@stdlib/iter/do-until-each/README.md

Standard iterator utilities.

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# iterDoUntilEach

> Create an iterator which, while a test condition is false, invokes a function for each iterated value before returning the iterated value.

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## Usage

```javascript
var iterDoUntilEach = require( '@stdlib/iter/do-until-each' );
```

#### iterDoUntilEach( iterator, predicate, fcn\[, thisArg] )

Returns an iterator which invokes a function for each iterated value **before** returning the iterated value until either a `predicate` function returns `true` or the iterator has iterated over all values. Note that the condition is evaluated **after** executing `fcn`; thus, `fcn` **always** executes at least once.

```javascript
var array2iterator = require( '@stdlib/array/to-iterator' );

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

function assert( v ) {
    if ( v !== v ) {
        throw new Error( 'should not be NaN' );
    }
}

var it = iterDoUntilEach( array2iterator( [ 1, 2, 3, 4 ] ), predicate, assert );
// returns {}

var r = it.next().value;
// returns 1

r = it.next().value;
// returns 2

r = it.next().value;
// undefined

// ...
```

The returned iterator protocol-compliant object has the following properties:

-   **next**: function which returns an iterator protocol-compliant object containing the next iterated value (if one exists) assigned to a `value` property and a `done` property having a boolean value indicating whether the iterator is finished.
-   **return**: function which closes an iterator and returns a single (optional) argument in an iterator protocol-compliant object.

Both the `predicate` function and the function to invoke for each iterated value are provided two arguments:

-   **value**: iterated value
-   **index**: iteration index (zero-based)

```javascript
var array2iterator = require( '@stdlib/array/to-iterator' );

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

function assert( v, i ) {
    if ( i > 2 ) {
        throw new Error( 'unexpected error' );
    }
}

var it = iterDoUntilEach( array2iterator( [ 1, 2, 3, 4 ] ), predicate, assert );
// returns <Object>

var r = it.next().value;
// returns 1

r = it.next().value;
// returns 2

r = it.next().value;
// undefined

// ...
```

To set the execution context for `fcn`, provide a `thisArg`.

<!-- eslint-disable no-invalid-this -->

```javascript
var array2iterator = require( '@stdlib/array/to-iterator' );

function assert( v ) {
    this.count += 1;
    if ( v !== v ) {
        throw new Error( 'should not be NaN' );
    }
}

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

var c = {
    'count': 0
};

var it = iterDoUntilEach( array2iterator( [ 1, 2, 3 ] ), predicate, assert, c );
// returns <Object>

var r = it.next().value;
// returns 1

r = it.next().value;
// returns 2

r = it.next().value;
// returns undefined

var count = c.count;
// returns 3
```

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## Notes

-   If an environment supports `Symbol.iterator` **and** a provided iterator is iterable, the returned iterator is iterable.

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## Examples

<!-- eslint no-undef: "error" -->

```javascript
var randu = require( '@stdlib/random/iter/randu' );
var isnan = require( '@stdlib/math/base/assert/is-nan' );
var iterDoUntilEach = require( '@stdlib/iter/do-until-each' );

function assert( v ) {
    if ( isnan( v ) ) {
        throw new Error( 'should not be NaN' );
    }
}

function predicate( v ) {
    return v <= 0.75;
}

// Create a seeded iterator for generating pseudorandom numbers:
var rand = randu({
    'seed': 1234,
    'iter': 10
});

// Create an iterator which validates generated numbers:
var it = iterDoUntilEach( rand, predicate, assert );

// Perform manual iteration...
var r;
while ( true ) {
    r = it.next();
    if ( r.done ) {
        break;
    }
    console.log( r.value );
}
```

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