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* @license Apache-2.0
*
* Copyright (c) 2025 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
'use strict';
// MODULES //
var isnan = require( '@stdlib/math/base/assert/is-nan' );
var sqrt = require( '@stdlib/math/base/special/sqrt' );
// MAIN //
/**
* Computes the standard deviation of a strided array using a two-pass algorithm.
*
* @private
* @param {PositiveInteger} N - number of indexed elements
* @param {number} correction - degrees of freedom adjustment
* @param {Object} x - input array object
* @param {Collection} x.data - input array data
* @param {Array<Function>} x.accessors - array element accessors
* @param {integer} strideX - strideX length
* @param {NonNegativeInteger} offsetX - starting index
* @returns {number} standard deviation
* @example
* var toAccessorArray = require( '@stdlib/array/base/to-accessor-array' );
* var arraylike2object = require( '@stdlib/array/base/arraylike2object' );
*
* var x = toAccessorArray( [ 2.0, 1.0, 2.0, -2.0, -2.0, 2.0, 3.0, 4.0 ] );
*
* var v = stdevpn( 4, 1, arraylike2object( x ), 2, 1 );
* // returns 2.5
*/
function stdevpn( N, correction, x, strideX, offsetX) {
var xbuf;
var xget;
var ix;
var mean;
var M2;
var delta;
var v;
var i;
// Cache reference to array data:
xbuf = x.data;
// Cache a reference to the element accessor:
xget = x.accessors[ 0 ];
if ( N <= 0 || (N - correction) <= 0) {
return NaN;
}
mean = 0.0;
M2 = 0.0;
ix = offsetX;
for ( i = 0; i < N; i++ ) {
v = xget( xbuf, ix );
delta = v - mean;
mean += delta / (i + 1);
M2 += delta * (v - mean);
ix += strideX;
}
var variance = M2 / (N - correction);
return sqrt( variance );
}
// EXPORTS //
module.exports = stdevpn;
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