Compute standard deviation for every 5 numbers in PHP - php

Question

I have an array of numbers shown below. However, I can only calculate the standard deviation of the whole array with only output 1 result as shown below:
Codes:
function standard_deviation_sample ($a)
{
//variable and initializations
$the_standard_deviation = 0.0;
$the_variance = 0.0;
$the_mean = 0.0;
$the_array_sum = array_sum($a); //sum the elements
$number_elements = count($a); //count the number of elements
//calculate the mean
$the_mean = $the_array_sum / $number_elements;
//calculate the variance
for ($i = 0; $i < $number_elements; $i++)
{
//sum the array
$the_variance = $the_variance + ($a[$i] - $the_mean) * ($a[$i] - $the_mean);
}
$the_variance = $the_variance / ($number_elements - 1.0);
//calculate the standard deviation
$the_standard_deviation = pow( $the_variance, 0.5);
//return the variance
return $the_standard_deviation;
}
$a = array(1,2,3,4,9,6,7,8,9,20,11,12,13,14,2,16,17,18,19,27);
$standard_deviation = standard_deviation_sample ($a);
echo "standard_deviation = $standard_deviation<br>";
Results:
standard_deviation = 7.10004
Does anyone know how to compute standard deviation for every 5 numbers instead? So that the output will be:
standard_deviation_1 = 3.11448
standard_deviation_2 = 5.70088
standard_deviation_3 = 4.82701
standard_deviation_4 = 4.39318

Answer 1

You're looking for array_chunk(); It'll split your array up into smaller arrays of a given size, so your code would now be:
$a = array_chunk(array(1,2,3,4,9,6,7,8,9,20,11,12,13,14,2,16,17,18,19,27), 5);
foreach($a as $b){
echo "standard_deviation: " . standard_deviation_sample($b);
}
Reference: http://uk1.php.net/array_chunk
If you require the number at the end of your standard_deviation_1 output, you could change the loop to a for() loop like so:
for($i = 0; $i < count($a); $i++){
echo "standard_deviation_" . ($i + 1) . " " . standard_deviation_sample($a[$i]);
}

Answer 2

The answer is to use array_chunk(), but there's a few more things you can improve on the code itself:
function standard_deviation_sample(array $a)
{
$the_mean = array_sum($a) / count($a);
return sqrt(array_reduce($a, function($result, $item) use ($the_mean) {
return $result + pow($item - $the_mean, 2);
}, 0) / (count($a) - 1));
}
The function itself can be greatly reduced by only using variables where they're needed; secondly, calculating the squared standard deviation is a great example of something you can solve using array_reduce(); you start with the initial value of 0 and the inner function keeps adding the squared differences together.
$a = array(1,2,3,4,9,6,7,8,9,20,11,12,13,14,2,16,17,18,19,27);
foreach (array_chunk($a, 5) as $k => $sample) {
printf("standard_deviation_%d = %.5f<br>\n",
$k + 1,
standard_deviation_sample($sample));
}