A hollow diamond star pattern prints only the outline of a diamond: spaces fill the interior, and stars sit on the diagonals. Half-height $rows means 2 * $rows - 1 printed lines.
Remember
Rule: upper i = 1..rows, then lower i = rows-1..1
On each row, print "*" where j == i or k == i
*
* *
* *
* *
* *
* *
* *
* *
* ← rows = 5 (9 lines)
Build it by stacking Program 7 (upper hollow V) and Program 8 (lower hollow V), starting the second outer loop at $rows - 1 so the waist prints once.
Approach
How to Solve It
Print one hollow row with two inner loops, then call that idea twice — ascending, then descending past the waist.
Method
Idea
Best for
Dual outer loops
Upper 1..$rows, lower $rows-1..1, same $j/$k bodies
Learning, interviews, clearest stack of P7 + P8
Helper + ternary
One print_row($i, $rows) called from both halves
Less duplication once the geometry clicks
Pseudocode
Pseudocode
printHollowRow(i, rows):
for j from rows down to 1:
print "*" if i == j else " "
for k from 2 to rows:
print "*" if i == k else " "
print newline
for i from 1 to rows: // upper half
printHollowRow(i, rows)
for i from rows - 1 down to 1: // lower half (skip waist)
printHollowRow(i, rows)
1. One row recipe.print_row owns the $j/$k diagonal logic and the row break.
2. Call it twice. Upper half walks $i up; lower half walks $i down from $rows - 1.
3. Same shape, less copy-paste. Learn the expanded loops first (Example 1), then refactor when the geometry feels familiar.
Edge Cases & Pitfalls
Check these before calling the solution done.
Lower starts at rows
Double waist
If the second outer loop starts at $i = $rows, the widest line prints twice. Use $rows - 1.
$k from 1
Extra center column
Right loop must start at $k = 2. Starting at 1 overlaps the left half’s last column and skews the diamond.
PHP_EOL inside
Broken outline
If echo PHP_EOL sits inside $j or $k, the outline collapses into a column. Call it only after both inner loops.
Proportional font
Looks skewed in the IDE
Spaces and stars need a monospace font (console default is fine). Proportional fonts make diagonals look uneven.
rows = 1
Single star
Upper prints one line; lower never runs. Output is just * — a good sanity check.
rows ≤ 0
Empty output
Neither outer loop runs. Validate and re-prompt for interactive programs.
Bad input
Use is_numeric
Bare (int) casts turn letters into 0 — prefer is_numeric and require $rows >= 1.
Analysis
Time and Space Complexity
Program
Time
Extra space
Dual loops (Examples 1–2)
O(rows²)
O(1)
Helper function (Example 3)
O(rows²)
O(1)
Lines printed = 2n - 1. Each line walks about 2n - 1 positions across the two inner loops, so work is still quadratic in n. Outline stars grow as 4(n - 1) for n ≥ 2 (and 1 when n = 1).
Remember
Key Takeaways
Compose halves: upper 1..$rows then lower $rows-1..1 — Programs 7 + 8 with one seam fix.
Diagonal rule: star only when $i == $j or $i == $k; everything else is a space.
Break the row: call echo PHP_EOL only after both inner loops.
Complexity:O(n²) time; O(1) extra space.
One line: print hollow rows for $i = 1..$rows, then again for $i = $rows-1..1, starring only the diagonals.
Frequently Asked Questions
Two sequential outer loops share the same inner structure. The first runs i from 1 to rows (upper half). The second runs i from rows minus 1 down to 1 (lower half). On each row, j and k print stars when they equal i.
The first part already prints the widest row when i equals rows. Starting the second part at rows again would duplicate that waist line. rows minus 1 mirrors the upper half without a double middle.
Yes, by mapping a loop index to an effective row i or by branching on upper vs lower half. Splitting into two loops matches Programs 7 and 8 mentally and keeps each block easy to read.
rows + (rows - 1) = 2 * rows - 1 lines. Each line is also 2 * rows - 1 characters wide.
echo stays on the same line. echo PHP_EOL ends the current line. Stars and spaces use echo; the row break uses echo PHP_EOL after both inner loops.
With n rows, about 2n - 1 printed lines, each with Theta(n) work across two inner loops, giving O(n²).
Program 9 draws only the outline (hollow). Program 10 fills every star in a solid diamond using spaces and 2*i-1 star runs.
Use trim(fgets(STDIN)) and check is_numeric($input) before casting to int so bad input does not produce unexpected results.
🤔
Did you know?
This hollow diamond is a direct composition: Program 7’s upper half plus Program 8’s lower half with the duplicate middle row removed by starting the second phase at rows - 1. Total printed lines = 2 * rows - 1.
