Number Pattern with Nested Loops

CSC-115 · Semester I · C Programming

beginner25 minAvailable

Cin-browser interpreter · runs locally

#include <stdio.h>

int main() {
  int rows = 5;
  for (int i = 1; i <= rows; i++) {
    for (int j = 1; j <= i; j++) {
      printf("%d ", j);
    }
    printf("\n");
  }
  return 0;
}

13 lines

Output

Write some code, then press Run. Stdout and errors appear here.

Lab brief

Objective

Print a right-angled triangle of numbers using nested for loops, and reason about inner vs outer loop counts.

Theory & concepts

Pattern problems are built from nested loops: the outer loop controls the rows, the inner loop controls the characters in one row. For a right triangle, row i should contain i numbers, so the inner loop runs j from 1 to i.

Rendering diagram…

Nested loops controlling rows and columns

for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= i; j++)
        printf("%d ", j);
    printf("\n");
}

Expected output is a triangle with rows '1', '1 2', '1 2 3', '1 2 3 4', and '1 2 3 4 5'. The number of items in each row equals its row number, which is exactly what the inner loop limit j <= i guarantees.

Prerequisites

Lesson: The for loop, break and continue
Lesson: Two-dimensional thinking with loops (nested loops)

Instructions

1Write a for loop for the rows, i from 1 to 5.
2Nest an inner for loop for the numbers in a row, j from 1 to i.
3Print j followed by a space inside the inner loop.
4Print a newline after each inner loop finishes (one per row).
5Run and compare the output with the expected triangle.

Tasks

Print a number triangle
Output the triangle with row i containing 1 2 ... i for i = 1 to 5.
Print the same number repeated
Change the inner loop to print i (not j) so each row repeats its row number.
Build an inverted triangle
Make the rows descend from 5 numbers down to 1 by changing the loop limits.
Explain the loop limits
Write a comment explaining why the inner loop must run to i and what would happen if it ran to a constant.

Self-check

The triangle shape matches the expected output exactly.
Row i contains exactly i numbers.
The program uses a newline correctly to separate rows.
The student can modify the pattern (repeat, invert) by changing loop limits.

Environment

editorstdout consoletest cases