PHP - How to turn a single array to multi-dimensional array? - php

Question

I have a single array like:
array (
'0' => 1,
'1' => 3,
'2' => 4
)
and I want to turn it to a multi-dimensional array like:
array (
'1' =>
array (
'3' => '4'
),
)
Another single array :
array (
'0' => 'a',
'1' => 'b',
'2' => 'c',
'3' => 'd'
)
to :
array (
'a' =>
array (
'b' =>
array (
'c' => 'd'
),
),
),
How can I achieve this?
Note:
The array is dynamic, not always containing 3 or 4 elements.

Answer 1

There are multiple ways to achieve this:
Simple for loop
First, use array_pop() to pop (i.e. store and remove) the last value from the array. Then iterate from the end of the array, successively creating an array with the values (via array_values()):
function transformArray($input) {
$accumulator = array_pop($input);
$values = array_values($input);
for($i = count($input)-1; $i > -1; $i--) {
$accumulator = array($values[$i] => $accumulator);
}
return $accumulator;
}
See it demonstrated in this playground example
foreach loop
Using the code from the previous example, the for loop can be replaced with a foreach loop by utilizing array_reverse(). That way we don't have to worry about when to start and stop the index values, index in to the array, etc.
function transformArray($input) {
$accumulator = array_pop($input);
$values = array_reverse(array_values($input));
foreach($values as $value) {
$accumulator = array($value => $accumulator);
}
return $accumulator;
}
See it demonstrated in this playground example.
Functional approach: array_reduce()
Array_reduce() can be used to simplify the iteration. The first argument of array_reduce() is the input array, the second is a callback function and the third is the initial value. The parameters for the callback function are the accumulator value (from previous iterations) and the array element at the current array index). The initial value is used as the accumulator value for the first iteration, so we can pass the value that was popped from the end of the array.
function transformArray($input) {
$accumulator = array_pop($input);
$values = array_reverse(array_values($input));
return array_reduce($values, function($accumulator, $element) {
return array($element => $accumulator);
}, $accumulator);
}
See it demonstrated in this playground example.
Recursive Approach
Perhaps the simplest technique is to use a recursive function by employing array_shift() to take the first element off the array and return an array with that element as the key and the value is the return value of calling the same function, unless there is only one item, in which case that one item is returned.
function transformArray($input) {
if (count($input) == 1) {
return $input[0];
}
return array(array_shift($input) => transformArray($input));
}
See it demonstrated in this playground example.

Answer 2

If it has always 3 elements:
$temp = array (
'0' => 1,
'1' => 3,
'2' => 4
);
$result = array();
$result[$temp['0']] = array($temp['1'] => $temp['2']);

Answer 3

Although I think there are better strategies, it's possible to use eval() for this purpose:
<?php
/* Example array */
$arr1 = array(
'0' => 'a',
'1' => 'b',
'2' => 'c',
'3' => 'd'
);
$array_string = '';
/* Build array string */
for($i = 0; $i < count($arr1); $i++){
if($i < (count($arr1) - 1)){
$array_string .= 'array ( \''. $arr1[$i] .'\' => ';
} else {
$array_string .= ' \''. $arr1[$i] .'\'';
}
}
/* Close array string */
for($y = 0; $y < count($arr1); $y++){
if($y < (count($arr1) - 2)){
$array_string .= ' ), ';
} else {
$array_string .= ' )';
break;
}
}
/* Evaluate array string as PHP code and put it in $result */
$result = eval("return $array_string;");
/* Print the result */
echo '<pre>';
print_r($result);
echo '</pre>';
?>
Result:
Array
(
[a] => Array
(
[b] => Array
(
[c] => d
)
)
)
But as I said, keep this quote in mind:
If eval() is the answer, you're almost certainly asking the wrong question. -- Rasmus Lerdorf, BDFL of PHP
Ask yourself why you need this. Is there another strategy available for what you want?