doWhileEachRight

While a test condition is true, invoke a function for each element in a collection, iterating from right to left.

Usage

var doWhileEachRight = require( '@stdlib/utils/do-while-each-right' );

doWhileEachRight( collection, fcn, predicate[, thisArg ] )

Invokes a function for each element in a collection, iterating from right to left, until either a predicate function returns false or the function has iterated over all collection elements. Note that the condition is evaluated after executing fcn; thus, fcn always executes at least once.

function predicate( value ) {
    return ( value === value );
}

function log( value, index ) {
    console.log( '%s: %d', index, value );
}

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

doWhileEachRight( arr, log, predicate );
/* =>
    4: 4
    3: 3
    2: 2
    1: NaN
*/

Both the predicate function and the function to apply are provided three arguments:

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

If provided an empty collection, both value and index are undefined.

function predicate( value ) {
    return ( value === value );
}

function log( value, index ) {
    console.log( '%s: %s', index, value );
}

var arr = [];

doWhileEachRight( arr, log, predicate );
/* =>
    undefined: undefined
*/

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 predicate( value ) {
    return ( value === value );
}

function log1( value, index, collection ) {
    console.log( '%s: %d', index, value );
    i += 1;
    if ( index === 0 && collection.length < 10 ) {
        collection.unshift( i+1 );
    }
}

doWhileEachRight( arr, log1, predicate );
/* =>
    3: 4
    2: 3
    1: 2
    0: 1
    0: 5
    0: 6
    0: 7
    0: 8
    0: 9
    0: 10
*/

function log2( value, index, collection ) {
    console.log( '%s: %d', index, value );
    collection.pop();
}

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

doWhileEachRight( arr, log2, predicate );
/* =>
    3: 4
    1: 2
*/

To set the function execution context for the applied function, provide a thisArg.

function predicate( value ) {
    return ( value > 2 );
}

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

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

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

doWhileEachRight( arr, sum, predicate, context );

var mean = context.sum / context.count;
// returns 3.0

Notes

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

  • The function returns the input collection.

  • The function does not skip undefined elements.

    function predicate( value ) {
        return ( value === value );
    }
    
    function log( value, index ) {
        console.log( '%s: %s', index, value );
    }
    
    var arr = [ 1, , , 4 ];
    
    doWhileEachRight( arr, log, predicate );
    /* =>
        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 randu = require( '@stdlib/random/base/randu' );
var floor = require( '@stdlib/math/base/special/floor' );
var doWhileEachRight = require( '@stdlib/utils/do-while-each-right' );

var arr;
var i;
var j;

function predicate( value ) {
    return ( value === value );
}

function log( value, index, collection ) {
    console.log( '%s: %d', index, value );
    i += 1;
    if ( isEven( i ) ) {
        collection.pop();
    } else {
        collection.unshift( i+1 );
    }
}

arr = new Array( 100 );
j = floor( randu()*arr.length );
for ( i = arr.length-1; i >= 0; i-- ) {
    if ( i === j ) {
        arr[ i ] = NaN;
    } else {
        arr[ i ] = i;
    }
}

i = 0;
doWhileEachRight( arr, log, predicate );