It reuses Program 3’s spacing idea, but uses odd star counts so the shape widens on both sides. The same row formula is the upper half of the filled diamond.
Approach
How to Solve It
Two inner loops per row — spaces then odd stars — or the same formulas with putchar.
Method
Idea
Best for
Nested loops
rows - i spaces, then 2 * i - 1 stars
Learning, interviews, exams
putchar
Same counts, character-at-a-time I/O
Leaner demos once loops click
Pseudocode
Pseudocode
for i from 1 to rows:
for j from 1 to (rows - i):
print " " (no newline)
for k from 1 to (2 * i - 1):
print "*" (no newline)
print newline
Change the row count and the pyramid updates instantly — including the perfect-square star total.
Whole numbers from 1 to 15. Base width = 2 * rows - 1; total stars = rows².
Live result5 rows · 25 stars
*
***
*****
*******
*********
Trace
Worked Walkthrough — rows = 4
Trace spaces and odd star counts for each outer-loop value of i.
i
Spaces rows - i
Stars 2 * i - 1
Printed row
1
3
1
*
2
2
3
***
3
1
5
*****
4
0
7
*******
Total stars: 1 + 3 + 5 + 7 = 16 = 4². Base width: 2×4 - 1 = 7.
Code
C Programs
Three complete programs: fixed rows, console input, and a putchar variant. Use View Output to reveal sample results.
Example 1 — Fixed rows = 5
Hard-coded height — space loop with rows - i, star loop with 2 * i - 1.
c
#include <stdio.h>
int main(void) {
int rows = 5;
int i, j, k;
for (i = 1; i <= rows; ++i) {
for (j = 1; j <= rows - i; ++j) {
printf(" ");
}
for (k = 1; k <= 2 * i - 1; ++k) {
printf("*");
}
printf("\n");
}
return 0;
}
Output
*
***
*****
*******
*********
How It Works
1. Set height.rows = 5 means five lines; the base has 2 * 5 - 1 = 9 stars.
2. Outer loop picks the row.i runs from 1 to rows.
3. Spaces then stars. Print rows - i spaces, then 2 * i - 1 stars via printf.
4. Break the line.printf("\n") after both inner loops starts the next row.
When i = 1: 4 spaces + 1 star. When i = 5: 0 spaces + 9 stars.
Example 2 — User Input Version
Read the row count at runtime. Prefer checking scanf’s return value (shown in the tip below).
c
#include <stdio.h>
int main(void) {
int rows;
int i, j, k;
printf("Enter the number of rows: ");
scanf("%d", &rows);
for (i = 1; i <= rows; ++i) {
for (j = 1; j <= rows - i; ++j) {
printf(" ");
}
for (k = 1; k <= 2 * i - 1; ++k) {
printf("*");
}
printf("\n");
}
return 0;
}
Output (when user enters 4)
Enter the number of rows: 4
*
***
*****
*******
How It Works
1. Prompt and read. Ask for a row count, then fill rows with scanf.
2. Same nested-loop core. Only the source of rows changes — the print logic matches Example 1.
3. Safer input tip. Ignoring scanf’s return leaves rows unset on bad input. Prefer:
Safer input
if (scanf("%d", &rows) != 1 || rows < 1) {
printf("Enter a positive whole number.\n");
return 1;
}
Example 3 — putchar for Spaces and Stars
Same formulas with character-at-a-time I/O — no format strings in the inner loops.
c
#include <stdio.h>
int main(void) {
int rows = 5;
int i, j;
for (i = 1; i <= rows; ++i) {
for (j = 1; j <= rows - i; ++j) {
putchar(' ');
}
for (j = 1; j <= 2 * i - 1; ++j) {
putchar('*');
}
putchar('\n');
}
return 0;
}
Output
*
***
*****
*******
*********
How It Works
1. Same outer loop. Still walk i from 1 to rows.
2. Same counts.rows - i spaces and 2 * i - 1 stars — only the print API changes to putchar.
3. Learn loops first. Use Examples 1–2 when you need to show nested bounds; treat this as a polish shortcut afterward.
Edge Cases & Pitfalls
Check these before calling the solution done.
2 * i
Even width
Use 2 * i - 1 (odd). Even widths lose the classic single-center peak.
j < rows - i
One space short
Space loop must be j <= rows - i. A strict < shifts the peak.
Only i stars
Program 3 shape
Same spaces with i stars is the right-aligned triangle, not a full pyramid.
rows = 1
Single star
0 spaces + 1 star — a good tip sanity check.
rows ≤ 0
Empty output
Outer loop never runs. Validate and re-prompt for interactive programs.
Bad scanf
Check the return
If scanf fails, rows is uninitialized — always test != 1.
Analysis
Time and Space Complexity
Program
Time
Extra space
Nested loops (Examples 1–2)
O(rows²)
O(1)
putchar variant (Example 3)
O(rows²)
O(1)
Each of n rows prints Θ(n) characters (spaces + stars). Total stars = 1 + 3 + … + (2n - 1) = n².
Remember
Key Takeaways
Formulas:rows - i spaces and 2 * i - 1 stars.
Odd widths: keep 2 * i - 1 for a single centered peak.
Break the row: print spaces/stars without a newline; call printf("\n") after both loops.
Complexity:O(n²) time; total stars = n²; O(1) extra space.
One line: print rows - i spaces, then 2 * i - 1 stars — that is the centered pyramid.
Frequently Asked Questions
2*i-1 gives odd lengths 1, 3, 5, … so each row adds one star on both sides. Using only i stars per row would not form the usual symmetric centered pyramid.
Printing (rows - i) spaces before the stars shifts the star block left as i grows, keeping the peak centered when the font is fixed-width.
Yes. Keep the same inner loops but run the outer loop from rows down to 1. The first printed row is the widest; later rows narrow toward the tip. See Program 6.
The last row has 2 * rows - 1 stars and no leading spaces when i equals rows.
printf("*") or printf(" ") stays on the same line. printf("\n") ends the current line. Spaces and stars use printf without a newline; the row break comes after both inner loops.
O(n²) for n rows. Each row prints Theta(n) characters in the worst case; there are n rows. Total stars equal n².
Program 3 uses the same (rows - i) spaces but only i stars. Program 5 uses 2*i-1 stars so the shape widens on both sides.
Check scanf's return value: if (scanf("%d", &rows) != 1) handle bad input. Unchecked scanf leaves rows uninitialized on failure.
🤔
Did you know?
Odd star counts 1, 3, 5, … come from 2 * i - 1. Their sum for n rows is n² — so total stars grow as a perfect square. This pyramid is also the upper half of the filled diamond.
