Indexed Arrays

A pill organizer has a numbered compartment for each day. One box, many values, and you find each one by its number.

A variable holds one value. But a class has many students, a cart has many items, a playlist has many songs. Making $student1, $student2, $student3, and so on would be hopeless. An array holds a whole list of values in a single variable.

Numbered compartments

<?php
$fruits = ["mango", "banana", "papaya"];

echo $fruits[0], "\n";
echo $fruits[2], "\n";

mango
papaya

• Square brackets [ ] with values separated by commas make an array.
• Each value sits at a numbered position, called its index (or key).
• Indexes start at 0, not 1. The first item is $fruits[0], the third is $fruits[2].

This kind of array, numbered from 0, is called an indexed array. (The next lesson, Associative Arrays, covers arrays with names instead of numbers.)

Reading an index that doesn't exist gives a warning:

<?php
$fruits = ["mango", "banana", "papaya"];
echo $fruits[3];
// error: Warning: Undefined array key 3

There are 3 items, so the last index is 2. Off-by-one mistakes like this are very common. The last index is always the count minus one.

Looking inside: print_r and var_dump

echo can't print a whole array. It prints the word Array with a warning. To see what's inside, use print_r for a quick look, or var_dump for the full details:

<?php
$scores = [88, 92, 75];
print_r($scores);
var_dump($scores);

Array
(
    [0] => 88
    [1] => 92
    [2] => 75
)
array(3) {
  [0]=>
  int(88)
  [1]=>
  int(92)
  [2]=>
  int(75)
}

Counting, changing, adding, removing

<?php
$playlist = ["Intro", "Sunrise", "Rain"];
echo count($playlist), " songs\n";

$playlist[1] = "Sunset";
$playlist[] = "Encore";
array_unshift($playlist, "Warm-up");
print_r($playlist);

3 songs
Array
(
    [0] => Warm-up
    [1] => Intro
    [2] => Sunset
    [3] => Rain
    [4] => Encore
)

• count($array) gives the number of items.
• $playlist[1] = "Sunset"; replaces the item at index 1.
• $playlist[] = "Encore"; with empty brackets adds to the end. This is the most common way to add an item.
• array_unshift adds to the start, and renumbers everything after it.

To take items off:

<?php
$queue = ["Ana", "Ben", "Carlo", "Dina"];

$last = array_pop($queue);
$first = array_shift($queue);
echo "Removed $first and $last\n";
print_r($queue);

Removed Ana and Dina
Array
(
    [0] => Ben
    [1] => Carlo
)

array_pop removes and returns the last item, and array_shift the first. Both give you the removed value, so you can use it.

Looping over an array

You met foreach in Loops. It's the natural way to go through an array:

<?php
$prices = [45, 120, 30];
$total = 0;

foreach ($prices as $i => $price) {
    echo "Item " . ($i + 1) . ": $price\n";
    $total += $price;
}
echo "Total: $total\n";

Item 1: 45
Item 2: 120
Item 3: 30
Total: 195

Adding 1 to the index gives the friendly numbering people expect, starting at 1.

Checking for a value

<?php
$allergies = ["peanuts", "shrimp"];

var_dump(in_array("shrimp", $allergies));
var_dump(in_array("milk", $allergies));
var_dump(array_search("shrimp", $allergies));

bool(true)
bool(false)
int(1)

in_array answers "is it in there?", and array_search gives the index where it was found (or false). To stay clear of type juggling, add true as a third argument to both, like in_array("5", $numbers, true), which makes them compare with ===.

From text to array, and back

Two functions connect strings and arrays, and you'll use them constantly:

• explode(separator, text) splits a string into an array.
• implode(separator, array) joins an array into a string.

<?php
$typed = "red,green,blue";
$colors = explode(",", $typed);
echo count($colors), " colors\n";
echo $colors[1], "\n";

echo implode(" / ", $colors), "\n";

3 colors
green
red / green / blue

Think of explode as cutting a string wherever the separator appears, and implode as gluing the pieces together with a new separator.

Try it

A class tracks who has turned in their project. Predict every line.

Running PHP here

Running PHP right in the browser is coming to Syntaxia soon. For now, you can read and edit the code here. If you have PHP installed on your computer (see Setting Up), save it as index.php and run it with php index.php.

Check your prediction

Class size: 5
Third student: Carlo
Still missing: Carlo and Dina
Most recent: Ana
Earlier ones: 2

After two additions, there are 5 students, and index 2 is the third one, Carlo. The loop builds a new $missing array from everyone not in $turnedIn. array_pop removes the last item, "Ana" (the last one in the typed text), leaving 2.

Try it yourself

1. Add "Fe" to the class list. Who's missing now?
2. Print the class list sorted alphabetically. Try sort($students); before printing it with implode.
3. Split the sentence "PHP is fun to learn" into words with explode(" ", ...), and print how many words it has.

Check your understanding

In $days = ["Mon", "Tue", "Wed"], what is $days[1]?

"Mon"

"Tue"

"Wed"

A warning

What does $list[] = "new"; do?

Empties the array

Adds "new" to the end of the array

Replaces the first item

Causes an error

What does implode("-", ["a", "b", "c"]) give?

["a-b-c"]

"abc"

"-a-b-c-"

"a-b-c"

Up next

Numbered positions are fine for a list, but what about a student's record, with a name, an age, and a grade? For that, you want labels instead of numbers, in Associative Arrays.