PHP Associative Arrays

Map names to values and loop over keys and values.

PHP

<?php
$scores = ["Ada" => 91, "Grace" => 88];
$scores["Linus"] = 79;
foreach ($scores as $name => $score) {
    echo "$name: $score\n";
}
echo count($scores), "\n";

Output

Ada: 91
Grace: 88
Linus: 79
3

Explanation

A PHP array is an ordered map. With string keys it works as a dictionary: ["Ada" => 91] pairs the key with a value, and $scores["Linus"] = 79 adds one more entry at the end.

foreach ($scores as $name => $score) gives the loop both the key and the value. Variables inside a double-quoted string are replaced with their values, and count returns the number of entries.

Explore the dry run

Guided walkthroughReady

1<?php

2$scores = ["Ada" => 91, "Grace" => 88];

3$scores["Linus"] = 79;

4foreach ($scores as $name => $score) {

5 echo "$name: $score\n";

6}

7echo count($scores), "\n";

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Two key-value pairs.	`scores = ["Ada" => 91, "Grace" => 88]`	—
A new key is appended.	`3 entries`	—
Each pass prints one pair, in insertion order.	`3 entries`	Ada: 91 Grace: 88 Linus: 79
Print the number of entries.	`3 entries`	Ada: 91 Grace: 88 Linus: 79 3

Cost and common mistakes

Lookup and insert are O(1) on average; the loop is O(n).

Reading a missing key raises a warning and gives null; test with isset or array_key_exists first.
Numeric-looking string keys such as "5" are converted to integer keys.

Try it yourself

Remove Grace before the loop.

Reveal answer and reasoning

Call unset($scores["Grace"]); the loop then prints two lines and count gives 2.

PHP Associative Arrays

Explanation

Follow the example

Cost and common mistakes

Try it yourself

Further reading