Reverse Counting Number Triangle in C++
What You’ll Learn
How to print a triangle where each row counts down from the row number to 1.
This helps practice loop boundaries and reverse counting with nested loops.
⭐ Pattern Output
For rows = 5, the output looks like this:
1
21
321
4321
54321Complete C++ Program
The inner loop counts down from i to 1 on each row.
#include <iostream>
using namespace std;
int main() {
int rows = 5;
for (int i = 1; i <= rows; i++) {
for (int j = i; j >= 1; j--) {
cout << j;
}
cout << "\n";
}
return 0;
}🧠 How It Works
Set the number of rows
int rows = 5; controls the height of the triangle.
Outer loop (row number)
for (int i = 1; i <= rows; i++) moves from row 1 to row 5.
Inner loop (count down)
for (int j = i; j >= 1; j--) prints i, i-1, ..., 1.
New line
cout << "\n"; moves the cursor to the next row.
Reverse counting triangle
Total prints are \(1 + 2 + \dots + n\) = n(n+1)/2, so time complexity is O(n²).
Variation — User Input Version
Read the number of rows at runtime using cin:
#include <iostream>
using namespace std;
int main() {
int rows;
cout << "Enter the number of rows: ";
cin >> rows;
for (int i = 1; i <= rows; i++) {
for (int j = i; j >= 1; j--) {
cout << j;
}
cout << "\n";
}
return 0;
}💡 Tips for Enhancement
Try These
- Add spaces:
cout << j << ' '; - Print a mirrored version by appending an ascending loop after the countdown
- Validate input (check
cin.fail()) for robustness - Change the outer loop direction to create other triangle orientations
Avoid
- Forgetting the newline after each row (output will run together)
- Using the wrong loop bounds (e.g., stopping at 0 won’t print 1)
- Negative/zero rows (handle with a guard if needed)
Key Takeaways
The outer loop chooses the row number i.
The inner loop counts down from i to 1.
Each row prints exactly i digits, so total prints are \(n(n+1)/2\).
This is a clean exercise for reverse loops and triangle shapes.
❓ Frequently Asked Questions
i from rows down to 1, instead of 1 up to rows.\n is usually faster because endl flushes the stream.Explore More C++ Number Patterns!
Try mixing reverse loops and spacing to build new triangle patterns.
Reverse loops (counting down) are frequently used in pattern printing and array traversal problems.
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