Jumping to an array key

PHP
1

I had just posted a thread on inbuilt iterator classes. W/o using an iterator class… is there a way move the array pointer to a specific value?

For example lets say I had an array with 1000+ items! I know I dont need to search through the first 550. is there a way to set the pointer to the 550th item and start the loop there?

BTW, I know a foreach resets the counter, but I am not sure if a while loop would work…

All input, as always, greatly appreciated.

2

Are the keys numerical?

3

yes numerical keys, but they may or may not be sequential…

4

This post on stack seemed to specify some things, one being you can not modify the pointer, but you can use key/next:
http://stackoverflow.com/questions/795625/how-to-set-an-arrays-internal-pointer-to-a-specific-position-php-xml

5

Ok, but it just seems like, if you had a large array, to go trough a percentage of the loop simply to REACH the part of the array you want to work with would be a somewhat inefficient use of computer power…

6

I mean, you wouldn’t necessarily have to use a foreach loop to cycle through the array starting at 550.


for ($i=550; $i<count($array); $i++)
{
     $array[$i] = "550th key.";
}

Or maybe even something like:


for ($i=(count($array)/2); $i<count($array); $i++)
{
echo $array[$i]; // In an array of 10 items, this would be the 5th!
}

That code might not work verbatim, but you get the logic.

7

ah, I think I understand… so the key (no pun) is not to rely on foreach?

Although there is one caveat to to your method. That is if the array is non sequential… $arr[1], $arr[3], $arr[5],$arr[7],$arr[10,$arr[12],$arr[15]…

8 
$a = array(
1=>'France',
6=>'England',
7=>'England',
8=>'England',
9=>'England',
2=>'France',
3=>'Germany',
4=>'France',
5=>'France');

$parts = array_chunk($a, 5, 1);

var_dump ($parts[1]);

//array
//  2 => string 'France' (length=6)
//  3 => string 'Germany' (length=7)
//  4 => string 'France' (length=6)
//  5 => string 'France' (length=6)

array_chunk( your array, size to split into, [BOOL preserve keys]);

In the above case, if you wanted to split the first 3 results you would have to join the remaining arrays back together again in order to have a new array without the first 3.

Would that work?

Edit:

Actually, array_slice might be better


$output = array_slice($a, 3, null, true);     

var_dump( $output );

//array
//  8 => string 'England' (length=7)
//  9 => string 'England' (length=7)
//  2 => string 'France' (length=6)
//  3 => string 'Germany' (length=7)
//  4 => string 'France' (length=6)
//  5 => string 'France' (length=6)
9 

<?php
function seek(array & $array, $key){
  do{
    if($key === key($array)){
      return true;
    }
  }while(each($array));
  
  return false;
}

$array = range(0, 100);

seek($array, 56);

echo current($array); #56
10

Anthony,
So I take it the only way to move the array pointer is one key at a time?

Zurev
incidentally… for seeking a particular key array , I came up with the following method. The only problem is it doesn’t actually move the pointer unless I add a loop…

function seek($arr, $order){
$kmap=array_keys($arr);
return $arr[$kmap[$order]];
}

I suppose I will have to stick to loops. It would be have been nice tho, if you could START an iteration or use each() at a specific point in the array (as oppose to walking up from 0)

11

With regards to seeking to an offset (not a key), there was some discussion about adding such a function on the PHP internals mailing list ([PHP-DEV] array_seek function) but nothing came of it.

My opinion for this thread is that you’re doing it wrong. If you only want to iterate over a part of the array, is there any reason that you can think of as to why you cannot make a new array containing only the items that you are interested in. array_slice() is fine for that. Of course there are other options like using a LimitIterator on an ArrayIterator, or wrestling with for/while/etc. loops.

12

Thanks Salathe,

It’s a shame this was never added, maybe in my naivety, I think a seek function to move the internal pointer would be an essential part of the standard tool-kit.

PHP has obviously done quite well without it, but it just seems natural to have to this.

I also agree with the point you made on the internal mailing list, a ‘truthy’ seek function would be more suited.

Hope you’re well.

Anthony.

13

no reason… now that i think of it other than the extra use of memory. :slight_smile: