How to create an array of every nth item? - php

Question

Say I have the following table:
name
----
A
B
C
D
E
F
G
H
I
I can extract this from MySQL and put it in an array:
$result = mysql_query("SELECT * FROM names ORDER BY name DESC");
while ($row = mysql_fetch_object($result)) {
$names[] = $row->name
}
What I am now looking for is a script that loops through $names and returns pairs, for instance every 3rd name:
array(
[A] => [C],
[B] => [D],
[C] => [E],
[D] => [F]
[E] => [G]
[F] => [H]
[G] => [I]
)
Or, for instance, every 4th name:
array(
[A] => [D],
[B] => [E],
[C] => [F],
[D] => [G]
[E] => [H]
[F] => [I]
)
The number of names in between (3 in the fist example, 4 in the second) should be variable. Does anybody know how to do this in php?

Answer 1

You can do this using a fairly simple for loop.
Here's an example. All you need to do is tweak $seperation:
<?php
$array = array('A','B','C','D','E','F','G','H','I');
$seperation = 3;
$assoc = array(); // stores result
for($i = 0; $i+$seperation < count($array); $i++) {
$assoc[$array[$i]] = $array[$i + $seperation];
}
print_r($assoc);
Demo

Answer 2

Another option is to use the array_slice() and array_combine() functions to create the key/value pairs that you are wanting.
function array_nth(array $array, $nth)
{
$keys = array_slice($array, 0, -($nth - 1));
$values = array_slice($array, $nth - 1);
return array_combine($keys, $values);
}
For example,
$names = range('A', 'I');
var_export(array_nth($names, 3));
Gives
array (
'A' => 'C',
'B' => 'D',
'C' => 'E',
'D' => 'F',
'E' => 'G',
'F' => 'H',
'G' => 'I',
)

Answer 3

I'm assuming these are indexed (by number)? Since in your example you just list [a, b, c]. Something like (might have an off by one bug but this is the idea):
function pairs($arr, $offset) {
$rv = array();
for ($i = 0; $i < (sizeof($arr) - $offset); $i++) {
$rv[$arr[$i]] = $arr[$i + $offset];
}
return $rv;
}
Think it's pretty self explanatory. (sizeof($arr) - $offset) just makes sure you don't go off the end whne you have the $arr[$i + $offset];