A rotating number pattern prints one full pass of 1..n on every row — but each row starts at i, continues to n, then wraps down i-1..1.
Rule: for i = 1..n
print i..n, then i-1..1
12345
23451
34521
45321
54321 ← n = 5After Program 38’s shrinking sequence, this one keeps a fixed width and rotates the starting digit each row.
Walk i from 1 to n; append the forward segment, then the wrap segment.
| Method | Idea | Best for |
|---|---|---|
| Two inner loops | Forward i..n; wrap i-1..1 | Learning, interviews |
| prompt n | Same loops; read n at runtime | Interactive practice |
| Compact n = 3 | Same structure; easier to trace by hand | Paper walkthrough |
for i from 1 to n:
line = ""
for j from i to n:
line += j
for k from i - 1 down to 1:
line += k
print line| Goal | Pattern |
|---|---|
| Set width | const n = 5; |
| Outer loop | for (let i = 1; i <= n; i++) |
| Forward | for (let j = i; j <= n; j++) line += j; |
| Wrap | for (let k = i - 1; k >= 1; k--) line += k; |
| End row | console.log(line); |
| Digits per row | Always n |
| vs Program 38 | Fixed width + rotate vs shrinking continuous k |
| API | Effect | Use for |
|---|---|---|
line += j / line += k | Stays on the same row | Each digit |
console.log(line) | Ends the line | After both halves |
Build the row with +=, then break once with console.log. Putting console.log inside an inner loop prints one digit per line.
Change n and the rotating pattern updates instantly.
n = 3Trace each outer i, the forward segment, the wrap segment, and the printed row.
i | Forward i..n | Wrap i-1..1 | Printed row |
|---|---|---|---|
1 | 1 2 3 | none | 123 |
2 | 2 3 | 1 | 231 |
3 | 3 | 2 1 | 321 |
Every row has exactly n digits. When i = 1, the wrap loop never runs.
Three complete programs: fixed n = 5, prompt input, and compact n = 3. Use View Output for sample results, or Try It Yourself to edit and run in the playground.
n = 5Hard-coded width — forward i..n, then wrap i-1..1.
const n = 5;
for (let i = 1; i <= n; i++) {
let line = "";
for (let j = i; j <= n; j++) {
line += j;
}
for (let k = i - 1; k >= 1; k--) {
line += k;
}
console.log(line);
}1. Outer loop. i is the starting digit of that row.
2. Forward. Append j from i up to n.
3. Wrap. Append k from i - 1 down to 1 (empty when i = 1); then console.log(line).
Read n at runtime with prompt and parseInt.
const n = parseInt(prompt("Enter n:"), 10);
if (!Number.isFinite(n) || n < 1) {
console.log("Please enter a positive integer.");
} else {
for (let i = 1; i <= n; i++) {
let line = "";
for (let j = i; j <= n; j++) {
line += j;
}
for (let k = i - 1; k >= 1; k--) {
line += k;
}
console.log(line);
}
}1. Prompt and validate. Parse the answer; reject non-positive or non-numeric input.
2. Same rotation. Both inner loops still use i and n — only the width changes.
3. Fixed width. Every row still prints exactly n digits.
n = 3Same two-loop structure with a smaller width — easy to trace on paper.
const n = 3;
for (let i = 1; i <= n; i++) {
let line = "";
for (let j = i; j <= n; j++) {
line += j;
}
for (let k = i - 1; k >= 1; k--) {
line += k;
}
console.log(line);
}1. Same structure. Outer and both inner loops match Example 1.
2. Only n changes. Total digits = 3 × 3 = 9 = n².
3. Trace first. Walk i = 1, 2, 3 on paper before scaling to 5 or more.
Check these before calling the solution done.
Using k = 1..i-1 instead of descending yields a pure cycle (23451 style) — a different pattern.
If the wrap loop includes i, you print i twice. Start wrap at i - 1.
If console.log is inside either inner loop, each digit lands on its own line.
Output is just 1 — the wrap loop never runs.
When n > 9, cells become two characters and the visual rotation breaks. Keep demos at n ≤ 9.
Letters or empty prompt yield NaN — check Number.isFinite(n) && n >= 1.
| Program | Time | Extra space |
|---|---|---|
| Examples 1–3 | O(n²) | O(n) for the current line string |
Each of n rows appends n digits → total n² → O(n²).
i..n, then i-1..1, on each row.n digits.line += j builds; console.log(line) breaks.n² → O(n²).One line: start at i, climb to n, then wrap down to 1.
Each row starts at i, appends i..n, then wraps with i-1..1. Row i always appends exactly n digits — total digits = n².
Continue with the next pattern in the JavaScript number-pattern series.
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