note 97413 modified in function.array-reduce by levim
| From: | levim@php.net | Date: | Mon, 24 Feb 2014 16:27:49 +0000 |
| Subject: | note 97413 modified in function.array-reduce by levim | ||
| References: | 1 | Groups: | php.notes |
| Request: | Send a blank email to php-notes+get-198630@lists.php.net to get a copy of this message | ||
If you do not provide $initial, the first value used in the iteration is NULL. This is not a problem
for callback functions that treat NULL as an identity (e.g. addition), but is a problem for cases
when NULL is not identity (such as boolean context).
Compare:
<?php
function andFunc($a, $b) {
return $a && $b;
}
$foo = array(true, true, true);
var_dump(array_reduce($foo, "andFunc"));
?>
returns false! One would expect that it would return true because
true && true
&& true == true!
Adding diagnostic output to andFunc() shows that the first call to andFunc is with the arguments
(NULL, true). This resolves to false (as (bool) null == false) and thereby corrupts the
whole reduction.
So in this case I have to set $initial = true so that the first call to andFunc() will
be (true, true). Now, if I were doing, say, orFunc(), I would have to set $initial =
false. Beware.
Note that the "rmul" case in the example sneakily hides this defect! They use an $initial
of 10 to get 10*1*2*3*4*5 = 12000. So you would assume that without an initial, you
would get 1200/10 = 120 = 1*2*3*4*5. Nope! You get big fat zero, because
int(null)==0, and 0*1*2*3*4*5 = 0!
I don't honestly see why array_reduce starts with a null argument. The first call to the
callback should be with arguments ($initial[0],$initial[1]) [or whatever the first two array entries
are], not (null,$initial[0]). That's what one would expect from the description.
Incidentally this also means that under the current implementation you will incur
count($input) number of calls to the callback, not count($input) - 1 as
you might expect.
--was--
It seems that $initial is a required value; if you do not provide $initial, the first value used in
the iteration is NULL.
Perhaps this is not a problem for callback functions that treat NULL as an identity (e.g. addition),
but is a problem for cases when NULL is not identity (such as boolean context).
Compare:
<?php
function andFunc($a,$b) {
return $a&&$b;
}
$foo=Array(true,true,true);
var_dump(array_reduce($foo,"andFunc"));
?>
returns false! One would expect that it would return true because true&&true&&true
== true!
Adding diagnostic output to andFunc() shows that the first call to andFunc is with the arguments
(NULL,true). This resolves to false (as (bool)null == false) and thereby corrupts the whole
reduction.
So in this case I have to set $initial=true so that the first call to andFunc() will be (true,true).
Now, if I were doing, say, orFunc(), I would have to set $initial=false. Beware.
Note that the "rmul" case in the example sneakily hides this defect! They use an $initial
of 10 to get 10*1*2*3*4*5=12000. So you would assume that without an initial, you would get 1200/10
= 120 = 1*2*3*4*5. Nope! You get big fat zero, because int(null)==0, and 0*1*2*3*4*5 = 0!
I don't honestly see why array_reduce starts with a null argument. The first call to the
callback should be with arguments ($initial[0],$initial[1]) [or whatever the first two array entries
are], not (null,$initial[0]). That's what one would expect from the description.
Incidentally this also means that under the current implementation you will incur count($input)
number of calls to the callback, not count($input)-1 as you might expect.
http://php.net/manual/en/function.array-reduce.php