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Date: Tue, 10 Feb 2004 00:30:02 -0000
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Message-ID: <20040210002856.1F4B8406E6B@mra02.ex.eclipse.net.uk>
Subject: [PATCH] Bug #24064: Standard deviation
From: webjedi@hudzilla.eclipse.co.uk ("Paul Hudson")

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Hi there,

Bug #24064 (submitted by tularis@php.net) requests a standard deviation
function for PHP.  I realise that any of you could implement this in 10
minutes, but according to the bug database it is still Open so I figured I
would give it a try myself!

There are probably a dozen errors in the code and/or places where it could
be better optimised, but I'm hoping one of you might be able to help with
that.  So, the attached diff file implements the function array_std_dev(),
to calculate standard deviation using the deviation method.

With the function in place, standard deviation is calculated like this:

<?php
  $scores = array(18,5,7,18,3,2,10);
  print array_std_dev($score);
  // prints 6.6833125519211
?>

My first attempt at implementing this was using an extra array to buffer the
deviations - this was more out of curiosity to see how the array stuff
works.  Sadly, it caused PHP to segfault and I couldn't figure out why - can
any of you help me spot the brain fart?  (I've attached the offending code
in bad_stddev_code.txt)

Yours,


Paul

PS: I'm not on the internals list, so I would appreciate it if you would CC
me on your reply.

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/* {{{ proto mixed array_std_dev(array input)
   Returns the standard deviation of the array entries */
PHP_FUNCTION(array_std_dev)
{
	zval **input,
		 **entry,
		 *entry_n,
		 *squareddev;
	int argc =3D ZEND_NUM_ARGS();
	HashPosition pos;
	double dval;
	double total;
	double mean;
	int numelements =3D 0; // note this is calcuated by hand, not using =
zend_hash_num_elements()
	double deviation;

	if (argc !=3D 1 || zend_get_parameters_ex(argc, &input) =3D=3D FAILURE) =
{
		WRONG_PARAM_COUNT;
	}

	if (Z_TYPE_PP(input) !=3D IS_ARRAY) {
		php_error_docref(NULL TSRMLS_CC, E_WARNING, "The argument should be an =
array");
		return;
	}

	ZVAL_LONG(return_value, 0);
	total =3D 0;

	// step one: sum the values of the array
	for (zend_hash_internal_pointer_reset_ex(Z_ARRVAL_PP(input), &pos);
		 zend_hash_get_current_data_ex(Z_ARRVAL_PP(input), (void **)&entry, =
&pos) =3D=3D SUCCESS;
		 zend_hash_move_forward_ex(Z_ARRVAL_PP(input), &pos)) {

		if (Z_TYPE_PP(entry) =3D=3D IS_ARRAY || Z_TYPE_PP(entry) =3D=3D =
IS_OBJECT)
			continue;

		entry_n =3D *entry;
		zval_copy_ctor(entry_n);
		convert_scalar_to_number(entry_n TSRMLS_CC);
		convert_to_double(entry_n);
		total +=3D Z_DVAL_P(entry_n);

		// this is incremented by hand so that it doesn't count object and =
array elements as an element
		numelements++;
	}

	// step two: calculate the mean of the input array
    mean =3D total / numelements;

	// step three: get the deviation from the mean of each number in the =
input array, and add it to the squareddev array
	MAKE_STD_ZVAL(squareddev);
	array_init(squareddev);

	for (zend_hash_internal_pointer_reset_ex(Z_ARRVAL_PP(input), &pos);
		zend_hash_get_current_data_ex(Z_ARRVAL_PP(input), (void **)&entry, =
&pos) =3D=3D SUCCESS;
		zend_hash_move_forward_ex(Z_ARRVAL_PP(input), &pos)) {

		if (Z_TYPE_PP(entry) =3D=3D IS_ARRAY || Z_TYPE_PP(entry) =3D=3D =
IS_OBJECT)
			continue;

		entry_n =3D *entry;
		zval_copy_ctor(entry_n);
		convert_scalar_to_number(entry_n TSRMLS_CC);
		convert_to_double(entry_n);

		Z_DVAL_P(entry_n) -=3D mean;

		zend_hash_next_index_insert(Z_ARRVAL_P(squareddev), entry_n, =
sizeof(zval *), NULL);

	}

	// step four: sum the squared deviation array
	total =3D 0;

	for (zend_hash_internal_pointer_reset_ex(Z_ARRVAL_P(squareddev), &pos);
		 zend_hash_get_current_data_ex(Z_ARRVAL_P(squareddev), (void =
**)&entry, &pos) =3D=3D SUCCESS;
		 zend_hash_move_forward_ex(Z_ARRVAL_P(squareddev), &pos)) {

		entry_n =3D *entry;
		zval_copy_ctor(entry_n);
		convert_scalar_to_number(entry_n TSRMLS_CC);
		convert_to_double(entry_n);
		total +=3D Z_DVAL_P(entry_n);
	}

	// step five: divide the sum of the squared deviation array by the =
number of elements - 1
	total /=3D numelements - 1;
	Z_DVAL_P(return_value) =3D sqrt(total);

}
/* }}} */
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