Infinite generators

PHP course · Expert · Lesson 4/54

Learn generators that produce infinite sequences and using them in finite loops.

Infinite generators

You can write a generator that produces values forever with while (true). Memory does not fill up because only one value is produced at a time; earlier values are not stored.

To use an infinite generator in a finite loop, you stop with break or take a fixed number of steps. Otherwise foreach would run forever.

Using an infinite generator finitely

If you iterate an infinite generator with foreach, the loop never stops. You stop it with break under a condition, for example after a fixed number of values.

This way you take only what you need from an infinite stream. The generator stops and memory releases; remaining values are never computed.

Exercise First 5 numbers with naturals

Write naturals(): \Generator: starting from 1, produce numbers forever with while (true), incrementing after each yield. Iterate naturals() with foreach; print the first 5 numbers, then stop with break.

Show example solution

In naturals, start with $n = 1, then in while (true) yield $n; and $n++. Iterate with foreach (naturals() as $n), keep a $count counter; break when $count >= 5.
Example solution:
<?php
function naturals(): \Generator {
    $n = 1;
    while (true) {
        yield $n;
        $n++;
    }
}

$count = 0;
foreach (naturals() as $n) {
    echo $n . "\n";
    $count++;
    if ($count >= 5) {
        break;
    }
}

Test yourself

What happens when an infinite generator with while (true) is called?
- All values are produced until memory overflows
- No value is ever produced
- Only the first value is returned
- One value is produced per request, forever
Show answer

One value is produced per request, forever — Correct. A generator is lazy; one value is produced per request and memory does not fill.

Infinite loop in the naturals generator

function naturals(): \Generator {
    $n = 1;
    ____ {
        yield $n;
        $n++;
    }
}

Show answer

function naturals(): \Generator {
    $n = 1;
    while (true) {
        yield $n;
        $n++;
    }
}

Match the infinite generator concepts

while (true)

Starts an infinite loop
yield

Produces a value each iteration
break

Stops the loop
foreach

Iterates the generator

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Infinite generators