Question

I’m trying to find the highest key of a PHP array, based on another value.

$values = array(0,0,50,100,200,400,800);

If I let’s say I have the value of 125, it should return 3. And it should return 5 if I have any value between 400 and 799.

$output = -1;
$input = 436;
$length = count($values);
for($i=1;$i<=$length;$i++){
    if($values[$i]<=$input) { $output++; }
}
// Returns 5

I can loop through the array, but there is 100 values, and that can slow the page down a lot because it is used ~20 times per page load per user. Is there a special function I’m missing? Or am I going to have to foreach the array?

The array is always in order and never changes.

Answer 1

You may be able to do this using array_flip().

[ghoti@pc ~]$ php -r '$n=150; $a=array(0,0,50,100,200,400,800); $a[]=$n; sort($a); $f=array_flip($a); print $f[$n]-1 . "\n";'
3
[ghoti@pc ~]$ php -r '$n=401; $a=array(0,0,50,100,200,400,800); $a[]=$n; sort($a); $f=array_flip($a); print $f[$n]-1 . "\n";'
5
[ghoti@pc ~]$ php -r '$n=200; $a=array(0,0,50,100,200,400,800); $a[]=$n; sort($a); $f=array_flip($a); print $f[$n]-1 . "\n";'
4
[ghoti@pc ~]$ php -r '$n=0; $a=array(0,0,50,100,200,400,800); $a[]=$n; sort($a); $f=array_flip($a); print $f[$n]-1 . "\n";'
1

How does this work?

Let’s break it down.

• $n=0; Our number.
• $a=array(0,0,50,100,200,400,800); Our array.
• $a[]=$n; We add our number to the array…
• sort($a); Sort the thing to place our number in the correct position…
• $f=array_flip($a); Then we exchange values with indices,
• print $f[$n]-1 And return the value (formerly the index) of the item before the one we added.

There may be confusion if $a contains multiple entries of the same number, for example 0 in your example. If this happens, you may need to adjust your adjustment (-1 in my example) because an index can only appear once in an array. (For example, if 0 appeared three times, you’d need to adjust your result with -2. This adjustment could be added with extra code, but I’m sure you can figure that part out yourself. 🙂

This avoids the loop, but be careful if $n=0.