Filling Multidimensional Arrays with Numbers in Order in PHP

In the previous examples, all numbers in the subarrays were the same. Let's now make it so that the numbers increase, like this:

<?php
	[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
?>

To do this, you need to create a special counter variable that will increase its value by 1 on each iteration of the inner loop. We will write the value of this counter into the array, like this:

<?php
	$arr = [];
	$k = 1; // counter
	
	for ($i = 0; $i < 3; $i++) {
		for ($j = 0; $j < 3; $j++) {
			$arr[$i][$j] = $k; // write the counter
			$k++; // increment the counter
		}
	}
	
	var_dump($arr);
?>

You can reduce the code by moving the counter definition into the first loop, and k++ - into the second:

<?php
	$arr = [];
	
	for ($i = 0, $k = 1; $i < 3; $i++) {
		for ($j = 0; $j < 3; $j++, $k++) {
			$arr[$i][$j] = $k;
		}
	}
	
	var_dump($arr);
?>

You can also increment the counter after assignment:

<?php
	$arr = [];
	
	for ($i = 0, $k = 1; $i < 3; $i++) {
		for ($j = 0; $j < 3; $j++) {
			$arr[$i][$j] = $k++;
		}
	}
	
	var_dump($arr);
?>

Please note that in this case it must be k++, and not ++k, as the second option will first increment the counter, and only then write to the array (so the array will start with two, not one).

Using two nested loops, form the following array:

<?php
	[[1, 2], [3, 4], [5, 6], [7, 8]]
?>

Using two nested loops, form the following array:

<?php
	[[2, 4, 6], [8, 10, 12], [14, 16, 18], [20, 22, 24]]
?>

Using three nested loops, form the following three-dimensional array:

<?php
	[[[1, 2], [3, 4]], [[5, 6], [7, 8]]]
?>