Finding square made of 1's in a 2D array/matrix - c

Question

Thanks for helping. I really appreciate it. I Have been searching for solution on SO, but nothing is exactly what I need. I need it in C.
My task is to find "largest square" of 1's in an array. The array consists of only 0's and 1's and looks, for example, like this:
4 4
0 1 1 1
0 1 0 1
0 1 1 1
1 0 1 0
Output should print [row][col] of "upper left corner" 1, and [row][col] of "lower right corner", So it should be, for my example, [0][1] and [2][3].
I am using my getcolor() function to get value on [row][col] spot.
Also, I have these functions to get longest horizontal and vertical lines. For some reason they only work with arrays with the same number of columns and rows. When I use, for example, an array with 4 cols and 5 rows, it does not work right. Aan you help me please?
void getHline(Bitmap *arr)
{
int i, j, k;
int line, line_start, line_end, line_max = 0;
// Horizontally
for (k = 0; k < arr->rows; k++)
{
for (i = 0; i < arr->rows; i++)
{
if(!getcolor(arr, k, i))
{
continue;
}
for(j = i; j < arr->cols; j++)
{
if(!getcolor(arr, k, j))
{
break;
}
}
j--;
if(j - i + 1 > line_max)
{
line = k;
line_start = i;
line_end = j;
line_max = line_end-line_start+1;
}
}
}
printf("horizontally\n");
printf("start: [%d][%d]\n", line_start, line);
printf("end: [%d][%d]\n", line_end, line);
}
void getVline(Bitmap *arr)
{
int i, j, k;
int col, col_start, col_end, col_max = 0;
for(k = 0; k < arr->cols; k++)
{
for (i = 0; i < arr->rows; i++)
{
if (!getcolor(arr,i,k)) continue;
for (j = i; j <arr->cols; j++)
{
if (!getcolor(arr,j,k)) break;
}
j--;
if (j - i + 1 >col_max)
{
col = k;
col_start = i;
col_end = j;
col_max = col_end-col_start+1;
}
}
}
printf("\nverticaly\n");
printf("start: [%d][%d]\n", col, col_start);
printf("end: [%d][%d]\n", col, col_end);
}

Answer 1

If you're trying to get the largest square this has noting to do with the longest horizontal and vertical lines, because they could be separated and no square associated with them.
When trying to solve a complex problem, don't try to solve it all at once.
The first thing we have, is that each point of the array is associated with a square (the largest one for each point). So we have to find that square: We take a point of the array, then we move by steps through a continuous horizontal and vertical lines. For each step we check if we get a square and repeat the process until we get the largest square associated with that single point.
Each time we get the largest square associated with a point we check if it's largest than the last largest square associated with some previous point.
After connecting these parts we get our final program.
Explanation of the variables used in the program:
Link to the program http://pastebin.com/Yw05Gbtg or view it here:
EDIT:
#include <stdio.h>
main()
{
int lines=4, cols=4;
int arr[4][4] = {
{0,1,1,1,},
{0,1,0,1,},
{0,1,1,1,},
{1,0,1,0,}
};
int x_start, y_start, x_end, y_end, d_max=0;
int i, j, k, l;
int col_start, line_start, col_end, line_end, checker;
for (y_start=0; y_start<lines; y_start++){
for (x_start=0; x_start<cols; x_start++){
x_end = x_start;
y_end = y_start;
for (i=x_start, j=y_start; i<cols && j<lines; i++, j++){ // moving horizontally and vertically
if (!arr[y_start][i] || !arr[j][x_start]){ // checking if the horizontal or vertical lines are not continuous
break;
}
else {
checker = 1;
for (k=x_start, l=y_start; k<i+1 && l<j+1; k++, l++){ // check if square
if (!arr[j][k] || !arr[l][i]){
checker = 0;
break;
}
}
if (checker){ // if square then
x_end = i;
y_end = j;
}
}
}
if ((x_end-x_start)>d_max){
col_start = x_start;
line_start = y_start;
col_end = x_end;
line_end = y_end;
d_max = col_end-col_start;
}
}
}
printf("The largest square is:\n[%d][%d] x [%d][%d]\n", line_start, col_start, line_end, col_end);
// this is only to check if the program is working properly
for (y_start=line_start; y_start<line_end+1; y_start++){
printf("\n ");
for (x_start=col_start; x_start<col_end+1; x_start++){
printf("%d ", arr[y_start][x_start]);
}
}
printf("\n");
}

Answer 2

There is confusion between rows and cols somewhere in your code. To find it, rename your variables i, j and k to something more meaningful, like row, col_start and col_end.
As for finding maximal square, you might want to use prefix sums.