C Program to Print Number Pattern

A school pattern exercise asks for number triangle. Describe what one row contains before putting that row inside the outer loop.

A number triangle resets its first value to one at the beginning of each row.

Before this exercise, review loop counters and stopping conditions. The worked program below applies those ideas to this specific problem.

Explain
Explain why number triangle uses this sequence of operations.
Trace
Trace the sample values and predict the printed result.
Apply
Print exactly two rows and predict both the characters and the leading spaces.

Number Triangle: follow the data

Given data
A school pattern exercise asks for number triangle. Describe what one row contains before putting that row inside the outer loop.
Rule
A number triangle resets its first value to one at the beginning of each row.
Observed result
1 1 2 1 2 3

A guided model of this example. The illustration explains the process; run the complete C program to check its output.

Understand the operation

Each row prints its column numbers; the counter does not continue from the preceding row.

The outer loop chooses a row; an inner loop chooses positions within that row. A newline belongs after the entire row, rather than after every printed character. Spaces are output data too, so count them explicitly.

Read the complete C program

The input values are written into this complete C17 example so the calculation can be reproduced without an unavailable service. Change the named data, rebuild and compare the actual output with your prediction.

C17 · complete program

#include <stdio.h>
int main(void) {
  int rows = 3;
  for (int row = 0; row < rows; row++) {
    for (int col = 0; col <= row; col++) {
      if (col != 0) putchar(' ');
      printf("%d", col + 1);
    }
    putchar('\n');
  }
  return 0;
}

Expected output

1
1 2
1 2 3

Run in C compiler →

Follow the values through the code

C17 · code used in this explanation

#include <stdio.h>
int main(void) {
  int rows = 3;
  for (int row = 0; row < rows; row++) {
    for (int col = 0; col <= row; col++) {
      if (col != 0) putchar(' ');
      printf("%d", col + 1);
    }
    putchar('\n');
  }
  return 0;
}

The sample requests 3 rows, with indexes zero through 2.

Each row prints its column numbers; the counter does not continue from the preceding row.

Follow the worked calculation

Start with the sample
The sample requests 3 rows, with indexes zero through 2.
Apply the rule
Each row prints its column numbers; the counter does not continue from the preceding row.
3
Check the result
1 1 2 1 2 3
```
1
1 2
1 2 3
```

A guided model of this example. The illustration explains the process; run the complete C program to check its output.

Try a small variation

C17 · code used in this explanation

#include <stdio.h>
int main(void) {
  int rows = 3;
  for (int row = 0; row < rows; row++) {
    for (int col = 0; col <= row; col++) {
      if (col != 0) putchar(' ');
      printf("%d", col + 1);
    }
    putchar('\n');
  }
  return 0;
}

Print exactly two rows and predict both the characters and the leading spaces.

Worked answer

The two-row answer reuses the row rule with a smaller requested count. Read the monospaced output: a leading space changes alignment even though it does not look like a printed symbol.

C17 · complete program

#include <stdio.h>
int main(void) {
  int rows = 2;
  for (int row = 0; row < rows; row++) {
    for (int col = 0; col <= row; col++) {
      if (col != 0) putchar(' ');
      printf("%d", col + 1);
    }
    putchar('\n');
  }
  return 0;
}

Expected output

1
1 2

Run in C compiler →

Recognize the important boundaries

Restrict row counts to a practical positive limit before calculating widths or generating large output.

Keep the newline after each completed row. Spaces position the characters; moving that newline inside the inner loop changes the shape into separate lines.

Choose the answer, then compare the reasoning for every option.

Use the idea independently

A number triangle resets its first value to one at the beginning of each row.

Continue with nested loop control.

Language reference: WG14 C11 committee draft. These examples use the C17-compatible subset of the explained language features.

Reaching the recap and completing the quizzes are separate milestones.

Next lesson: C Program to Print Character Pattern →

C Program to Print Number Pattern