Fill matrix with two numbers in a spiral - arrays

Question

In the middle of matrix is number x. Matrix is filled with x in a spiral, while the rest is filled with y.
Example:
int x=4, y=6, v=7, s=9;
OUTPUT:
6 6 6 6 6 6 6 6 4
4 4 4 4 4 4 4 6 4
4 6 6 6 6 6 4 6 4
4 6 4 4 x 6 4 6 4
4 6 4 6 6 6 4 6 4
4 6 4 4 4 4 4 6 4
4 6 6 6 6 6 6 6 4
Note: In the output instead of the letter x program should print 4, but I wrote x to make it easier for understanding task.
Spiral filling looks like --> this
#include <stdio.h>
int main() {
int v = 7, s = 9, x = 4, y = 6, mat[7][9], i, j, l, k;
for (i = 0; i < v; i++)
for (j = 0; j < s; j++)
mat[i][j] = y;
for (i = 0; i < v; i++) {
for (j = 0; j < s; j++) {
// spiral goes here
// mat[i][j] = x;
}
}
for (i = 0; i < v; i++) {
for (j = 0; j < s; j++)
printf("%4d", mat[i][j]);
printf("\n");
}
return 0;
}
EDIT: After applying suggestions, now the whole matrix is filled with y. How to make it fill spiral with x?

Answer 1

Below is a program for constructing a spiral from the center in an s by v matrix.
Here's the algorithm:
The matrix is ​​filled with y values
Calculating the center, the cursor cx, cy is set there, 0 is written
The spiral drawing direction is set, dir, 1 - left, 2 - down, 3 - right, 4 - up, first 1
Draw 1 step by setting x value to matrix
If the value in the matrix for dir+1 (if dir+1=5 then dir=1) = y (empty) then dir is incremented by +1. If not (center (0) or already drawn (x)) then dir remains the same.
If beyond the edge of the matrix, then dir is incremented +1 as if empty, but nothing is drawn.
Step 4 is repeated s*v/2 times
The program is compiled with the gcc compiler.
Output:
cx - 4, xy - 3
6 6 6 6 6 6 6 6 4
4 4 4 4 4 4 4 6 4
4 6 6 6 6 6 4 6 4
4 6 4 4 0 6 4 6 4
4 6 4 6 6 6 4 6 4
4 6 4 4 4 4 4 6 4
4 6 6 6 6 6 6 6 4
Required:
6 6 6 6 6 6 6 6 4
4 4 4 4 4 4 4 6 4
4 6 6 6 6 6 4 6 4
4 6 4 4 x 6 4 6 4
4 6 4 6 6 6 4 6 4
4 6 4 4 4 4 4 6 4
4 6 6 6 6 6 6 6 4
#include <stdio.h>
/*
Increase dir
*/
int incdir (int dir) {
// direction, 1 - left, 2 - down, 3 - right, 4 - up
dir++;
if (dir==5) dir=1;
return dir;
}
/*
Is coord in matrix
*/
int inmat (int y, int x, int v, int s) {
// direction, 1 - left, 2 - down, 3 - right, 4 - up
if (x<0 || x>=s) return 0;
if (y<0 || y>=v) return 0;
return 1;
}
/*
Value in some direction
*/
int dirval(int cx, int cy, int v, int s, int dir, int mat[v][s]) {
// direction, 1 - left, 2 - down, 3 - right, 4 - up
if (dir==1) {
if (!inmat(cy, cx-2, v, s)) return -1;
return mat[cy][cx-2];
}
if (dir==2) {
if (!inmat(cy+2, cx, v, s)) return -1;
return mat[cy+2][cx];
}
if (dir==3) {
if (!inmat(cy, cx+2, v, s)) return -1;
return mat[cy][cx+2];
}
if (dir==4) {
if (!inmat(cy-2, cx, v, s)) return -1;
return mat[cy-2][cx];
}
}
int main() {
int v = 7, s = 9, x = 4, y = 6, mat[7][9], i, j, l, k;
// cursor
int cx, cy;
cx=((int)s/2);
cy=((int)v/2);
printf("cx - %4d, xy - %4d\n", cx, cy);
int dir=1; // direction, 1 - left, 2 - down, 3 - right, 4 - up
int dv;
for (i = 0; i < v; i++)
for (j = 0; j < s; j++)
mat[i][j] = y;
mat[cy][cx]=0; // start
// spiral goes here
for (i = 0; i < s*v/2; i++) {
if (dir==1) {
if (inmat(cy, cx-1, v, s)) mat[cy][cx-1]=x;
if (inmat(cy, cx-2, v, s)) mat[cy][cx-2]=x;
cx=cx-2;
}
if (dir==2) {
if (inmat(cy+1, cx, v, s)) mat[cy+1][cx]=x;
if (inmat(cy+2, cx, v, s)) mat[cy+2][cx]=x;
cy=cy+2;
}
if (dir==3) {
if (inmat(cy, cx+1, v, s)) mat[cy][cx+1]=x;
if (inmat(cy, cx+2, v, s)) mat[cy][cx+2]=x;
cx=cx+2;
}
if (dir==4) {
if (inmat(cy-1, cx, v, s)) mat[cy-1][cx]=x;
if (inmat(cy-2, cx, v, s)) mat[cy-2][cx]=x;
cy=cy-2;
}
dv=dirval(cx, cy, v, s, incdir(dir), mat);
if (dv==y || dv==-1) dir=incdir(dir);
}
for (i = 0; i < v; i++) {
for (j = 0; j < s; j++)
printf("%4d", mat[i][j]);
printf("\n");
}
return 0;
}

Answer 2

In the first iteration of the inner loop if (i == v / 2 && j == s / 2) is always false so you keep doing:
mat[l][k--] = x; // Decrement k
mat[l++][j] = y; // Increment l
Now add the inner-loop:
------ Increment k
|
for (k = j; k < s; k++) {
//if (i == v / 2 && j == s / 2) // Always false when i is zero
// mat[i][j] = x;
//else
{
mat[l][k--] = x; // Decrement k
mat[l++][j] = y; // Increment l
}
}
So you decrement k and the increment k in the for loop. In other words - k doesn't change and you have an endless loop. In that endless loop you keep incrementing l and use it as array index. That will access the array out of bounds and crash your program.