C Decreasing Number Triangle Pattern (Right-Aligned)

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6 min read
Updated: Sep 2026
3 programs
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Right-aligned decreasing number triangle from 5 down to 5 4 3 2 1 in C programming
Definition

What Is This Pattern?

A right-aligned decreasing triangle prints rows down to i on each row. An indent loop adds two spaces per step, and %2d keeps columns aligned.

Remember
Rule: for i from rows down to 1
      print 2 spaces for j = 1..i-1
      print j with %2d for j = rows..i

         5
       5 4
     5 4 3
   5 4 3 2
 5 4 3 2 1     ← rows = 5

Two inner loops — indent then numbers. Follows the incremental triangle in Program 35; next is Program 37.

Approach

How to Solve It

Start with fixed rows = 5, then generalize with scanf — optionally demo a smaller count for tracing.

MethodIdeaBest for
Indent + %2dSpaces for j < i; then print rows..iLearning, interviews, exams
User input rowsSame loops; read rows with scanfReusable demos and labs

Pseudocode

Pseudocode
for i from rows down to 1:
    for j from 1 to i-1:
        print 2 spaces
    for j from rows down to i:
        print j (width 2)
    print newline

Cheat sheet

GoalPattern
Shrink outer boundfor (i = rows; i >= 1; i--)
Indent the rowfor (j = 1; j < i; j++) printf(" ");
Print rows..ifor (j = rows; j >= i; j--) printf("%2d", j);
End the rowprintf("\n");
Continuous counter insteadSee Program 35 — k++ with %3d

Printing Numbers vs Starting a New Line

APIEffectUse for
printf(" ") / printf("%2d", j)Stays on the same lineIndent spaces and each number
printf("\n")Ends the current lineAfter both inner loops

Print spaces and numbers without a newline, then end the row once.

Try it

Live Preview

Change the row count and the right-aligned decreasing triangle updates instantly — each number uses a width-2 field like %2d.

Whole numbers from 1 to 9. Tap a chip or type a value — the preview redraws as you go.

Live result 5 rows · 15 numbers
         5
       5 4
     5 4 3
   5 4 3 2
 5 4 3 2 1
Trace

Worked Walkthrough — rows = 4

Trace each outer-loop value of i from 4 down to 1, listing indent steps and numbers printed.

iIndent stepsNumbers (rows..i)Printed row
4344
324, 34 3
214, 3, 24 3 2
104, 3, 2, 14 3 2 1

Numbers printed for n rows = n(n + 1)/2. Indent work is also triangular — why time is O(n²).

Code

C Programs

Three complete programs: fixed rows, scanf input, and a small demo. Use View Output to reveal sample results.

Example 1 — Fixed rows = 5

Hard-coded row count — indent loop then %2d countdown from rows.

C
#include <stdio.h>

int main(void)
{
    int rows = 5;
    int i, j;

    for (i = rows; i >= 1; i--)
    {
        for (j = 1; j < i; j++)
            printf("  ");

        for (j = rows; j >= i; j--)
            printf("%2d", j);

        printf("\n");
    }

    return 0;
}

How It Works

1. Outer loop shrinks i. i runs from 5 down to 1 — first row is shortest.

2. Indent loop. j runs from 1 to i - 1, printing two spaces each time.

3. Number loop. j runs from rows down to i with %2d.

When i = 5 you get a padded 5; when i = 1 you get 5 4 3 2 1 with no indent.

Example 2 — User Input Version

Read rows with scanf and reject invalid input. Same dual inner-loop core.

C
#include <stdio.h>

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

    printf("Enter rows: ");
    if (scanf("%d", &rows) != 1 || rows < 1)
        return 0;

    for (i = rows; i >= 1; i--)
    {
        for (j = 1; j < i; j++)
            printf("  ");

        for (j = rows; j >= i; j--)
            printf("%2d", j);

        printf("\n");
    }

    return 0;
}

How It Works

1. Prompt and validate. Exit early if scanf fails or rows < 1.

2. Same core. Only the hard-coded 5 becomes rows in every loop bound.

3. Safer input tip. Prefer an explicit message instead of a silent exit:

Safer input
if (scanf("%d", &rows) != 1 || rows < 1)
{
    printf("Enter a positive whole number of rows.\n");
    return 1;
}

Example 3 — Compact rows = 3

Same indent and number loops with a smaller row count for quick paper tracing.

C
#include <stdio.h>

int main(void)
{
    int rows = 3;
    int i, j;

    for (i = rows; i >= 1; i--)
    {
        for (j = 1; j < i; j++)
            printf("  ");

        for (j = rows; j >= i; j--)
            printf("%2d", j);

        printf("\n");
    }

    return 0;
}

How It Works

1. Same structure. Only rows changes from 5 to 3 — indent and %2d stay identical.

2. Trace on paper. For i = 2: one indent pair, then 3 and 2 → 3 2.

3. Scale up next. Once the small demo is clear, use Examples 1–2 for five rows or user input.

Edge Cases & Pitfalls

Check these before calling the solution done.

i++

Upside-down triangle

If the outer loop counts up, the longest row prints first. Use for (i = rows; i >= 1; i--).

1 space

Broken column alignment

A single space instead of " " does not match %2d width. Always print two spaces per indent step.

no %2d

Multi-digit drift

Plain printf("%d", j) crowds columns when values reach 10+. Keep %2d.

\n inside

Broken rows

If printf("\n") sits inside either inner loop, each cell lands on its own line. Call the newline only after both loops finish.

rows = 1

Smallest triangle

Output is a single padded 1 with no indent — a good sanity check.

scanf

Check the return value

If scanf fails, rows may be uninitialized — always test scanf(...) == 1.

Analysis

Time and Space Complexity

ProgramTimeExtra space
Fixed / input (Examples 1–2)O(rows²)O(1)
Small demo (Example 3)O(rows²)O(1)

Numbers printed = n(n + 1)/2; indent steps form another triangular sum. Extra memory stays constant aside from loop counters.

Remember

Key Takeaways

  • Rule: indent with " " for j < i, then print rows..i with %2d.
  • Start at rows: i = rows makes the first line a single padded digit.
  • Match widths: two spaces for indents and %2d for numbers so columns stay aligned.
  • Complexity: O(n²) time; O(1) extra space.

One line: for i from rows down to 1, indent i - 1 times, print rows..i with %2d, then printf("\n").

Frequently Asked Questions

A right-aligned decreasing triangle: the first row prints 5, then 5 4, then 5 4 3, down to 5 4 3 2 1.
An indentation loop prints two spaces while j < i before the number loop runs, pushing shorter rows to the right.
The format specifier reserves 2 columns per number (right-aligned), keeping columns stable in the console output.
The number loop runs for (j = rows; j >= i; j--), so every row begins at rows and counts down to the current i.
Program 35 uses a continuous counter k. Program 36 restarts from rows on each row with a separate indentation loop.
Replace 5 with rows in the outer and number-loop bounds — see Example 2.
O(n²) for n rows because total indent steps plus number prints grow quadratically; numbers alone sum to n(n+1)/2.
Check scanf's return value and reject non-positive row counts so the outer loop has a valid range.

Did you know?

Each row starts from rows and counts down to i. An indentation loop prints two spaces per step (j = 1..i-1), then %2d keeps number columns aligned.

Next: Palindrome Number Triangle

Continue with a triangle that mirrors descending and ascending digits on each row.

Program 37 tutorial →

About the author

Mari Selvan M P
Mari Selvan M P 🔗

Developer, cloud engineer, and technical writer

  • Experience 12 years building web and cloud systems
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I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

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C Number Pattern 35 — Right-Aligned Incremental Triangle C Number Pattern 37 — Palindrome Number Triangle