@stdlib/array-to-fancy

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array2fancy

Convert an array to an object supporting fancy indexing.

An array supporting fancy indexing is an array which supports slicing via indexing expressions for both retrieval and assignment.

var array2fancy = require( '@stdlib/array-to-fancy' );

// Create a plain array:
var x = [ 1, 2, 3, 4, 5, 6, 7, 8 ];

// Turn the plain array into a "fancy" array:
var y = array2fancy( x );

// Select the first 3 elements:
var v = y[ ':3' ];
// returns [ 1, 2, 3 ]

// Select every other element, starting from the second element:
v = y[ '1::2' ];
// returns [ 2, 4, 6, 8 ]

// Select every other element, in reverse order, starting with the least element:
v = y[ '::-2' ];
// returns [ 8, 6, 4, 2 ]

// Set all elements to the same value:
y[ ':' ] = 9;

// Create a shallow copy by selecting all elements:
v = y[ ':' ];
// returns [ 9, 9, 9, 9, 9, 9, 9, 9 ]

Installation

npm install @stdlib/array-to-fancy

Usage

var array2fancy = require( '@stdlib/array-to-fancy' );

array2fancy( x[, options] )

Converts an array to an object supporting fancy indexing.

var Slice = require( '@stdlib/slice-ctor' );

var x = [ 1, 2, 3, 4 ];

var y = array2fancy( x );
// returns <Array>

// Normal element access:
var v = y[ 0 ];
// returns 1

v = y[ 1 ];
// returns 2

// Using negative integers:
v = y[ -1 ];
// returns 4

v = y[ -2 ];
// returns 3

// Using subsequence expressions:
v = y[ '1::2' ];
// returns [ 2, 4 ]

// Using Slice objects:
v = y[ new Slice( 1, null, 2 ) ];
// returns [ 2, 4 ]

// Assignment:
y[ '1:3' ] = 5;
v = y[ ':' ];
// returns [ 1, 5, 5, 4 ]

The function supports the following options:

• cache: cache for resolving array index objects. Must have a get method which accepts a single argument: a string identifier associated with an array index.

  If an array index associated with a provided identifier exists, the get method should return an object having the following properties:

  • data: the underlying index array.
  • type: the index type. Must be either 'mask', 'bool', or 'int'.
  • dtype: the data type of the underlying array.

  If an array index is not associated with a provided identifier, the get method should return null.

  Default: ArrayIndex.

• strict: boolean indicating whether to enforce strict bounds checking. Default: false.

By default, the function returns a fancy array which does not enforce strict bounds checking. For example,

var y = array2fancy( [ 1, 2, 3, 4 ] );

var v = y[ 10 ];
// returns undefined

To enforce strict bounds checking, set the strict option to true.

var y = array2fancy( [ 1, 2, 3, 4 ], {
    'strict': true
});

var v = y[ 10 ];
// throws <RangeError>

array2fancy.factory( [options] )

Returns a function for converting an array to an object supporting fancy indexing.

var fcn = array2fancy.factory();

var x = [ 1, 2, 3, 4 ];

var y = fcn( x );
// returns <Array>

var v = y[ ':' ];
// returns [ 1, 2, 3, 4 ]

The function supports the following options:

• cache: default cache for resolving array index objects. Must have a get method which accepts a single argument: a string identifier associated with an array index.

  If an array index associated with a provided identifier exists, the get method should return an object having the following properties:

  • data: the underlying index array.
  • type: the index type. Must be either 'mask', 'bool', or 'int'.
  • dtype: the data type of the underlying array.

  If an array index is not associated with a provided identifier, the get method should return null.

  Default: ArrayIndex.

• strict: boolean indicating whether to enforce strict bounds checking by default. Default: false.

By default, the function returns a function which, by default, does not enforce strict bounds checking. For example,

var fcn = array2fancy.factory();

var y = fcn( [ 1, 2, 3, 4 ] );

var v = y[ 10 ];
// returns undefined

To enforce strict bounds checking by default, set the strict option to true.

var fcn = array2fancy.factory({
    'strict': true
});
var y = fcn( [ 1, 2, 3, 4 ] );

var v = y[ 10 ];
// throws <RangeError>

The returned function supports the same options as above. When the returned function is provided option values, those values override the factory method defaults.

array2fancy.idx( x[, options] )

Wraps a provided array as an array index object.

var x = [ 1, 2, 3, 4 ];

var idx = array2fancy.idx( x );
// returns <ArrayIndex>

For documentation and usage, see ArrayIndex.

---

Notes

• A fancy array shares the same data as the provided input array. Hence, any mutations to the returned array will affect the underlying input array and vice versa.
• For operations returning a new array (e.g., when slicing or invoking an instance method), a fancy array returns a new fancy array having the same configuration as specified by options.
• A fancy array supports indexing using positive and negative integers (both numeric literals and strings), Slice instances, subsequence expressions, and index arrays (boolean, mask, and integer).
• A fancy array supports all properties and methods of the input array, and, thus, a fancy array can be consumed by any API which supports array-like objects.
• Indexing expressions provide a convenient and powerful means for creating and operating on array views; however, their use does entail a performance cost. Indexing expressions are best suited for interactive use (e.g., in the REPL) and scripting. For performance critical applications, prefer equivalent functional APIs supporting array-like objects.
• In older JavaScript environments which do not support Proxy objects, the use of indexing expressions is not supported.

Bounds Checking

By default, fancy arrays do not enforce strict bounds checking across index expressions. The motivation for the default fancy array behavior stems from a desire to maintain parity with plain arrays; namely, the returning of undefined when accessing a single non-existent property.

Accordingly, when strict is false, one may observe the following behaviors:

var x = array2fancy( [ 1, 2, 3, 4 ], {
    'strict': false
});

// Access a non-existent property:
var v = x[ 'foo' ];
// returns undefined

// Access an out-of-bounds index:
v = x[ 10 ];
// returns undefined

v = x[ -10 ];
// returns undefined

// Access an out-of-bounds slice:
v = x[ '10:' ];
// returns []

// Access one or more out-of-bounds indices:
var i = array2fancy.idx( [ 10, 20 ] );
v = x[ i ];
// throws <RangeError>

When strict is true, fancy arrays normalize index behavior and consistently enforce strict bounds checking.

var x = array2fancy( [ 1, 2, 3, 4 ], {
    'strict': true
});

// Access a non-existent property:
var v = x[ 'foo' ];
// returns undefined

// Access an out-of-bounds index:
v = x[ 10 ];
// throws <RangeError>

v = x[ -10 ];
// throws <RangeError>

// Access an out-of-bounds slice:
v = x[ '10:' ];
// throws <RangeError>

// Access one or more out-of-bounds indices:
var i = array2fancy.idx( [ 10, 20 ] );
v = x[ i ];
// throws <RangeError>

Broadcasting

Fancy arrays support broadcasting in which assigned scalars and single-element arrays are repeated (without additional memory allocation) to match the length of a target array instance.

var y = array2fancy( [ 1, 2, 3, 4 ] );

// Broadcast a scalar:
y[ ':' ] = 5;
var v = y[ ':' ];
// returns [ 5, 5, 5, 5 ]

// Broadcast a single-element array:
y[ ':' ] = [ 6 ];
v = y[ ':' ];
// returns [ 6, 6, 6, 6 ]

Fancy array broadcasting follows the same rules as for ndarrays. Consequently, when assigning arrays to slices, the array on the right-hand-side must be broadcast-compatible with number of elements in the slice. For example, each assignment expression in the following example follows broadcast rules and is thus valid.

var y = array2fancy( [ 1, 2, 3, 4 ] );

y[ ':' ] = [ 5, 6, 7, 8 ];
var v = y[ ':' ];
// returns [ 5, 6, 7, 8 ]

y[ '1::2' ] = [ 9, 10 ];
v = y[ ':' ];
// returns [ 5, 9, 7, 10 ]

y[ '1::2' ] = [ 11 ];
v = y[ ':' ];
// returns [ 5, 11, 7, 11 ]

y[ '1::2' ] = 12;
v = y[ ':' ];
// returns [ 5, 12, 7, 12 ]

// Out-of-bounds slices (i.e., slices with zero elements):
y[ '10:20' ] = [ 13 ];
v = y[ ':' ];
// returns [ 5, 12, 7, 12 ]

y[ '10:20' ] = 13;
v = y[ ':' ];
// returns [ 5, 12, 7, 12 ]

y[ '10:20' ] = [];
v = y[ ':' ];
// returns [ 5, 12, 7, 12 ]

However, the following assignment expressions are not valid.

var y = array2fancy( [ 1, 2, 3, 4 ] );

y[ ':' ] = [ 5, 6 ];
// throws <Error>

// Out-of-bounds slice (i.e., a slice with zero elements):
y[ '10:20' ] = [ 8, 9, 10, 11 ];
// throws <Error>

Casting

Fancy arrays support (mostly) safe casts (i.e., any cast which can be performed without overflow or loss of precision, with the exception of floating-point arrays which are also allowed to downcast from higher precision to lower precision).

var Uint8Array = require( '@stdlib/array-uint8' );
var Int32Array = require( '@stdlib/array-int32' );

var x = new Int32Array( [ 1, 2, 3, 4 ] );
var y = array2fancy( x );

// 8-bit unsigned integer values can be safely cast to 32-bit signed integer values:
y[ ':' ] = new Uint8Array( [ 5, 6, 7, 8 ] );

When attempting to perform an unsafe cast, fancy arrays will raise an exception.

var Uint8Array = require( '@stdlib/array-uint8' );

var x = new Uint8Array( [ 1, 2, 3, 4 ] );
var y = array2fancy( x );

// Attempt to assign a non-integer value:
y[ ':' ] = 3.14;
// throws <TypeError>

// Attempt to assign a negative value:
y[ ':' ] = -3;
// throws <TypeError>

When assigning a real-valued scalar to a complex number array (e.g., Complex128Array or Complex64Array), a fancy array will cast the real-valued scalar to a complex number argument having an imaginary component equal to zero.

var Complex128Array = require( '@stdlib/array-complex128' );
var real = require( '@stdlib/complex-real' );
var imag = require( '@stdlib/complex-imag' );

var x = new Complex128Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 ] );
var y = array2fancy( x );

// Retrieve the first element:
var v = y[ 0 ];
// returns <Complex128>

var re = real( v );
// returns 1.0

var im = imag( v );
// returns 2.0

// Assign a real-valued scalar to the first element:
y[ 0 ] = 9.0;

v = y[ 0 ];
// returns <Complex128>

re = real( v );
// returns 9.0

im = imag( v );
// returns 0.0

---

Examples

var Uint8Array = require( '@stdlib/array-uint8' );
var Int32Array = require( '@stdlib/array-int32' );
var array2fancy = require( '@stdlib/array-to-fancy' );

var x = [ 1, 2, 3, 4, 5, 6 ];
var y = array2fancy( x );
// returns <Array>

// Slice retrieval:
var z = y[ '1::2' ];
// returns [ 2, 4, 6 ]

z = y[ '-2::-2' ];
// returns [ 5, 3, 1 ]

z = y[ '1:4' ];
// returns [ 2, 3, 4 ]

// Slice assignment:
y[ '4:1:-1' ] = 10;
z = y[ ':' ];
// returns [ 1, 2, 10, 10, 10, 6 ]

y[ '2:5' ] = [ -10, -9, -8 ];
z = y[ ':' ];
// returns [ 1, 2, -10, -9, -8, 6 ]

// Array index retrieval:
var idx = array2fancy.idx;

var i = idx( [ 1, 3, 4 ] ); // integer index array
z = y[ i ];
// returns [ 2, -9, -8 ]

i = idx( [ true, false, false, true, true, true ] ); // boolean array
z = y[ i ];
// returns [ 1, -9, -8, 6 ]

i = idx( new Uint8Array( [ 0, 0, 1, 0, 0, 1 ] ) ); // mask array
z = y[ i ];
// returns [ 1, 2, -9, -8 ]

i = idx( new Int32Array( [ 0, 0, 1, 1, 2, 2 ] ) ); // integer index array
z = y[ i ];
// returns [ 1, 1, 2, 2, -10, -10 ]

---

See Also

• @stdlib/array-slice: return a shallow copy of a portion of an array.
• @stdlib/ndarray-fancy: fancy multidimensional array constructor.

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Notice

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For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.

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License

See LICENSE.

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