compute-indexspace

indexspace

Generates a linearly spaced index array from a subsequence string.

Installation

$ npm install compute-indexspace

For use in the browser, use browserify.

Usage

var indexspace = require( 'compute-indexspace' );

indexspace( str, len )

Generates a linearly spaced index array from a subsequence string. len specifies the reference array length, which is needed to properly interpret the subsequence string. If len = 0, the function returns an empty array.

var arr = indexspace( ':', 5 );
// returns [ 0, 1, 2, 3, 4 ]
 
arr = indexspace( ':', 0 );
// returns []

The subsequence string syntax is similar to Python's slice notation.

var str = '<start>:<stop>:<increment>';

Notes about the notation:

• If an increment is not specified, the default increment is 1. An increment of zero is not allowed.
• The start index is inclusive, while the stop index is exclusive.
• Both start and stop indices are optional. If not provided, start and stop default to index extremes.
• Both start and stop can be negative, in which case the index is subtracted from len.

var arr = indexspace( '-3:', 5 );
// returns [ 2, 3, 4 ];
 
arr = indexspace( ':-2', 5 );
// returns [ 0, 1, 2 ]

The function also recognizes the end keyword, which refers to the last index; i.e., len-1. If specified as the stop index, end is inclusive and equivalent to <start>::<increment>.

var arr = indexspace( 'end::-1', 5 );
// returns [ 4, 3, 2, 1, 0 ]
 
arr = indexspace( ':end', 5 );
// returns [ 0, 1, 2, 3, 4 ]

Basic arithmetic (subtraction and division) may be performed on the end keyword. The result from division is rounded up to the next integer.

var arr = indexspace( 'end-2::-1', 5 );
// returns [ 2, 1, 0 ];
 
arr = indexspace( ':end/2', 5 );
// returns [ 0, 1 ]
 
arr = indexspace( 'end/2:', 5 );
// returns [ 2, 3, 4 ]
 
arr = indexspace( 'end/3::-1', 5 );
// returns [ 2, 1, 0 ];
 
arr = indexspace( '1:end:2', 5 );
// returns [ 1, 3 ];

Note: unlike Matlab, but like Python, the subsequence string is upper-bound exclusive. For example, in Python, 0:2 corresponds to the index array [0,1]. In Matlab, 1:3 corresponds to [1,2,3].

This implementation chooses to follow the Python convention such that :n combined with n: is equivalent to :. Using the Matlab convention, the two subsequences would overlap by one element.

Examples

var indexspace = require( 'compute-indexspace' );
 
var arr = indexspace( ':', 5 );
// returns [ 0, 1, 2, 3, 4 ] 
 
arr = indexspace( '2:', 5 );
// returns [ 2, 3, 4 ]
 
arr = indexspace( ':3', 5 );
// returns [ 0, 1, 2 ]
 
arr = indexspace( '2:4', 5 );
// returns [ 2, 3 ]
 
arr = indexspace( '1:4:2', 5 );
// returns [ 1, 3 ]
 
arr = indexspace( '2::2', 5 );
// returns [ 2, 4 ]
 
arr =  indexspace( ':10:3', 20 );
// returns [ 0, 3, 6, 9 ]
 
arr = indexspace( ':-2', 5 );
// returns [ 0, 1, 2 ]
 
arr = indexspace( ':-1:2', 5 );
// returns [ 0, 2 ]
 
arr = indexspace( '-4:-1:2', 5 );
// returns [ 1, 3 ]
 
arr = indexspace( '-5:-1', 5 );
// returns [ 0, 1, 2, 3 ]
 
arr = indexspace( '::-1', 5 );
// returns [ 4, 3, 2, 1, 0 ]
 
arr = indexspace( ':0:-1', 5 );
// returns [ 4, 3, 2, 1 ]
 
arr = indexspace( '3:0:-1', 5 );
// returns [ 3, 2, 1 ]
 
arr = indexspace( '-1:-4:-2', 5 );
// returns [ 4, 2 ]
 
arr = indexspace( ':end', 5 );
// returns [ 0, 1, 2, 3, 4 ]
 
arr = indexspace( ':end-1', 5 );
// returns [ 0, 1, 2, 3 ]
 
arr = indexspace( ':end/2', 5 );
// returns [ 0, 1 ]
 
arr = indexspace( 'end-2::-1', 5 );
// returns [ 2, 1, 0 ]
 
arr = indexspace( 'end/2:', 5 );
// returns [ 2, 3, 4 ]

To run the example code from the top-level application directory,

$ node ./examples/index.js

Notes

The motivation for this module stems from wanting to create an API for arrays similar to Python and Matlab; e.g., A = B[1:6:2];. JavaScript only supports basic indexing; e.g., A = B[3];.

The workaround provided by this module is to express the subsequence syntax as a string, which, when provided with a reference array length, is parsed and then converted into an index array. A consumer can then iterate through the index array to extract the desired elements.

var indexspace = require( 'compute-indexspace' );
 
// Create an array...
var len = 10,
    arr;
 
arr = new Array( len );
for ( var i = 0; i < len; i++ ) {
    arr[ i ] = i;
}
 
// Create an index array...
var idx = indexspace( '::-1', len );
 
// From the original array, create a reversed array...
var rev = new Array( len );
for ( var j = 0; j < len; j++ ) {
    rev[ j ] = arr[ idx[j] ];
}
console.log( arr.join( ',' ) );
console.log( rev.join( ',' ) );

Tests

Unit

Unit tests use the Mocha test framework with Chai assertions. To run the tests, execute the following command in the top-level application directory:

$ make test

All new feature development should have corresponding unit tests to validate correct functionality.

Test Coverage

This repository uses Istanbul as its code coverage tool. To generate a test coverage report, execute the following command in the top-level application directory:

$ make test-cov

Istanbul creates a ./reports/coverage directory. To access an HTML version of the report,

$ make view-cov

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License

MIT license.

Copyright

Copyright © 2015. Athan Reines.