Right-Aligned Number Triangle in C++

Beginner
⏱️ 6 min read
📚 Updated: Aug 2025
🎯 2 Code Examples
Nested Loops
Right-aligned number triangle output (1, 1 2, 1 2 3, 1 2 3 4, 1 2 3 4 5) in C++ programming

What You’ll Learn

How to print a right-aligned triangle where row i prints the sequence 1..i.

This pattern is a neat way to practice:

  • Using one loop to print leading spaces
  • Using another loop to print increasing numbers
  • Formatting columns with setw()

⭐ Pattern Output

For rows = 5, the pattern looks like this:

Output
    1
   1 2
  1 2 3
 1 2 3 4
1 2 3 4 5
1

Complete C++ Program

First print spaces to align the row, then print numbers 1 to i using setw(2).

C++
#include <iostream>
#include <iomanip>
using namespace std;

int main() {
    int i, j, k;

    for (i = 1; i <= 5; i++) {
        for (j = 5; j > i; j--)
            cout << " ";

        for (k = 1; k <= i; k++)
            cout << setw(2) << k;

        cout << "\n";
    }

    return 0;
}

🧠 How It Works

1

Choose the size

We print 5 rows in this example (i = 1..5).

Setup
2

Print leading spaces

The first inner loop prints spaces while j > i. That makes the first rows shift to the right.

Alignment
3

Print numbers 1..i

The second inner loop prints k from 1 to i, producing rows like 1 2 3.

Number printing
4

Format with setw(2)

setw(2) keeps spacing consistent between numbers.

Formatting
=

Right-aligned triangle

Total printed numbers are 1+2+…+n, so runtime is O(n²) for n rows.

2

Variation — User Input Version

Let the user choose the number of rows at runtime.

C++
#include <iostream>
#include <iomanip>
using namespace std;

int main() {
    int rows;
    cout << "Enter the number of rows: ";
    cin >> rows;

    if (rows <= 0) return 0;

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

        for (int k = 1; k <= i; k++)
            cout << setw(2) << k;

        cout << "\n";
    }

    return 0;
}

💡 Tips for Enhancement

Try These

  • Print i..1 to create a decreasing number triangle
  • Use a different alignment by changing the space loop
  • Increase setw() if you print numbers above 9
  • Replace numbers with letters to make an alphabet version
  • Validate input using cin.fail() when taking user input

Avoid

  • Mixing tabs and spaces (alignment may break)
  • Forgetting the newline after each row
  • Hard-coding the row count when you want a reusable solution
  • Using endl in tight loops (extra flushing)

Key Takeaways

1

Row i prints the sequence 1..i.

2

Leading spaces right-align the triangle.

3

setw(2) improves column alignment.

4

Total prints are n(n+1)/2, giving O(n²) time.

❓ Frequently Asked Questions

Spaces shift shorter rows to the right so the triangle becomes right-aligned.
Yes. Remove the leading-space loop and just print 1..i on each row.
iomanip provides setw(), which helps format and align numbers in neat columns.
O(n²) for n rows due to the triangular number of prints.

Explore More C++ Number Patterns!

Once you can control spaces and loop bounds, you can align patterns in many different ways.

All Number Patterns →
Did you know?

If you replace the leading spaces with leading zeros or dots, you can “see” the alignment logic more clearly while debugging pattern programs.

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
  • Focus Full Stack Development, AWS, and Developer Education

I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

AWS Certified Developer – Associate (Credly)

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