How to Check if Matrix is Circular / Target Matrix? - arrays

Question

Enter N (dimension of square matrix) such that N is odd and in interval [1,100]. For entered matrix, check if it is a target matrix: if yes, print YES; if no, print NO.
Target Matrix is a matrix that is organized in concentric circles starting from the centre. Each concentric circle has -1 the value of the previous one. Examples:
and
I've tried approaching this problem by using a while loop to increase the radius which starts at centre increases. inside, I've used two for loops to go through that part and check if the values are according the the rule given above.
I'm not really sure if this is a good approach. Do you have some suggestions?
#include <stdio.h>
#define DIM 100
int main() {
int matrix[DIM][DIM];
int N;
int targetMatrix = 1;
int matrixCenter;
int radius;
do{
printf("Enter N: ");
scanf("%d", &N);
if (N % 2 == 0 || N < 1 || N > 100){
printf("Invalid value of N.\n");
}
} while (N % 2 == 0 || N < 1 || N > 100);
// Matrix Entry
printf("Enter the matrix: ");
int i, j;
for (i = 0; i < N; i++){
for (j = 0; j < N; j++){
scanf("%d", &matrix[i][j]);
}
}
// Value at Center
matrixCenter = matrix[N/2][N/2];
radius = 1;
// (N - 1) / 2 is the distance from center of matrix to its side
while (radius <= (N - 1) / 2)
{
for(i = N/2 - radius; i <= N/2 + radius; i++){
for(j = N/2 - radius; j <= N/2 + radius; j++){
if (i == N/2 && j == N/2) // Center Value
continue;
if (matrix[i][j] != matrixCenter - radius)
targetMatrix = 0;
}
}
if (targetMatrix == 0){
printf("NO: This is not a target matrix"); // If not a target matrix
return 1;
}
radius++;
}
printf("YES: this is a target matrix"); // If it is a target matrix
return 0;
}

Answer 1

Testing shell of submatrix only :
int first = N/2 - radius;
int last = N/2 + radius;
for (int i = first; i <= last;i++) {
// first on last line: increment by one
// other : increment by radius * 2
for (int j = first; j <= last;j += ((i==first)||(i==last) ? 1 :radius*2)) {
// test i,j here
}
}