of char.

do it like this:

void main()
{


    int i,j, m, n;

    m = 5;
    n = 6;



    //create my array

    char mineGrid[n][m];

    //set all fields in to safe (.)

    for (j = 0; j <= n; j++)
    {
        for (i = 0; i <= m; i++)
        {
            mineGrid[j][i] = '.';
        }
    }
    //print a grid of dots

    for (j = 0; j <= n; j++)
    {
        for (i = 0; i <= m; i++)
        {
            printf("%c", mineGrid[j][i]);
        }
        printf("\n");
    }
}

answered Mar 30, 2013 at 3:17

shaish

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2 Comments

Mahesh Over a year ago

I don't think you are correct. You are thinking m,n are constants after assignment. But they are not. Variable Length Arrays are allowed from C99 standard.

user968127 Over a year ago

Derr. Looked at that for a while. Thanks a lot!

Civa · Accepted Answer · 2013-03-30 03:29:48Z

0

you created n X m elements but used n+1 X m+1 elements in array. use like bellow

a

for (j = 0; j < n; j++)
{
    for (i = 0; i < m; i++)
    {
        mineGrid[j][i] = '.';
    }
}

answered Mar 30, 2013 at 3:29

Civa

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Comments

Mahesh · Accepted Answer · 2013-03-30 03:36:04Z

0

That's because for an array of size N, the valid array indexes 0 to N-1. But you are accessing N th element which is not a valid array index and accessing it invokes undefined behavior.

for (j = 0; j <= n; j++)
{
   for (i = 0; i <= m; i++) // Array out of bounds in either condition check

With that said, you have issues with your input as well.

scanf("%d", m ); // Missing & operator before m.

edited Mar 30, 2013 at 3:36

answered Mar 30, 2013 at 3:30

Mahesh

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