PHP Standard Deviation. Array_map function not found inside class function

Question

I'm trying to perform a Standard Deviation with PHP.

I used this function I founded there because the stats_standard_deviation doesn't seem to be reliable

    <?php 
    class TestController extends \BaseController {

         function MyFunction() {
             $array = array('1','2','3','4');
             function sd_square($x, $mean) { return pow($x - $mean,2); } 

             function sd($array) { 
                      return sqrt(array_sum(array_map("sd_square", $array, array_fill(0,count($array), (array_sum($array) / count($array)) ) ) ) / (count($array)) ); 
             } 
    } return sd($array)
} ?>

The problem is that it returns me array_map() expects parameter 1 to be a valid callback, function 'sd_square' not found or invalid function name when I try to use it.

Looking at some post here I tried to change "sd_square" for "self::sd_square" or array($this, "sq_square").

Please provide the full context. Is this in a class? Both in the same? — kero, Dec 09 '13 at 17:09
There does not seem to be any issue in your code. Are these functions residing inside any class. Can you post the full code. — Sabari, Dec 09 '13 at 17:10
The statistics extension is not bundled with PHP by default. You have installed it haven't you? http://www.php.net/manual/en/stats.installation.php — vascowhite, Dec 09 '13 at 17:17
You're in a class, so `sd_square()` is actually `TestController::sd_square()` and you must therefore use it as a callback like: `array_map(array('TestController', 'sd_square'), ....)` More examples are in the [PHP docs on callbacks](http://php.net/manual/en/language.types.callable.php) — Michael Berkowski, Dec 09 '13 at 17:22
@Michael - but is it? `sd_square` is inside `MyFunction` as well. I don't think `TestController::sd_square()` is right — jon_darkstar, Dec 09 '13 at 17:28
@jon_darkstar Indeed. I didn't notice that due to bad indentation. — Michael Berkowski, Dec 09 '13 at 17:30
@Wistar How are you calling this function from outside the class? How do you pass $array to sd. It should work fine. Shows no error for me. Since your functions are inside MyFunction it can be accessed in that scope. — Sabari, Dec 09 '13 at 17:32
@jon_darkstar Almost looks like transliterated JavaScript than PHP. — Michael Berkowski, Dec 09 '13 at 17:32
Did you try `array($this, "sd_square")`? You wrote you've tried `array($this, "sq_square")`!? — l-x, Dec 09 '13 at 17:33
@Michael Hah yes it kind of does. Try adding `()` at the end maybe? =P — jon_darkstar, Dec 09 '13 at 17:40
@l-x Yes I'v tried `array($this, "sd_square")` and it returns me "undefined variable this" — Wistar, Dec 09 '13 at 17:43
@Wistar return sd($array) is inside function sd () {} is that a typo ?? it should be outside that function. — Sabari, Dec 09 '13 at 17:47

jon_darkstar · Accepted Answer · 2013-12-09T17:33:54.303

1

This kind of sidesteps the issue (and you should address it), but sq_square is small enough you may want to consider simply inlining

Change:

return sqrt(array_sum(array_map("sd_square", $array, array_fill(0,count($array), (array_sum($array) / count($array)) ) ) ) / (count($array)) );

To:

return sqrt(array_sum(array_map(function ($x, $mean) { return pow($x - $mean,2); }, $array, array_fill(0,count($array), (array_sum($array) / count($array)) ) ) ) / (count($array)) );

Or:

$sd_square = function ($x, $mean) { return pow($x - $mean,2); };
return sqrt(array_sum(array_map($sd_square, $array, array_fill(0,count($array), (array_sum($array) / count($array)) ) ) ) / (count($array)) );

edited Dec 09 '13 at 17:33

answered Dec 09 '13 at 17:25

jon_darkstar

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This is NOT to say you shouldn't figure out the scope problems going on, but this is the kind of use case where I like anonymous functions (you can also consider assigning to a variable right beforehand like `$f = function ($x, $mean) { return pow($x - $mean,2); }`, this line is pretty damn long already) – jon_darkstar Dec 09 '13 at 17:35
I guess that my class issue comes from the framework I'm using. I'll figure that out with the documentation. For now, the anonymous function will do the job. Thanks. – Wistar Dec 09 '13 at 17:53
1

Glad this works for you, but I really don't think the framework is the reason for your issue. I'd suggest you reorganize/refactor it all, and really look into the scope issues, when you have the time. They are likely to come up again. – jon_darkstar Dec 09 '13 at 17:58
@jon_darkstar, your formulas should end by `/ (count($array)-1) );` not `/ (count($array)) );`. Note the `-1`! (square root of sum of squares devided by N-1). See: https://www.php.net/manual/en/function.stats-standard-deviation.php#97369 – SAVAFA Dec 13 '20 at 23:20

PHP Standard Deviation. Array_map function not found inside class function

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