php get subarray between two array keys

Question

What is the most efficient way to get a subarray of an array between two keys.

So for example,

$arr=array();
$arr['2014-03-01']='something';
$arr['2014-03-03']='something';
$arr['2014-02-04']='something';
$arr['2014-03-05']='something';
$arr['2014-03-07']='something';
$arr['2014-03-09']='something';
$arr['2014-01-04']='something';
$arr['2014-03-31']='something';

Get the subarray between two keys i.e. start key:2014-02-04 and end key:2014-03-07 should return an array with only:

$arr['2014-02-04']='something';
$arr['2014-03-05']='something';
$arr['2014-03-07']='something';

Is there a quick and efficient way to do this without looping through the entire array?

UPDATE: I did a benchmark here is the results:

$arr=array();
for ($i=1;$i<=1000000;$i++) {
    $arr["$i"]=$i;
}

$time_start=microtime_float();

$start = '20000';
$end   = '20010';

$offset = array_search($start, array_keys($arr));
$length = array_search($end, array_keys($arr)) - $offset + 1;
$output = array_slice($arr, $offset, $length);
print_r($output);
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "TIME=$time\n";
echo "\n============\n";
$time_start=microtime_float();

$result = array();
$start = '20000';
$end   = '20010';

foreach ($arr as $key => $value) {
  if ($key >= $start && $key <= $end)
    $result[$key] = $value;
}
print_r($output);
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "TIME=$time\n";

exit;

RESULTS:

Array
(
    [0] => 20000
    [1] => 20001
    [2] => 20002
    [3] => 20003
    [4] => 20004
    [5] => 20005
    [6] => 20006
    [7] => 20007
    [8] => 20008
    [9] => 20009
    [10] => 20010
)
TIME=1.8481030464172

============
Array
(
    [0] => 20000
    [1] => 20001
    [2] => 20002
    [3] => 20003
    [4] => 20004
    [5] => 20005
    [6] => 20006
    [7] => 20007
    [8] => 20008
    [9] => 20009
    [10] => 20010
)
TIME=1.700336933136

Hence, a simple loop seems to be slightly faster. The advantage increases if I make the start further down the array. You could also use break; once the latter point is reached.

Umm, well, unless the keys are sorted in advance, there is no way you can forego the 'iterates over each values' problem. — Achrome
– Achrome, Commented Mar 19, 2014 at 8:30
You're not actually trying to get a "subarray between two keys"! You're trying to filter your data set to only include values between two boundaries, which is a different problem. Also one you'll never solve without looking at each value individually in a loop. — deceze
– deceze ♦, Commented Mar 19, 2014 at 8:43

xdazz · Accepted Answer · 2014-03-19 08:35:45Z

4

The most efficient way is to use a loop.

$result = array();
$start = '2014-02-04';
$end = '2014-03-07';

foreach ($arr as $key => $value) {
  // your date format is string comparable, otherwise use strtotime to convert to unix timestamp.
  if ($key >= $start && $key <= $end) {
    $result[$key] = $value;
  }
}

Or less efficient way is using array_flip to exchange the key and value, then use array_filter to the required keys, then use array_intersect_key to get the result.

edited Mar 19, 2014 at 8:35

answered Mar 19, 2014 at 8:30

xdazz

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8 Comments

Ray S. Over a year ago

using a huge array. anyway to do this without looping?

deceze Over a year ago

@Ray You will never be able to work with any array without looping.

deceze Over a year ago

@xdazz Only works as long as the keys are comparable and sorted though. Not necessarily a general solution.

xdazz Over a year ago

@deceze The op's date format could be compared as string. And sorted? do you mean sort the result?

deceze Over a year ago

@xdazz I mean, what about the array ['f' => 'foo', 'd' => 'bar', 'z' => 'baz']? Your solution won't work with that. Just noting that though, since it works for the OP's case.

|

hsz · Accepted Answer · 2014-03-19 08:31:33Z

2

You can try with ksort, array_slice and array_search:

$start = '2014-02-04';
$end   = '2014-03-07';

ksort($arr);
$offset = array_search($start, array_keys($arr));
$length = array_search($end, array_keys($arr)) - $offset + 1;
$output = array_slice($arr, $offset, $length);

var_dump($output);

Output:

array (size=5)
  '2014-02-04' => string 'something' (length=9)
  '2014-03-01' => string 'something' (length=9)
  '2014-03-03' => string 'something' (length=9)
  '2014-03-05' => string 'something' (length=9)
  '2014-03-07' => string 'something' (length=9)

answered Mar 19, 2014 at 8:31

hsz

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5 Comments

Achrome Over a year ago

This solution assumes that the array is sorted. :)

hsz Over a year ago

@Achrome It is. Look at ksort.

Achrome Over a year ago

I am aware of ksort. I doubt that it's slower than a simple loop construct.

MSadura Over a year ago

Question just out of curiosity - is that method anyhow faster than simple foreach? I mean wont ksort, array_slice, array_search loop that array "in the background" anyway a few times?

deceze Over a year ago

This will be looping a whole lot.

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