README.md (4863B)
1 <!-- 2 3 @license Apache-2.0 4 5 Copyright (c) 2020 The Stdlib Authors. 6 7 Licensed under the Apache License, Version 2.0 (the "License"); 8 you may not use this file except in compliance with the License. 9 You may obtain a copy of the License at 10 11 http://www.apache.org/licenses/LICENSE-2.0 12 13 Unless required by applicable law or agreed to in writing, software 14 distributed under the License is distributed on an "AS IS" BASIS, 15 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 16 See the License for the specific language governing permissions and 17 limitations under the License. 18 19 --> 20 21 # grev 22 23 > Reverse a strided array in-place. 24 25 <section class="usage"> 26 27 ## Usage 28 29 ```javascript 30 var grev = require( '@stdlib/blas/ext/base/grev' ); 31 ``` 32 33 #### grev( N, x, stride ) 34 35 Reverses a strided array `x` in-place. 36 37 ```javascript 38 var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ]; 39 40 grev( x.length, x, 1 ); 41 // x => [ -3.0, -1.0, 0.0, 4.0, -5.0, 3.0, 1.0, -2.0 ] 42 ``` 43 44 The function has the following parameters: 45 46 - **N**: number of indexed elements. 47 - **x**: input [`Array`][mdn-array] or [`typed array`][mdn-typed-array]. 48 - **stride**: index increment. 49 50 The `N` and `stride` parameters determine which elements in `x` are accessed at runtime. For example, to reverse every other element 51 52 ```javascript 53 var floor = require( '@stdlib/math/base/special/floor' ); 54 55 var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ]; 56 var N = floor( x.length / 2 ); 57 58 grev( N, x, 2 ); 59 // x => [ -1.0, 1.0, 4.0, -5.0, 3.0, 0.0, -2.0, -3.0 ] 60 ``` 61 62 Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][mdn-typed-array] views. 63 64 ```javascript 65 var Float64Array = require( '@stdlib/array/float64' ); 66 var floor = require( '@stdlib/math/base/special/floor' ); 67 68 // Initial array... 69 var x0 = new Float64Array( [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ] ); 70 71 // Create an offset view... 72 var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element 73 var N = floor( x0.length/2 ); 74 75 // Reverse every other element... 76 grev( N, x1, 2 ); 77 // x0 => <Float64Array>[ 1.0, -6.0, 3.0, -4.0, 5.0, -2.0 ] 78 ``` 79 80 #### grev.ndarray( N, x, stride, offset ) 81 82 Reverses a strided array `x` in-place using alternative indexing semantics. 83 84 ```javascript 85 var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ]; 86 87 grev.ndarray( x.length, x, 1, 0 ); 88 // x => [ -3.0, -1.0, 0.0, 4.0, -5.0, 3.0, 1.0, -2.0 ] 89 ``` 90 91 The function has the following additional parameters: 92 93 - **offset**: starting index. 94 95 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 `x` 96 97 ```javascript 98 var x = [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ]; 99 100 grev.ndarray( 3, x, 1, x.length-3 ); 101 // x => [ 1.0, -2.0, 3.0, -6.0, 5.0, -4.0 ] 102 ``` 103 104 </section> 105 106 <!-- /.usage --> 107 108 <section class="notes"> 109 110 ## Notes 111 112 - If `N <= 0`, both functions return `x` unchanged. 113 - Where possible, one should "reverse" a strided array by negating its stride, which is an `O(1)` operation, in contrast to performing an in-place reversal, which is `O(N)`. However, in certain circumstances, this is not tenable, particularly when interfacing with libraries which assume and/or expect a specific memory layout (e.g., strided array elements arranged in memory in ascending order). In general, when working with strided arrays, only perform an in-place reversal when strictly necessary. 114 - Depending on the environment, the typed versions ([`drev`][@stdlib/blas/ext/base/drev], [`srev`][@stdlib/blas/ext/base/srev], etc.) are likely to be significantly more performant. 115 116 </section> 117 118 <!-- /.notes --> 119 120 <section class="examples"> 121 122 ## Examples 123 124 <!-- eslint no-undef: "error" --> 125 126 ```javascript 127 var round = require( '@stdlib/math/base/special/round' ); 128 var randu = require( '@stdlib/random/base/randu' ); 129 var Float64Array = require( '@stdlib/array/float64' ); 130 var grev = require( '@stdlib/blas/ext/base/grev' ); 131 132 var rand; 133 var sign; 134 var x; 135 var i; 136 137 x = new Float64Array( 10 ); 138 for ( i = 0; i < x.length; i++ ) { 139 rand = round( randu()*100.0 ); 140 sign = randu(); 141 if ( sign < 0.5 ) { 142 sign = -1.0; 143 } else { 144 sign = 1.0; 145 } 146 x[ i ] = sign * rand; 147 } 148 console.log( x ); 149 150 grev( x.length, x, 1 ); 151 console.log( x ); 152 ``` 153 154 </section> 155 156 <!-- /.examples --> 157 158 <section class="links"> 159 160 [mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array 161 162 [mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray 163 164 [@stdlib/blas/ext/base/drev]: https://www.npmjs.com/package/@stdlib/blas/tree/main/ext/base/drev 165 166 [@stdlib/blas/ext/base/srev]: https://www.npmjs.com/package/@stdlib/blas/tree/main/ext/base/srev 167 168 </section> 169 170 <!-- /.links -->