How does the spiral pattern algorithm work?

We fill the matrix in four directions: top row left-to-right, right column top-to-bottom, bottom row right-to-left, and left column bottom-to-top. After each cycle, we shrink the boundaries inward.

Why do we need row_start, row_end, col_start, and col_end?

These boundaries define the current outer layer of the spiral. After filling a layer, we update them (row_start++, row_end--, col_start++, col_end--) to move inward.

Can I make a counter-clockwise spiral?

Yes. Change the order and direction of the four filling loops to rotate the spiral direction.

What is the time complexity?

O(n^2) because we fill and print n×n elements.

Spiral Matrix Number Pattern in C

Beginner

⏱️ 8 min read

📚 Updated: Aug 2025

🎯 2 Code Examples

2D Arrays

Spiral matrix number pattern output in C programming

What You’ll Learn

How to fill a square matrix in a clockwise spiral using boundary pointers. This is a classic exercise for mastering 2D arrays and loop control.

⭐ Pattern Output

For a 10×10 matrix, the spiral looks like this:

Output

1    2    3    4    5    6    7    8    9   10
36  37   38   39   40   41   42   43   44   11
35  64   65   66   67   68   69   70   45   12
34  63   84   85   86   87   88   71   46   13
33  62   83   96   97   98   89   72   47   14
32  61   82   95  100   99   90   73   48   15
31  60   81   94   93   92   91   74   49   16
30  59   80   79   78   77   76   75   50   17
29  58   57   56   55   54   53   52   51   18
28  27   26   25   24   23   22   21   20   19

Complete C Program (Fixed Size)

This version uses #define SIZE 10 and fills numbers from 1 to 100 in a spiral.

#include <stdio.h>

#define SIZE 10

int main() {
    int matrix[SIZE][SIZE];
    int i, j, num = 1;
    int row_start = 0, row_end = SIZE - 1;
    int col_start = 0, col_end = SIZE - 1;

    while (num <= SIZE * SIZE) {
        for (i = col_start; i <= col_end; i++)
            matrix[row_start][i] = num++;
        row_start++;

        for (i = row_start; i <= row_end; i++)
            matrix[i][col_end] = num++;
        col_end--;

        for (i = col_end; i >= col_start; i--)
            matrix[row_end][i] = num++;
        row_end--;

        for (i = row_end; i >= row_start; i--)
            matrix[i][col_start] = num++;
        col_start++;
    }

    for (i = 0; i < SIZE; i++) {
        for (j = 0; j < SIZE; j++)
            printf("%4d", matrix[i][j]);
        printf("\n");
    }

    return 0;
}

Variation — User Input Size

This version reads the size and fills a variable-length matrix.

#include <stdio.h>

int main() {
    int size;
    int i, j, num = 1;
    int row_start, row_end, col_start, col_end;

    printf("Enter the size of the matrix: ");
    scanf("%d", &size);

    int matrix[size][size];
    row_start = 0;
    row_end = size - 1;
    col_start = 0;
    col_end = size - 1;

    while (num <= size * size) {
        for (i = col_start; i <= col_end; i++)
            matrix[row_start][i] = num++;
        row_start++;

        for (i = row_start; i <= row_end; i++)
            matrix[i][col_end] = num++;
        col_end--;

        for (i = col_end; i >= col_start; i--)
            matrix[row_end][i] = num++;
        row_end--;

        for (i = row_end; i >= row_start; i--)
            matrix[i][col_start] = num++;
        col_start++;
    }

    for (i = 0; i < size; i++) {
        for (j = 0; j < size; j++)
            printf("%4d", matrix[i][j]);
        printf("\n");
    }

    return 0;
}