An alphabet right-angled triangle prints a left-aligned staircase of letters: row i runs from A through the i-th letter (A, AB, ABC, …).
Remember
Rule: on row i, print A through the i-th letter
A
AB
ABC
ABCD
ABCDE ← 5 rows
In C you solve it with two nested for loops over char: the outer loop picks the end letter, the inner loop restarts at A and prints through that end letter, then printf("\n") moves to the next line. Once this clicks, inverted letter triangles and other alphabet shapes become much easier.
Approach
How to Solve It
Two ways to emit the same shape — start with nested char loops and printf, then optionally polish with putchar.
Method
Idea
Best for
Nested loops + printf
Outer = end letter; inner = A..end via printf("%c", j)
Learning, interviews, exams
Nested loops + putchar
Same loops; putchar(j) per letter
Slightly leaner demos once loops click
Pseudocode
Pseudocode
for end from 'A' to lastLetter: // lastLetter depends on rows
for ch from 'A' to end:
print ch (no newline)
print newline
Change the row count and the alphabet triangle updates instantly — capped at 26 letters (A–Z).
Whole numbers from 1 to 26. Tap a chip or type a value — the preview redraws as you go.
Live result5 rows · 15 letters
A
AB
ABC
ABCD
ABCDE
Trace
Worked Walkthrough — rows = 4
Trace each outer-loop end letter and count how many times the inner loop prints.
End i
Inner j
Printed row
Letters
'A'
A..A
A
1
'B'
A..B
AB
2
'C'
A..C
ABC
3
'D'
A..D
ABCD
4
Total letter prints: 1 + 2 + 3 + 4 = 10 = 4×5/2. That triangular sum is why time is O(n²).
Code
C Programs
Three complete programs: fixed end letter, scanf input, and a putchar shortcut. Use View Output to reveal sample results.
Example 1 — Fixed through 'E'
Hard-coded end letter — ideal for first demos and screenshots (5 rows).
C
#include <stdio.h>
int main(void)
{
char i, j;
for (i = 'A'; i <= 'E'; i++)
{
for (j = 'A'; j <= i; j++)
{
printf("%c", j);
}
printf("\n");
}
return 0;
}
Output
A
AB
ABC
ABCD
ABCDE
How It Works
1. Outer loop picks the end letter.i runs from 'A' to 'E' — one row per end letter.
2. Inner loop restarts at A. For each i, j runs from 'A' to i, so the row is A..i.
3. Print letters, then break the line.printf("%c", j) stays on the row; printf("\n") after the inner loop starts the next row.
When i = 'A' you get A; when i = 'B' you get AB; up through ABCDE.
Example 2 — User Input Version
Read the row count at runtime with scanf. Always check its return value in real apps (shown in the tip below).
C
#include <stdio.h>
int main(void)
{
int rows;
char i, j, last;
printf("Enter the number of rows: ");
scanf("%d", &rows);
last = (char)('A' + rows - 1);
for (i = 'A'; i <= last; i++)
{
for (j = 'A'; j <= i; j++)
{
printf("%c", j);
}
printf("\n");
}
return 0;
}
Output (when user enters 4)
Enter the number of rows: 4
A
AB
ABC
ABCD
How It Works
1. Prompt and read. Ask for a row count, then store it with scanf("%d", &rows).
2. Map rows to an end letter.last = (char)('A' + rows - 1) — for rows = 4, last is 'D'.
3. Same nested-loop core. Only the source of last changes — the print logic matches Example 1.
4. Safer input tip. Unchecked scanf leaves rows uninitialized on bad input. Prefer:
Safer input
if (scanf("%d", &rows) != 1 || rows < 1 || rows > 26)
{
printf("Enter a whole number from 1 to 26.\n");
return 1;
}
Example 3 — putchar(j)
Same nested loops — write each letter with putchar instead of printf.
C
#include <stdio.h>
int main(void)
{
char i, j;
for (i = 'A'; i <= 'E'; i++)
{
for (j = 'A'; j <= i; j++)
{
putchar(j);
}
putchar('\n');
}
return 0;
}
Output
A
AB
ABC
ABCD
ABCDE
How It Works
1. Same outer and inner bounds. Still walk i from 'A' to 'E' and j from 'A' to i.
2. One character at a time.putchar(j) writes a letter without a format string.
3. End the row.putchar('\n') advances to the next line.
Learn the printf version first (Examples 1–2) so you can explain both bounds in an interview; treat putchar as a polish shortcut afterward.
Edge Cases & Pitfalls
Check these before calling the solution done.
\n inside
Column of letters
If printf("\n") is inside the inner loop, each letter lands on its own line. Print letters without a newline; call printf("\n") only after the inner loop.
No restart
Letters keep advancing
The inner loop must start at 'A' every row. A single advancing counter across rows produces a different pattern.
No newline
One endless line
Omitting the row break glues every letter onto a single line.
rows > 26
Past Z
'A' + rows - 1 leaves A–Z when rows > 26. Clamp or reject in interactive programs.
rows = 1
Single A
Output is just A on one line — a good sanity check.
scanf
Check the return value
If scanf fails, rows may be uninitialized — always test scanf(...) == 1 and prefer 1–26.
Analysis
Time and Space Complexity
Program
Time
Extra space
Nested loops + printf (Examples 1–2)
O(rows²)
O(1)
putchar (Example 3)
O(rows²)
O(1)
Total letters printed = 1 + 2 + … + n = n(n+1)/2, which is still quadratic in n.
Remember
Key Takeaways
Rule: row with end letter i prints A through i.
Two loops: outer = end letter, inner = A..end with printf("%c", j).
Break the row: call printf("\n") only after the inner loop.
Complexity:O(n²) time from the triangular letter count; O(1) extra space.
One line: for each end letter i, print A through i with printf("%c", j), then printf("\n").
Frequently Asked Questions
The outer loop picks the last letter on each row. The inner loop always restarts at A and prints up through that last letter, so row 1 is A, row 2 is AB, row 3 is ABC, and so on.
Each row is a fresh sequence from A to the current end letter. If you kept advancing a single char across rows, you would get a different pattern (not this right-angled alphabet triangle).
printf("%c", j) prints a letter and stays on the same line. printf("\n") ends the current line. Letters use %c; the row break uses \n after the inner loop.
Shrink the end letter each row — for example walk the outer loop from the last letter down toward A. See Alphabet Pattern Program 2.
O(n²) where n is the number of rows. Total letter prints equal 1+2+…+n = n(n+1)/2.
Yes. putchar(j) writes one character without a format string. Nested loops with putchar are a common, slightly leaner style; keep printf if your course examples use it.
Check scanf's return value: if (scanf("%d", &rows) != 1) handle bad input. Unchecked scanf leaves rows uninitialized on failure. Clamp to 26 for A–Z demos.
Char math can produce characters beyond Z. Clamp to 26 for A–Z demos, or define a clear wrap/error policy.
🤔
Did you know?
Row i prints letters from A through the i-th letter (A, AB, ABC, …). Total letters for n rows is the triangular number n(n+1)/2 — the same count that makes this pattern O(n²).
