C Increasing Number Triangle Pattern (Right-Aligned)
Beginner
5 min read
Updated: Sep 2026
3 programs
Live preview
Definition
What Is This Pattern?
A right-aligned ascending triangle prints 1 through i on each row, with leading spaces so shorter rows sit against the right edge. %2d keeps each number two columns wide.
Remember
Rule: for i from 1 to rows
print (rows - i) spaces
print k from 1 to i with %2d
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5 ← rows = 5
Start with fixed rows = 5, then generalize with scanf — optionally drop the space loop for a left-aligned contrast.
Method
Idea
Best for
Spaces then numbers
Indent while j > i; print 1..i with %2d
Learning, interviews, exams
Left-aligned
Same numbers; skip the space loop
Seeing what indentation adds
Pseudocode
Pseudocode
for i from 1 to rows:
for j from rows down to i + 1:
print one space
for k from 1 to i:
print k in width 2
print newline
Cheat sheet
Goal
Pattern
Walk each row
for (i = 1; i <= rows; i++)
Leading spaces
for (j = rows; j > i; j--) printf(" ");
Print 1..i
for (k = 1; k <= i; k++) printf("%2d", k);
End the row
printf("\n");
Space count
rows - i
Printing Numbers vs Starting a New Line
API
Effect
Use for
printf(" ") / printf("%2d", k)
Stays on the same line
Each space and each number
printf("\n")
Ends the current line
After 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 triangle updates instantly — each number uses a width-2 field like %2d.
Whole numbers from 1 to 9 so every value stays within %2d. Tap a chip or type a value — the preview redraws as you go.
Live result5 rows · 15 numbers
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5
Trace
Worked Walkthrough — rows = 4
Trace each outer-loop value of i, how many spaces print, and which numbers follow.
i
Spaces
Numbers
Printed row
1
3
1
1
2
2
1, 2
1 2
3
1
1, 2, 3
1 2 3
4
0
1, 2, 3, 4
1 2 3 4
Number prints for n rows = n(n + 1)/2 (here 10). Space cells add more work of the same order — why time is O(n²).
Code
C Programs
Three complete programs: fixed rows, scanf input, and a left-aligned contrast. Use View Output to reveal sample results.
Example 1 — Fixed rows = 5
Hard-coded row count — spaces while j > i, then %2d for 1..i.
C
#include <stdio.h>
int main(void)
{
int rows = 5;
int i, j, k;
for (i = 1; i <= rows; i++)
{
for (j = rows; j > i; j--)
printf(" ");
for (k = 1; k <= i; k++)
printf("%2d", k);
printf("\n");
}
return 0;
}
Output
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5
How It Works
1. Outer loop grows i. Each pass adds one more number and drops one leading space.
2. Spaces then digits. The j loop prints rows - i spaces; the k loop prints 1..i with %2d.
3. End the line.printf("\n") runs only after both inner loops finish.
When i = 1 you get four spaces then 1; when i = 5 you get 1 2 3 4 5 with no indent.
Example 2 — User Input Version
Read rows with scanf and reject non-positive values. Same space-and-number core.
C
#include <stdio.h>
int main(void)
{
int rows;
int i, j, k;
printf("Enter number of rows: ");
if (scanf("%d", &rows) != 1 || rows <= 0)
{
printf("Please enter a positive integer.\n");
return 1;
}
for (i = 1; i <= rows; i++)
{
for (j = rows; j > i; j--)
printf(" ");
for (k = 1; k <= i; k++)
printf("%2d", k);
printf("\n");
}
return 0;
}
Output (when user enters 3)
Enter number of rows: 3
1
1 2
1 2 3
How It Works
1. Prompt and validate. Reject bad input with a clear message instead of looping on garbage.
2. Same core. Only the hard-coded 5 becomes the user value in both loops.
3. Safer input tip. You can also clamp demos to single-digit values:
Safer input
if (scanf("%d", &rows) != 1 || rows < 1 || rows > 9)
{
printf("Enter a whole number from 1 to 9.\n");
return 1;
}
Example 3 — Left-Aligned Triangle
Same ascending sequence without leading spaces — numbers start flush left.
C
#include <stdio.h>
int main(void)
{
int rows = 5;
int i, k;
for (i = 1; i <= rows; i++)
{
for (k = 1; k <= i; k++)
printf("%2d", k);
printf("\n");
}
return 0;
}
Output
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5
How It Works
1. Same numbers. The k loop and %2d formatting stay identical to Example 1.
2. Drop the indent. Removing the space loop leaves every row flush left — useful for comparing alignment.
3. Restore spaces next. Once the digit sequence is clear, put the j loop back for the right-aligned look.
Edge Cases & Pitfalls
Check these before calling the solution done.
no spaces
Left-aligned by accident
Skipping the j loop removes the indent. Keep for (j = rows; j > i; j--) for right alignment.
no %2d
Crowded columns
Plain printf("%d", k) makes two-digit values crowd earlier columns. Keep %2d (or wider for larger rows).
\n inside
Broken rows
If printf("\n") sits inside either inner loop, each token lands on its own line. Call the newline only after both loops.
rows > 9
%2d overflow
Values from 10 upward need three columns. Clamp demos to 1–9, or switch to %3d.
rows = 1
Smallest triangle
Output is a single width-2 1 with no leading spaces. 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
Program
Time
Extra space
Fixed / input (Examples 1–2)
O(rows²)
O(1)
Left-aligned (Example 3)
O(rows²)
O(1)
Number prints = n(n + 1)/2; space cells are also triangular. For rows = 5 that is 15 numbers (plus 10 indent spaces).
Remember
Key Takeaways
Rule: print rows - i spaces, then 1..i with %2d.
Two inner loops: indent first, digits second — order matters for the right edge.
Break the row: call printf("\n") only after both inner loops.
Complexity:O(n²) time from spaces plus numbers; O(1) extra space.
One line: for each i, print rows - i spaces, then %2d for 1..i, then printf("\n").
Frequently Asked Questions
A right-aligned ascending triangle: row 1 prints 1, row 2 prints 1 2, row 3 prints 1 2 3, and so on until 1 2 3 4 5.
Before printing numbers, the program prints spaces while j > i. Smaller rows get more spaces, pushing digits to the right edge.
The format specifier reserves 2 columns per number (right-aligned), keeping columns stable when values become two digits.
Remove the space loop that prints leading spaces. Then print numbers 1 to i directly — see Example 3.
Program 42 prints a hollow square grid with border conditions. Program 43 prints an ascending number triangle with indentation spaces.
Use a rows variable and loop i from 1 to rows — see Example 2.
O(n²) for n rows because total number prints are 1 + 2 + ... + n = n(n+1)/2 (plus O(n²) space cells).
Check scanf's return value and reject non-positive row counts so the outer loop has a valid range.
🤔
Did you know?
Each row prints numbers 1 to i. A space loop runs while j > i before the number loop; %2d keeps columns aligned in the output.
