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

License

MIT license.

compute-io/indexspace