C Palindrome Number Pattern (Increasing-Decreasing)
Beginner
5 min read
Updated: Sep 2026
3 programs
Live preview
Definition
What Is This Pattern?
An increasing-decreasing pyramid prints each row as a palindrome: count up from i to the peak 2i - 1, then back down to i. Row i always has 2i - 1 digits.
Remember
Rule: m = i each row
print ascending i..(2i-1) with m++
m = m - 2 ← skip the peak
print descending with m--
1
232
34543
4567654
567898765 ← rows = 5
Follows the alternating triangle in Program 51; next is the mirror diagonal pattern in Program 53.
Approach
How to Solve It
Set m = i each row. Print the ascending half with m++, step back with m = m - 2, then print the descending half with m--.
Method
Idea
Best for
Two halves + step-back
Ascend with m++, m -= 2, descend with m--
Learning, interviews, exams
Compact trace
Dry-run with rows = 3 before coding rows = 5
Paper tracing and labs
Pseudocode
Pseudocode
for i from 1 to rows:
m = i
for j from 1 to i:
print m, then m = m + 1
m = m - 2
for k from 1 to (i - 1):
print m, then m = m - 1
print newline
Cheat sheet
Goal
Pattern
Pick each row
for (i = 1; i <= rows; i++)
Start value
m = i;
Ascending half
for (j = 1; j <= i; j++) printf("%d", m++);
Skip the peak
m = m - 2;
Descending half
for (k = 1; k < i; k++) printf("%d", m--);
Digits on row i
2 * i - 1
Printing Numbers vs Starting a New Line
API
Effect
Use for
printf("%d", m++)
Stays on the same line
Each digit in both halves
printf("\n")
Ends the current line
After both inner loops
Print all digits without a newline, then end the row once.
Try it
Live Preview
Change the row count and the pyramid updates instantly — capped at 5 so every digit stays a single character (peak ≤ 9).
Whole numbers from 1 to 5. Tap a chip or type a value — the preview redraws as you go.
Live resultrows = 5 · 25 digits
1
232
34543
4567654
567898765
Trace
Worked Walkthrough — Row i = 3
Trace both halves for row 3 — ascending 345, step back, then descending 43.
Step
State
Prints
Start
m = 3
—
Ascend (3 times)
m++ prints 3, 4, 5
345
After ascend
m = 6, then m = m - 2 → 4
—
Descend (2 times)
m-- prints 4, then 3
43
Full row: 34543. Digits on row i = 2i - 1; total = n² → O(n²).
Code
C Programs
Three complete programs: fixed rows = 5, scanf input, and a compact rows = 3 demo. Use View Output to reveal sample results.
Example 1 — Fixed rows = 5
Hard-coded height — ascend with m++, step back with m = m - 2, descend with m--.
C
#include <stdio.h>
int main(void)
{
int rows = 5;
int i, j, k, m;
for (i = 1; i <= rows; i++)
{
m = i;
for (j = 1; j <= i; j++)
printf("%d", m++);
m = m - 2;
for (k = 1; k < i; k++)
printf("%d", m--);
printf("\n");
}
return 0;
}
Output
1
232
34543
4567654
567898765
How It Works
1. Start each row. Set m = i so row 3 begins at 3, row 5 at 5.
2. Ascend to the peak. Print i digits with m++ — up to 2i - 1.
3. Step back, then descend.m = m - 2 skips the peak; the second loop prints i - 1 digits going down.
Example 2 — User Input Rows
Read rows with scanf and reject non-positive values.
C
#include <stdio.h>
int main(void)
{
int rows;
int i, j, k, m;
printf("Enter the 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++)
{
m = i;
for (j = 1; j <= i; j++)
printf("%d", m++);
m = m - 2;
for (k = 1; k < i; k++)
printf("%d", m--);
printf("\n");
}
return 0;
}
Output (when user enters 4)
Enter the number of rows: 4
1
232
34543
4567654
How It Works
1. Prompt and validate. Reject bad input with a clear message.
2. Same core. Only the outer loop limit comes from the user — both halves stay identical.
3. Safer input tip. Cap demos so the peak stays a single digit (2 * rows - 1 ≤ 9):
Safer input
if (scanf("%d", &rows) != 1 || rows < 1 || rows > 5)
{
printf("Enter a whole number from 1 to 5.\n");
return 1;
}
Example 3 — Compact rows = 3
Same two-half structure with a smaller height for quick paper tracing.
C
#include <stdio.h>
int main(void)
{
int rows = 3;
int i, j, k, m;
for (i = 1; i <= rows; i++)
{
m = i;
for (j = 1; j <= i; j++)
printf("%d", m++);
m = m - 2;
for (k = 1; k < i; k++)
printf("%d", m--);
printf("\n");
}
return 0;
}
Output
1
232
34543
How It Works
1. Three rows. Row 1 skips descending; row 2 prints 232; row 3 prints 34543.
2. Trace on paper. Confirm m = m - 2 after the peak so the middle digit is not doubled.
3. Scale up next. Once the small demo is clear, use Examples 1–2 for five rows or user input.
Edge Cases & Pitfalls
Check these before calling the solution done.
m - 1
Doubled peak
If you use m = m - 1 (or skip the step-back), the peak digit prints twice. Keep m = m - 2.
k <= i
Extra descending digit
The descending loop must run i - 1 times: for (k = 1; k < i; k++). Using k <= i adds one too many.
\n inside
Broken rows
If printf("\n") sits inside either half, each digit lands on its own line. Call the newline only after both loops.
rows = 1
Single row
Output is just 1 — the descending loop never runs. A good sanity check.
rows > 5
Multi-digit values
Past 5, the peak exceeds 9 and values like 10 break the tight look. Cap demos at 5 or use spaced / fixed-width format.
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(n²)
O(1)
Compact rows = 3 (Example 3)
O(n²)
O(1)
Row i prints 2i - 1 digits. Total = 1 + 3 + 5 + … + (2n - 1) = n². For n = 5 that is 25 digits.
Remember
Key Takeaways
Palindrome row: ascend i..(2i-1), then descend back to i.
Skip the peak: after ascending, m = m - 2 so the middle digit is not printed twice.
Break the row: call printf("\n") only after both halves.
Complexity:O(n²) time from n² digits; O(1) extra space.
One line:m = i; print ascending with m++, m -= 2, print descending with m--, then printf("\n").
Frequently Asked Questions
Row 3 starts at 3, prints up to 5 (345), then prints back down to 3 (43) after m = m - 2 — producing 34543.
Each row counts up from i to the peak 2i-1, then counts back down to i. The sequence reads the same left-to-right on each line.
After the increasing loop, m is one past the peak. Subtracting 2 moves it to the value just before the peak so the decreasing loop does not repeat the peak digit.
Step back with m = m - 2 before the decreasing loop. The decreasing loop then runs i-1 times, skipping the peak.
Change rows or read it from user input with scanf — see Example 2.
O(n²) for n rows because row i prints 2i-1 digits and 1+3+5+...+(2n-1) = n² total prints.
Program 51 uses a continuous counter with alternating direction across rows. Program 52 resets m = i each row and builds a palindromic line per row.
One row prints 1 — the decreasing loop k < i never runs when i = 1.
🤔
Did you know?
Each row is palindromic: print i..(2i-1) ascending, then back down with m = m - 2 to skip the peak. Row 3 prints 34543 — total digits = 1+3+5+…+(2n-1) = n² for n rows.
