inmapRight

Invoke a function for each element in a collection and update the collection in-place, iterating from right to left.

Usage

var inmapRight = require( '@stdlib/utils/inmap-right' );

inmapRight( collection, fcn[, thisArg ] )

Invokes a function for each element in a collection and updates the collection in-place, iterating from right to left.

function scale( value, index ) {
    return value * index;
}

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

var out = inmapRight( arr, scale );
// returns [ 0, 2, 6, 12 ]

var bool = ( out === arr );
// returns true

The invoked function is provided three arguments:

  • value: collection element
  • index: collection index
  • collection: input collection

Basic support for dynamic collections is provided. Note, however, that index incrementation is not guaranteed to be monotonically decreasing.

var arr = [ 1, 2, 3, 4 ];
var i = 0;

function scale1( value, index, collection ) {
    i += 1;
    if ( index === 0 && collection.length < 10 ) {
        collection.unshift( i+1 );
    }
    return value * index;
}

var out = inmapRight( arr, scale1 );
// returns [ 0, 0, 0, 0, 0, 0, 0, 2, 6, 12 ]

function scale2( value, index, collection ) {
    collection.shift();
    return value * index;
}

arr = [ 1, 2, 3, 4 ];

out = inmapRight( arr, scale2 );
// returns [ 3, 12 ]

To set the function execution context, provide a thisArg.

function sum( value ) {
    this.sum += value;
    this.count += 1;
    return value;
}

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

var context = {
    'sum': 0,
    'count': 0
};

var out = inmapRight( arr, sum, context );
// returns [ 1, 2, 3, 4 ]

var mean = context.sum / context.count;
// returns 2.5

Notes

  • A collection may be either an Array, Typed Array, or an array-like Object (excluding strings and functions).

  • The function differs from Array.prototype.map in the following ways:

    • The function returns the input collection.

    • The function modifies collection elements in-place.

    • The function does not skip undefined elements.

      function log( value, index ) {
          console.log( '%s: %s', index, value );
          return value;
      }
      
      var arr = [ 1, , , 4 ];
      
      var out = inmapRight( arr, log );
      /* =>
          3: 4
          2: undefined
          1: undefined
          0: 1
      */
      
    • The function provides limited support for dynamic collections (i.e., collections whose length changes during execution).

Examples

var isEven = require( '@stdlib/assert/is-even' ).isPrimitive;
var inmapRight = require( '@stdlib/utils/inmap-right' );

var bool;
var arr;
var out;
var i;

function scale( value, index, collection ) {
    i += 1;
    if ( isEven( i ) ) {
        collection.pop();
    } else {
        collection.unshift( i+1 );
    }
    return value * index;
}

arr = new Array( 100 );
for ( i = 0; i < arr.length; i++ ) {
    arr[ i ] = i;
}

i = 0;
out = inmapRight( arr, scale );

bool = ( out === arr );
console.log( bool );

console.log( out );