How the array of array works in php?

Question

I looked a question in php related with array. But the question is in array of array. The output of the question is "78". But I don't know how it is work. Please explain it...

<?php
    $arr= array(1,2,3,5,8,13,21,34,55);
    $sum = 0;

    for($i=0; $i<5; $i++)
    {
        $sum += $arr[$arr[$i]];
    }
        echo$sum;
?>

$arr[$i] is just the expression you use as the key of $arr?! — Rizier123
– Rizier123, Commented Dec 21, 2016 at 19:46
$sum += $arr[$arr[$i]] is the same as $index = $arr[$i]; $sum += $arr[$index]; — jonhopkins
– jonhopkins, Commented Dec 21, 2016 at 19:58
Well you loop through the numbers 0-4. You use those as index for $arr with the variable $i. So you access the first 5 array elements of the array and use those again as key again. Means the first 5 elements 1,2,3,5,8 are used as key again with which you access the numbers: 2,3,5,13,55 and add them together. — Rizier123
– Rizier123, Commented Dec 21, 2016 at 19:59
@Rizier Thank you for your grand visualize in Sam Onela's answers. — S.Sakthybaalan
– S.Sakthybaalan, Commented Dec 21, 2016 at 20:22

Rizier123 · Accepted Answer · 2016-12-21 20:15:10Z

4

It is adding the element at index $arr[$i], which is not the same as the element at $i.

╔═════════════╦════╦═══════════════════╦══════════════╦══════╗
║ Iteration   ║ $i ║ $index = $arr[$i] ║ $arr[$index] ║ $sum ║
╠═════════════╬════╬═══════════════════╬══════════════╬══════╣
║ before loop ║ -  ║ -                 ║ -            ║ 0    ║
║ 1           ║ 0  ║ 1                 ║ 2            ║ 2    ║
║ 2           ║ 1  ║ 2                 ║ 3            ║ 5    ║
║ 3           ║ 2  ║ 3                 ║ 5            ║ 10   ║
║ 4           ║ 3  ║ 5                 ║ 13           ║ 23   ║
║ 5           ║ 4  ║ 8                 ║ 55           ║ 78   ║
╚═════════════╩════╩═══════════════════╩══════════════╩══════╝

You can see this illustrated in this phpfiddle example.

edited Dec 21, 2016 at 20:15

Rizier123

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answered Dec 21, 2016 at 19:58

Sᴀᴍ Onᴇᴌᴀ

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1 Comment

S.Sakthybaalan Over a year ago

Thank you for your kind explantion.

D Lowther · Accepted Answer · 2016-12-21 21:50:47Z

I changed some variable names and spaced actions out for clarity, and added a lot of comments. I hope this helps clarify what's happening inside the for loop :).

<?php
  /**
   * Think of this as [0 => 1, 1 => 2, ...8 => 55,] or more abstractly as
   * [index => value, index => value] where Array indices
   * start at 0 and climb by every additional value.
   */
  $arrayVariable = [1,2,3,5,8,13,21,34,55,];
  $sumOfArrayParts = 0;

  /* Use for loop to create a bounded iteration (in this case run 5 times) */
  for ($arrayIndex = 0; $arrayIndex < 5; $arrayIndex++) {
    /**
     * Separating this into a separate step for clarity, 
     * set the index to whatever number is at the index given by $arrayIndex 
     */
    $chosenIndex = $arrayVariable[$arrayIndex];
    /* Index Values: 1, 2, 3, 5, 8 */
    $chosenNumber = $arrayVariable[$chosenIndex];
    /* Number Values: 2, 3, 5, 13, 55 */

    /* Add current value at array index */
    $sumOfArrayParts += $chosenNumber;
    /**
     * Iteration values:
     * 1) 0 + 2         // $sumOfArrayParts = 2
     * 2) 2 + 3         // $sumOfArrayParts = 5         
     * 3) 5 + 5         // $sumOfArrayParts = 10
     * 4) 10 + 13       // $sumOfArrayParts = 23
     * 5) 23 + 55       // $sumOfArrayParts = 78
     */
  }
  echo $sumOfArrayParts;
?>

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