How to convert array of strings to an array of chunks with variable chunk sizes?

Question

How would one convert the following array:

[
  "prefix1 foo",
  "prefix2 bar",
  "prefix1 aaa",
  "prefix2 bbb",
  "prefix3 ccc",
  "prefix1 111",
  "prefix2 222"
]

into the following data structure:

[
  [
    "prefix1" => "foo",
    "prefix2" => "bar"
  ],
  [
    "prefix1" => "aaa",
    "prefix2" => "bbb",
    "prefix3" => "ccc"
  ],
  [
    "prefix1" => "111",
    "prefix2" => "222"
  ],
]

array_chunk would be perfect if not for the fact that the chunks are of variable sizes. The prefixes are known ahead of time and each "chunk" would have a length of either two or three.

Loop through and explode(), then build a new array with the pieces — ivanivan
– ivanivan, Commented Nov 3, 2018 at 1:02
In the future please include your best failed coding attempt. — mickmackusa
– mickmackusa ♦, Commented Nov 3, 2018 at 3:25

trincot · Accepted Answer · 2018-11-03 01:16:08Z

1

You could do it like this (given $input):

$result = [];
$prev = $input[0];
foreach($input as $line) {
    list($key, $value) = explode(" ", $line);
    if ($key < $prev) $result[] = [];
    $result[count($result)-1][$key] = $value;
    $prev = $key;
}

It assumes that the keys, which should end up in the same subarray, are alphabetically ordered in the input, so that a break into a next subarray happens when this order is not maintained.

answered Nov 3, 2018 at 1:16

trincot

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Comments

Nick · Accepted Answer · 2018-11-03 02:40:15Z

This solution allows prefixes to be arbitrarily ordered by storing them in an array in the order in which they are expected to be encountered in the input data (assumed to be in $input):

$prefixes = ['prefix1', 'prefix2', 'prefix3'];
$output = array();
$lastidx = count($prefixes);
foreach ($input as $item) {
    list($prefix, $value) = explode(' ', $item);
    $index = array_search($prefix, $prefixes);
    if ($index < $lastidx) $output[] = array();
    $output[count($output)-1][$prefix] = $value;
    $lastidx = $index;
}
print_r($output);

For your sample input output is:

Array ( 
    [0] => Array ( 
        [prefix1] => foo
        [prefix2] => bar 
    )
    [1] => Array (
        [prefix1] => aaa
        [prefix2] => bbb
        [prefix3] => ccc
    )
    [2] => Array (
        [prefix1] => 111
        [prefix2] => 222
    ) 
)

Demo on 3v4l.org

mickmackusa · Accepted Answer · 2018-11-03 03:19:22Z

isset() is faster than array_search()

Code: (Demo)

$array = [
  "prefix1 foo",
  "prefix2 bar",
  "prefix1 aaa",
  "prefix2 bbb",
  "prefix3 ccc",
  "prefix1 111",
  "prefix2 222"
];

foreach ($array as $v) {
    [$prefix, $value] = explode(' ', $v, 2);  // explode and perform "array destructuring"
    if (isset($batch[$prefix])) {     // check if starting new batch
        $result[] = $batch;           // store old batch
        $batch = [$prefix => $value]; // start new batch
    } else{
        $batch[$prefix] = $value;     // store to current batch
    }
}
$result[] = $batch;                   // store final batch
var_export($result);

or

foreach ($array as $v) {
    [$prefix, $value] = explode(' ', $v, 2);
    if (isset($batch[$prefix])) {
        $result[] = $batch;
        unset($batch);
    }
    $batch[$prefix] = $value;
}
$result[] = $batch;

Output:

array (
  0 => 
  array (
    'prefix1' => 'foo',
    'prefix2' => 'bar',
  ),
  1 => 
  array (
    'prefix1' => 'aaa',
    'prefix2' => 'bbb',
    'prefix3' => 'ccc',
  ),
  2 => 
  array (
    'prefix1' => '111',
    'prefix2' => '222',
  ),
)

splash58 · Accepted Answer · 2018-11-03 07:01:31Z

1

You can do it a little easier

$res = [];
foreach ($array as $v) {
    list($prefix, $value) = explode(' ', $v, 2);
    $res[$prefix][] = [$prefix => $value];
}
print_r($res);
// if you want numeric index
$res = array_values($res);
print_r($res);

demo

answered Nov 3, 2018 at 7:01

splash58

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