A filled diamond star pattern is a centered solid diamond: an upper pyramid of odd-length star rows, mirrored below, with the widest row printed only once.
Remember
Rule: spaces = rows - i, stars = 2 * i - 1
Grow i to rows, then shrink from rows - 1
*
***
*****
*******
*********
*******
*****
***
* ← rows = 5 (half-height)
In PHP you print each row with two inner loops (spaces, then stars), then echo PHP_EOL;. Run that once upward and once downward so the shape closes into a diamond.
Approach
How to Solve It
Reuse one row formula for both halves — start with nested loops, then optionally shorten with str_repeat.
Method
Idea
Best for
Nested loops
Spaces loop + stars loop, twice (up then down)
Learning, interviews, exams
str_repeat
Build spaces and stars as whole strings
Shorter demos once loops click
Pseudocode
Pseudocode
for i from 1 to rows: // upper half
print (rows - i) spaces
print (2 * i - 1) stars
print newline
for i from rows - 1 down to 1: // lower half
print (rows - i) spaces
print (2 * i - 1) stars
print newline
1. One helper for a row.printRow writes $rows - $i spaces, then a line of 2 * $i - 1 stars.
2. Same outer structure. Call it upward for $i = 1..$rows, then downward from $rows - 1.
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.
Lower at rows
Doubled middle
If the lower loop starts at $rows, the widest line prints twice. Start at $rows - 1.
Even stars
Lost center
Use 2 * $i - 1 (odd counts). Even widths break left–right symmetry.
Wrong spaces
Not centered
Spaces must be $rows - $i. Using $i spaces leans the diamond the wrong way.
rows = 1
Single star
Upper prints *; lower never runs — correct tiny diamond.
rows ≤ 0
Empty output
Both outer loops skip. Validate and re-prompt for interactive programs.
Bad input
Check is_numeric
Bare (int) casts turn non-numeric strings into 0 — prefer is_numeric before casting.
Analysis
Time and Space Complexity
Program
Time
Extra space
Nested loops (Examples 1–2)
O(rows²)
O(1)
str_repeat helper (Example 3)
O(rows²)
O(rows) per temporary row string
About 2 * rows - 1 lines; each line does Θ(rows) work for spaces and stars. Total stars = rows² + (rows - 1)².
Remember
Key Takeaways
Row formula:rows - i spaces and 2 * i - 1 stars.
Two phases: grow i to rows, then shrink from rows - 1.
Break the row:echo for spaces/stars; echo PHP_EOL after both inner loops.
Complexity:O(n²) time for half-height n; O(1) extra space for nested loops.
One line: spaces = rows - i, stars = 2*i - 1, grow then shrink from rows - 1.
Frequently Asked Questions
The first outer loop runs i from 1 to rows. On each row it prints (rows - i) spaces, then (2 * i - 1) stars. That builds the upper centered pyramid. The second outer loop runs i from (rows - 1) down to 1 with the same two inner loops, mirroring the shape so the diamond closes.
The first part already prints the widest row when i equals rows. Starting the second part at rows - 1 continues with the next narrower rows without repeating the middle line.
The filled diamond prints full runs of stars using 2*i-1 stars per row. The hollow diamond uses diagonal conditions so only the outline has stars. Both use an upper phase and a lower phase starting at rows - 1.
Odd widths keep a single center star on each row and grow by one star on each side per step, which keeps left–right symmetry.
With n equal to rows (half-height), there are 2n - 1 printed lines. Each line does Theta(n) work for spaces and stars combined, so overall time is O(n²).
Exactly 2 * rows - 1 lines. The widest line has 2 * rows - 1 stars.
Yes. Upper half is Program 5; lower half uses the same inner loops with i running like Program 6’s inverted idea, but starting at rows - 1.
Use trim(fgets(STDIN)) and check is_numeric($input) before casting to int so bad input does not produce unexpected results.
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Did you know?
The filled diamond is Program 5’s pyramid plus its mirror: same (rows - i) spaces and (2 * i - 1) stars, with the lower half starting at rows - 1 so the widest row prints only once.
