C++ Decreasing Number Triangle Pattern (Right-Aligned)
Beginner
5 min read
Updated: Sep 2026
3 programs
Live preview
Definition
What Is This Pattern?
A right-aligned decreasing number triangle prints rows..i on each row — shorter rows get leading spaces so the right edge lines up.
Remember
Rule: for i from rows down to 1
print 2 spaces, (i - 1) times
print j from rows down to i (width 2)
5
5 4
5 4 3
5 4 3 2
5 4 3 2 1 ← rows = 5
Follows the continuous counter in Program 35; next is the centered square number triangle in Program 37.
Approach
How to Solve It
Outer loop shrinks i from rows to 1. First inner loop indents; second prints rows..i with setw(2).
Method
Idea
Best for
Indent + descending digits
Spaces for j < i, then j = rows..i
Learning, interviews, exams
setw(2) columns
Matches pad width of " "
Clean right alignment
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 in width 2
print newline
Cheat sheet
Goal
Pattern
Shrink start digit
for (i = rows; i >= 1; i--)
Indent
for (j = 1; j < i; j++) cout << " ";
Print rows..i
for (j = rows; j >= i; j--)
Fixed columns
cout << setw(2) << j;
End of row
cout << "\n";
Printing Numbers vs Starting a New Line
API
Effect
Use for
cout << " " / setw(2) << j
Stays on the same line
Each pad or number on the row
cout << "\n"
Ends the current line
After both inner loops finish
Print characters without a newline, then end the row once.
Try it
Live Preview
Change the row count and the right-aligned decreasing triangle updates instantly — capped at 9 for readable demos.
Whole numbers from 1 to 9. Tap a chip or type a value — the preview redraws as you go.
Live resultrows = 5 · 15 numbers
5
5 4
5 4 3
5 4 3 2
5 4 3 2 1
Trace
Worked Walkthrough — rows = 5
Trace how indents shrink and each row always starts at 5, counting down to i.
i
Indents
Numbers
Prints
5
4
5
5
4
3
5 4
5 4
3
2
5 4 3
5 4 3
2
1
5..2
5 4 3 2
1
0
5..1
5 4 3 2 1
Indents = i − 1. Numbers on the row = rows − i + 1. Total = n(n+1)/2 → O(n²).
Code
C++ Programs
Three complete programs: fixed rows = 5, cin input, and a compact rows = 3 demo. Use View Output to reveal sample results.
Example 1 — Fixed rows = 5
Hard-coded height — indent loop, then descending digits with setw(2).
C++
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
int rows = 5;
int i, j;
for (i = rows; i >= 1; i--)
{
for (j = 1; j < i; j++)
cout << " ";
for (j = rows; j >= i; j--)
cout << setw(2) << j;
cout << "\n";
}
return 0;
}
Output
5
5 4
5 4 3
5 4 3 2
5 4 3 2 1
How It Works
1. Outer starts at rows. First printed line is the shortest (i = 5 → just 5).
2. Indent then digits. Print i - 1 pairs of spaces, then rows..i with width 2.
3. Newline once. Call cout << "\n" only after both inner loops finish.
Example 2 — User Input Rows
Read rows with cin, validate, then use the same dual inner-loop core.
C++
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
int rows;
int i, j;
cout << "Enter rows: ";
if (!(cin >> rows) || rows <= 0)
{
cout << "Please enter a positive integer.\n";
return 1;
}
for (i = rows; i >= 1; i--)
{
for (j = 1; j < i; j++)
cout << " ";
for (j = rows; j >= i; j--)
cout << setw(2) << j;
cout << "\n";
}
return 0;
}
Output (when user enters 3)
Enter rows: 3
3
3 2
3 2 1
How It Works
1. Prompt and validate. Reject failed reads and non-positive values before printing.
2. Same core. Indent + setw(2) digits match Example 1 — only rows comes from the user.
3. Safer input tip. Cap demos for readable output:
Safer input
if (!(cin >> rows) || rows < 1 || rows > 9)
{
cout << "Enter a whole number from 1 to 9.\n";
return 1;
}
Example 3 — Compact rows = 3
Same structure with only three rows — easy to confirm indents and the restart at rows.
C++
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
int rows = 3;
int i, j;
for (i = rows; i >= 1; i--)
{
for (j = 1; j < i; j++)
cout << " ";
for (j = rows; j >= i; j--)
cout << setw(2) << j;
cout << "\n";
}
return 0;
}
Output
3
3 2
3 2 1
How It Works
1. Three rows.i = 3 → 3; i = 2 → 3 2; i = 1 → 3 2 1.
2. Trace on paper. If the number loop starts at i instead of rows, every row loses its shared peak digit.
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.
no indent
Missing space loop
Without the indent loop the triangle left-aligns. Keep j = 1..i-1 with " ".
j = i
Wrong number start
Starting the number loop at i instead of rows prints i..i only. Use j = rows..i.
width ≠ 2
Pad mismatch
Leading pads must match number width — use " " with setw(2).
\n inside
Broken rows
If cout << "\n" sits inside either inner loop, the row splits. Call it only after both finish.
rows = 1
Single value
Output is just 1 with no indents. A good sanity check for input validation.
cin
Check the stream
Validate cin >> rows before looping — a failed read leaves rows unset or stale.
Analysis
Time and Space Complexity
Program
Time
Extra space
Fixed / input (Examples 1–2)
O(n²)
O(1)
Compact rows = 3 (Example 3)
O(n²)
O(1)
Row with start i prints rows − i + 1 numbers. Across all rows the total is 1 + 2 + … + n = n(n+1)/2 → O(n²) time. Only a few loop variables are needed.
Remember
Key Takeaways
Two inner loops: indent with spaces, then print rows..i.
Start at rows: every row begins at the peak digit and counts down to i.
setw(2): match pad width to number width so the right edge stays aligned.
Complexity:O(n²) time; O(1) extra space.
One line: for each i from rows down to 1, indent then print rows..i with setw(2), then newline.
Frequently Asked Questions
A right-aligned decreasing triangle: for rows=5 the first line is 5, then 5 4, then 5 4 3, and so on until 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.
setw(2) 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.
cout << setw(2) << j stays on the same line. cout << "\n" ends the row after both inner loops finish.
After prompting, use if (!(cin >> rows) || rows <= 0) to reject bad input before printing.
O(n²) for n rows because total prints are 1 + 2 + … + n = n(n+1)/2.
🤔
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 setw(2) keeps number columns aligned.
