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feat: add blas/ext/base/dlast-index-of-truthy
PR-URL: #13001 Co-authored-by: Athan Reines <kgryte@gmail.com> Reviewed-by: Athan Reines <kgryte@gmail.com> Signed-off-by: Athan Reines <kgryte@gmail.com> Closes: stdlib-js/metr-issue-tracker#857
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‎lib/node_modules/@stdlib/blas/ext/base/dlast-index-of-truthy/README.md‎

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<!--
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@license Apache-2.0
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Copyright (c) 2026 The Stdlib Authors.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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# dlastIndexOfTruthy
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> Return the index of the last truthy element in a double-precision floating-point strided array.
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<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->
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<section class="intro">
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</section>
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<!-- /.intro -->
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<!-- Package usage documentation. -->
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<section class="usage">
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## Usage
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```javascript
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var dlastIndexOfTruthy = require( '@stdlib/blas/ext/base/dlast-index-of-truthy' );
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```
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#### dlastIndexOfTruthy( N, x, strideX )
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Returns the index of the last truthy element in a double-precision floating-point strided array.
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```javascript
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var Float64Array = require( '@stdlib/array/float64' );
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var x = new Float64Array( [ 0.0, 0.0, 3.0, 0.0, 4.0, 0.0, -1.0, 3.0 ] );
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var idx = dlastIndexOfTruthy( x.length, x, 1 );
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// returns 7
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```
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The function has the following parameters:
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- **N**: number of indexed elements.
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- **x**: input [`Float64Array`][@stdlib/array/float64].
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- **strideX**: stride length.
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If the function is unable to find a truthy element, the function returns `-1`.
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```javascript
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var Float64Array = require( '@stdlib/array/float64' );
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var x = new Float64Array( [ 0.0, 0.0, 0.0, 0.0 ] );
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var idx = dlastIndexOfTruthy( x.length, x, 1 );
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// returns -1
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```
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The `N` and stride parameters determine which elements in the strided array are accessed at runtime. For example, to access every other element:
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```javascript
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var Float64Array = require( '@stdlib/array/float64' );
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var x = new Float64Array( [ 0.0, 0.0, 3.0, 0.0, 4.0, 0.0, -1.0, 3.0 ] );
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var idx = dlastIndexOfTruthy( 4, x, 2 );
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// returns 3
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```
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Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][mdn-typed-array] views.
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```javascript
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var Float64Array = require( '@stdlib/array/float64' );
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// Initial array...
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var x0 = new Float64Array( [ 0.0, 0.0, 0.0, 3.0, 5.0, 0.0 ] );
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// Create an offset view...
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var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
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// Find index...
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var idx = dlastIndexOfTruthy( 3, x1, 2 );
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// returns 1
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```
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#### dlastIndexOfTruthy.ndarray( N, x, strideX, offsetX )
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Returns the index of the last truthy element in a double-precision floating-point strided array using alternative indexing semantics.
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```javascript
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var Float64Array = require( '@stdlib/array/float64' );
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var x = new Float64Array( [ 0.0, 0.0, 3.0, 0.0, 4.0, 0.0, -1.0, 3.0 ] );
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var idx = dlastIndexOfTruthy.ndarray( x.length, x, 1, 0 );
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// returns 7
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```
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The function has the following additional parameters:
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- **offsetX**: starting index.
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While [`typed array`][mdn-typed-array] views mandate a view offset based on the underlying buffer, the offset parameter supports indexing semantics based on a starting index. For example, to access only the last three elements of the strided array:
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```javascript
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var Float64Array = require( '@stdlib/array/float64' );
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var x = new Float64Array( [ 0.0, 0.0, 3.0, 0.0, 4.0, 0.0, 0.0, 3.0 ] );
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var idx = dlastIndexOfTruthy.ndarray( 3, x, 1, x.length-3 );
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// returns 2
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```
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</section>
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<!-- /.usage -->
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<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
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<section class="notes">
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## Notes
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- Both functions explicitly treat `NaN` values as falsy.
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</section>
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<!-- /.notes -->
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<!-- Package usage examples. -->
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<section class="examples">
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## Examples
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<!-- eslint no-undef: "error" -->
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```javascript
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var bernoulli = require( '@stdlib/random/array/bernoulli' );
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var dlastIndexOfTruthy = require( '@stdlib/blas/ext/base/dlast-index-of-truthy' );
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var x = bernoulli( 10, 0.3, {
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'dtype': 'float64'
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});
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console.log( x );
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var idx = dlastIndexOfTruthy( x.length, x, 1 );
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console.log( idx );
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```
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</section>
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<!-- /.examples -->
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<!-- C interface documentation. -->
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* * *
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<section class="c">
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## C APIs
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<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->
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<section class="intro">
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</section>
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<!-- /.intro -->
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<!-- C usage documentation. -->
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<section class="usage">
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### Usage
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```c
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#include "stdlib/blas/ext/base/dlast_index_of_truthy.h"
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```
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#### stdlib_strided_dlast_index_of_truthy( N, \*X, strideX )
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Returns the index of the last truthy element in a double-precision floating-point strided array.
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```c
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const double x[] = { 0.0, 0.0, 3.0, 4.0 };
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int idx = stdlib_strided_dlast_index_of_truthy( 4, x, 1 );
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// returns 3
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```
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The function accepts the following arguments:
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- **N**: `[in] CBLAS_INT` number of indexed elements.
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- **X**: `[in] double*` input array.
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- **strideX**: `[in] CBLAS_INT` stride length.
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```c
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CBLAS_INT stdlib_strided_dlast_index_of_truthy( const CBLAS_INT N, const double *X, const CBLAS_INT strideX );
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```
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#### stdlib_strided_dlast_index_of_truthy_ndarray( N, \*X, strideX, offsetX )
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Returns the index of the last truthy element in a double-precision floating-point strided array using alternative indexing semantics.
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```c
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const double x[] = { 0.0, 0.0, 3.0, 4.0 };
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int idx = stdlib_strided_dlast_index_of_truthy_ndarray( 4, x, 1, 0 );
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// returns 3
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```
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The function accepts the following arguments:
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- **N**: `[in] CBLAS_INT` number of indexed elements.
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- **X**: `[in] double*` input array.
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- **strideX**: `[in] CBLAS_INT` stride length.
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- **offsetX**: `[in] CBLAS_INT` starting index.
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```c
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CBLAS_INT stdlib_strided_dlast_index_of_truthy_ndarray( const CBLAS_INT N, const double *X, const CBLAS_INT strideX, const CBLAS_INT offsetX );
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```
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</section>
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<!-- /.usage -->
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<!-- C API usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
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<section class="notes">
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### Notes
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- Both functions explicitly treat `NaN` values as falsy.
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</section>
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<!-- /.notes -->
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<!-- C API usage examples. -->
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<section class="examples">
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### Examples
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```c
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#include "stdlib/blas/ext/base/dlast_index_of_truthy.h"
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#include <stdio.h>
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int main( void ) {
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// Create a strided array:
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const double x[] = { 0.0, 0.0, 3.0, 0.0, 4.0, 0.0, -1.0, 3.0 };
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// Specify the number of indexed elements:
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const int N = 8;
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// Specify a stride:
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const int strideX = 1;
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// Find the index of the last truthy element:
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int idx = stdlib_strided_dlast_index_of_truthy( N, x, strideX );
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// Print the result:
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printf( "index value: %d\n", idx );
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}
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```
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</section>
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<!-- /.examples -->
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</section>
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<!-- /.c -->
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<!-- Section to include cited references. If references are included, add a horizontal rule *before* the section. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
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<section class="references">
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</section>
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<!-- /.references -->
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<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->
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<section class="related">
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</section>
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<!-- /.related -->
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<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
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<section class="links">
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[@stdlib/array/float64]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/array/float64
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[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
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</section>
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<!-- /.links -->

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