Thread: help with searching a string on a char matrix.

So i started learning c quite recently and have a question.

I want to make a program that receives the number of lines and collumns of a matrix, a matrix of char, the number of pairs of coordinates and the coordinates. the program has to return the char (or chars) that are on those coordinates on the matrix.

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example:

receives

2 3
ABC
DEF
2
1 1
1 2

returns

AB

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this is how i tried to solve this problem:

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Code:

#include 
#define MAX 1000

int main() {
  int nlin, ncol;
  char mat[MAX][MAX];
  int x[MAX], y[MAX];
  int ncoords;
  int l, c, n;

/* receiving variables and storing matrix.*/

  scanf("%d%d\n", &nlin, &ncol);
  for(l = 0; l < nlin; l++) {
    for(c = 0; c < ncol; c++)
      mat[l][c] = getchar();
    getchar(); /* read the \n at the end of each line */
  }
  scanf("%d", &ncoords);
  for(n = 0; n < ncoords; n++) {
    scanf("%d%d", &x[n], &y[n]);
  }

  /*finding string with the received coordinates*/

    int f;
    int e;
    int r;
    char result [1000];

    for ( r=0; ncoords != 0; ncoords--,r++,f++) {
        for(l=1,f=0;x[f]!=l;l++);
        for(c=1,e=0;y[e]!=c;c++)
        result[r]=mat[f][e];
    }
    printf("found string '%s'\n",result);
    return 0;
}

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For some reason that i can't seem to find, this solution is not right, may someone explain me why and point me into the right direction?

Code:

        for(l=1,f=0;x[f]!=l;l++);
        for(c=1,e=0;y[e]!=c;c++)
        result[r]=mat[f][e];

The ; at the end of the first loop means it does nothing.
What are the values of f and e when those loops exit?

It seems an excessively complicated version of something like
result[r] = mat[x[r]-1][y[r]-1];


> printf("found string '%s'\n",result);
You need to make sure your result has a \0 in the right place.

Originally Posted by Lired

Why do you say it does nothing?

Well, let's write this:

Code:

for(l=1,f=0;x[f]!=l;l++);
for(c=1,e=0;y[e]!=c;c++)
result[r]=mat[f][e];

In an equivalent form, with proper indentation:

Code:

for (l = 1, f = 0; x[f] != l; l++)
{
    /* do nothing in the loop body */
}

for (c = 1, e = 0; y[e] != c; c++)
{
    result[r] = mat[f][e];
}

Obviously, the first for loop has no net effect beyond incrementing l. If this is not what you intended, then the semi-colon at the end of the first for loop is a mistake.