A palindrome number triangle prints ascending digits 1..i, then mirrors i−1..1 — each row reads the same forwards and backwards.
Rule: for i = 1..rows
print 1..i, then i−1..1
1
121
12321
1234321
123454321 ← rows = 5Two inner loops keep the peak digit from repeating — a clear follow-up after the diagonal star in Program 26.
For each row i, print 1..i, then print i−1..1, then end the line.
| Method | Idea | Best for |
|---|---|---|
| Two inner loops | Ascend, then mirror from i - 1 | Learning, interviews |
| input() rows | Same loops; read row count at runtime | Interactive practice |
| Spaced digits | print(..., end=" ") in both halves | Readable output |
for i from 1 to rows:
for j from 1 to i:
print j (same line, no spaces)
for k from i - 1 down to 1:
print k (same line, no spaces)
print newline| Goal | Pattern |
|---|---|
| Set rows | rows = 5 |
| Outer loop | for i in range(1, rows + 1): |
| Ascending half | for j in range(1, i + 1): print(j, end="") |
| Mirror half | for k in range(i - 1, 0, -1): print(k, end="") |
| End row | print() |
| Spaced | print(j, end=" ") / print(k, end=" ") |
| API | Effect | Use for |
|---|---|---|
print(j, end="") / print(k, end="") | Stays on the same line | Each digit |
print() | Ends the current line | After both inner loops |
Glue digits with end="", then break once with print(). Putting print() inside an inner loop prints one digit per line.
Change the row count and the palindrome triangle updates instantly.
rows = 3Trace each i, the ascending half, the mirror half, and the full row.
i | Ascend 1..i | Mirror i−1..1 | Printed row |
|---|---|---|---|
1 | 1 | none | 1 |
2 | 12 | 1 | 121 |
3 | 123 | 21 | 12321 |
Starting the mirror at i − 1 keeps the peak digit from appearing twice in the middle.
Three complete programs: fixed rows = 5, input() rows, and spaced digits. Use View Output for sample results.
rows = 5Hard-coded height — ascend 1..i, then mirror from i − 1.
rows = 5
for i in range(1, rows + 1):
for j in range(1, i + 1):
print(j, end="")
for k in range(i - 1, 0, -1):
print(k, end="")
print()1. Outer rows. i runs from 1 to rows — the peak digit of that palindrome.
2. Ascend. Print j from 1 to i with end="".
3. Mirror. Print k from i − 1 down to 1; then print() ends the row.
Read the row count at runtime; both inner loops stay the same.
rows = int(input("Enter rows: "))
for i in range(1, rows + 1):
for j in range(1, i + 1):
print(j, end="")
for k in range(i - 1, 0, -1):
print(k, end="")
print()1. Read rows. Convert the prompt answer with int().
2. Same two loops. Ascend and mirror logic matches Example 1 for the chosen height.
3. Safer input tip. Bare int(input()) raises ValueError on letters. Prefer:
raw = input("Enter rows: ").strip()
try:
rows = int(raw)
except ValueError:
print("Enter a positive whole number.")
raise SystemExit(1)
if rows < 1:
print("Enter a positive whole number.")
raise SystemExit(1)Keep rows = 5 but print a space after each digit in both halves.
rows = 5
for i in range(1, rows + 1):
for j in range(1, i + 1):
print(j, end=" ")
for k in range(i - 1, 0, -1):
print(k, end=" ")
print()1. Same structure. Outer loop and both half-loops match Example 1.
2. Only end= changes. Use end=" " in both the ascending and mirror loops.
3. Still a palindrome. Values and length stay the same — only spacing differs.
Check these before calling the solution done.
Starting the mirror at range(i, 0, -1) repeats the middle digit (123321). Use range(i - 1, 0, -1).
Skipping the descending loop prints only 1, 12, 123 — not palindromes.
If print() is inside either inner loop, each digit lands on its own line. Call it only after both loops.
Output is just 1 — the mirror loop never runs.
The outer loop never runs. Guard interactive input with rows >= 1.
Letters crash bare int(input()) — wrap in try/except and require rows >= 1.
| Program | Time | Extra space |
|---|---|---|
| Examples 1–3 | O(n²) | O(1) |
Row i prints 2i − 1 digits; total = 1 + 3 + … + (2n − 1) = n² → O(n²).
1..i, then mirror i − 1..1.print() advances after each row.O(n²) for n rows.One line: climb to i, then walk back down from i − 1 so the row is a palindrome.
This palindrome triangle prints 1..i and then i-1..1 on each row. The second loop mirrors the first, producing outputs like 12321 and 123454321.
Continue with the next pattern in the Python number-pattern series.
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