PHP lesson 23: Sorting: bubble sort and sort()

What your child will learn

We are learning to trace a bubble sort, pass by pass.

Success looks like

• I can trace a bubble sort, pass by pass.
• I can write the swap of two items.
• I can use PHP's own sort() and rsort() and know what they return.

Key words

sorting · bubble sort · pass · swap · comparison · sort()

What to have ready

• A device with a web browser (PHP runs in the browser – nothing to install)
• Paper for tracing
• A keyboard helps for typing code (a tablet or phone works too)

The big ideas (for the adult)

A bubble sort goes through the array comparing each item with the NEXT one and swapping them if they are the wrong way round. After the first pass, the biggest item has "bubbled" to the end. Repeat until a pass makes no swaps.

To swap two items you need a spare variable: $t = $a[$j]; $a[$j] = $a[$j + 1]; $a[$j + 1] = $t;.

Real programs usually call PHP's sort() – it is faster and already tested. It changes the array itself and returns only true, so $a = sort($a); is a bug that throws your data away.

Worked example – see it

<?php
$a = ["museum", "school", "guitar", "dragon"];
sort($a);
echo implode(" ", $a);

It prints:

dragon guitar museum school

The lesson, step by step