Number Pyramid 1, 121, 12321 in C

Algorithm

1. Read n and check that it is between 1 and 9, so every number in a row is a single digit.
2. For each row i from 1 to n, print n - i spaces to centre the row.
3. Print the digits 1 to i in increasing order.
4. Print the digits i - 1 down to 1 in decreasing order.
5. End the row with a newline and move to the next row.

C program

#include <stdio.h>

int main(void) {
    int rows, row, j;

    printf("Enter the number of rows (1-9):\n");
    if (scanf("%d", &rows) != 1 || rows < 1 || rows > 9) {
        printf("Error: rows must be between 1 and 9\n");
        return 1;
    }

    for (row = 1; row <= rows; row++) {
        for (j = 1; j <= rows - row; j++) {
            printf(" ");
        }
        for (j = 1; j <= row; j++) {
            printf("%d", j);
        }
        for (j = row - 1; j >= 1; j--) {
            printf("%d", j);
        }
        printf("\n");
    }
    return 0;
}

Sample input and output

4

Enter the number of rows (1-9):
   1
  121
 12321
1234321

1

Enter the number of rows (1-9):
1

10

Enter the number of rows (1-9):
Error: rows must be between 1 and 9

How the program works

Each row is built from three parts: leading spaces that centre it, an ascending run of digits and a descending run that mirrors it. Row i always has i digits on the way up and i - 1 on the way down, so the row reads the same forwards and backwards.

The limit of nine rows is deliberate. Once a row reaches 10, the number 10 would print as two characters and break the pattern, so the input is restricted to one-digit values.

Viva questions

Common mistakes

• Printing n spaces instead of n - i, which shifts every row to the right.
• Starting the descending loop at row instead of row - 1, which repeats the peak digit.
• Forgetting the newline after each row, so the whole pattern prints on one line.