A mirrored number pattern prints left digits 1..i and right digits i..1 inside two fixed-width loops — spaces fill unused columns so both halves stay aligned.
Remember
Rule: for i = 1..rows
left = j if j <= i else space (j = 1..rows)
right = space if k > i else k (k = rows..1)
1 1
12 21
123 321
1234 4321
1234554321 ← rows = 5
Unlike Program 27’s tight palindrome, this version keeps a shrinking gap — a natural step after the 0-centered mirror in Program 28.
Approach
How to Solve It
Walk i from 1 to rows; fill a left half of width rows, then a right half of width rows.
Method
Idea
Best for
Two fixed loops
Left digits/spaces + right spaces/digits
Learning, interviews
prompt rows
Same loops; ternary conditions
Interactive practice
Compact rows = 3
Same structure; easier to trace by hand
Paper walkthrough
Pseudocode
Pseudocode
for i from 1 to rows:
line = ""
for j from 1 to rows:
line += j if j <= i else " "
for k from rows down to 1:
line += " " if k > i else k
print line (with newline)
Cheat sheet
Goal
Pattern
Set rows
const rows = 5;
Outer loop
for (let i = 1; i <= rows; i++)
Left half
if (j <= i) line += j; else line += " ";
Right half
if (k > i) line += " "; else line += k;
End row
console.log(line);
Ternary form
line += (j <= i) ? j : " ";
User input
const rows = parseInt(prompt(...), 10);
Printing Numbers vs Starting a New Line
API
Effect
Use for
line += j / line += " "
Stays on the same row
Each digit or space
console.log(line)
Ends the line
After both halves
Build the full row with +=, then break once with console.log. Putting console.log inside an inner loop prints one character per line.
Try it
Live Preview
Change the row count and the spaced mirror updates instantly.
Whole numbers from 1 to 9. Tap a chip or type a value — the preview redraws as you go.
Live resultrows = 5 · width = 10
1 1
12 21
123 321
1234 4321
1234554321
Trace
Worked Walkthrough — rows = 3
Trace each outer i, the left half, the right half, and the printed row.
i
Left j=1..3
Right k=3..1
Printed row
1
1 + 2 spaces
2 spaces + 1
1 1
2
12 + 1 space
1 space + 21
12 21
3
123
321
123321
Gap spaces between digit groups = 2 * (rows - i) — zero when i = rows.
Code
JavaScript Programs
Three complete programs: fixed rows = 5, prompt rows, and compact rows = 3. Use View Output for sample results, or Try It Yourself to edit and run in the playground.
Example 1 — Fixed rows = 5
Hard-coded row count — left loop j = 1..5, right loop k = 5..1, with if/else for digits vs spaces.
JavaScript
const rows = 5;
for (let i = 1; i <= rows; i++) {
let line = "";
for (let j = 1; j <= rows; j++) {
if (j <= i) {
line += j;
} else {
line += " ";
}
}
for (let k = rows; k >= 1; k--) {
if (k > i) {
line += " ";
} else {
line += k;
}
}
console.log(line);
}
1. Prompt and validate. Parse the answer; reject non-positive or non-numeric input.
2. Same shape. Both inner loops still run rows times — only the bound changes.
3. Ternaries.(j <= i) ? j : " " and (k > i) ? " " : k replace if/else.
Example 3 — Compact rows = 3
Same if/else structure with a smaller row count — easy to trace on paper.
JavaScript
const rows = 3;
for (let i = 1; i <= rows; i++) {
let line = "";
for (let j = 1; j <= rows; j++) {
if (j <= i) {
line += j;
} else {
line += " ";
}
}
for (let k = rows; k >= 1; k--) {
if (k > i) {
line += " ";
} else {
line += k;
}
}
console.log(line);
}
1. Same structure. Outer and both inner loops match Example 1.
2. Only rows changes. Width is 3 per half — final row is 123321 with no gap.
3. Trace first. Walk i = 1, 2, 3 on paper before scaling to 5 or more.
Edge Cases & Pitfalls
Check these before calling the solution done.
no spaces
Missing filler spaces
Printing only digits collapses the gap — the right half slides left and the mirror no longer looks centered.
wrong bound
Inner loop to i only
Both loops must run to rows, not i. Stopping early removes the alignment spaces.
log inside
Vertical output
If console.log is inside either inner loop, each character lands on its own line. Log only after both halves.
rows = 1
Smallest mirror
Output is 11 — left prints 1, right prints 1, no gap.
flip test
Swapped conditions
Using j > i for digits on the left (or k <= i for spaces on the right) breaks the shape.
NaN input
Validate parseInt
Letters or empty prompt yield NaN — check Number.isFinite(rows) && rows >= 1.
Analysis
Time and Space Complexity
Program
Time
Extra space
Examples 1–3
O(n²)
O(n) for the current line string
n rows, each with two loops of width n → O(n²).
Remember
Key Takeaways
Rule: left 1..i (pad spaces), right i..1 (pad spaces).
Fixed width: both inner loops always run rows times.
Write vs log:line += … builds; console.log(line) breaks.
Complexity:O(n²) for n rows.
One line: grow digits from both sides while spaces shrink the middle gap until the last row joins.
Frequently Asked Questions
Spaces keep the left and right halves aligned so the pattern looks symmetric. Without them, the right half shifts left each row.
The right loop runs k from rows down to 1. When k > i it appends a space; otherwise it appends k — building i..1 on the right.
Both inner loops always run rows times. Extra positions are filled with spaces so columns stay aligned.
Program 27 prints a tight palindrome with no alignment spaces. Program 29 uses fixed-width loops and spaces for a symmetric spaced mirror.
When i equals rows, both halves fill all columns — 12345 on the left and 54321 on the right meet with no space between.
Replace 5 with rows in both inner loop bounds — see Example 2.
O(n²) for n rows because each row runs two inner loops of width n.
Use parseInt with Number.isFinite so bad input does not produce NaN.
Yes — line += (j <= i) ? j : " " compacts the if/else logic in JavaScript.
🤔
Did you know?
This pattern prints an increasing left half (1..i), then a mirrored right half (i..1). Spaces in the fixed-width loops keep both halves aligned until the final row joins without a gap.
