# Name

MochiKit.Iter - itertools for JavaScript; iteration made HARD, and then easy

# Synopsis

    theSum = sum(takewhile(
            partial(operator.gt, 10),
            imap(
                partial(operator.mul, 2),
                count()
            )
        )
    ));

    assert( theSum == (0 + 0 + 2 + 4 + 6 + 8) );

# Description

All of the functional programming missing from MochiKit.Base lives here. The functionality in this module is largely inspired by Python's iteration protocol [1], and the itertools module [2].

MochiKit.Iter defines a standard way to iterate over anything, that you can extend with registerIterator, or by implementing the `.iter()` protocol. Iterators are lazy, so it can potentially be cheaper to build a filter chain of iterators than to build lots of intermediate arrays. Especially when the data set is very large, but the result is not.

# Dependencies

- MochiKit.Base

# Overview

## Iteration for JavaScript

The best overview right now is in my Iteration for JavaScript [3] blog entry. This information will migrate here eventually.

# API Reference

## Errors

StopIteration:

> The singleton MochiKit.Base.NamedError that signifies the end of an iterator

## Functions

applymap(fun, seq[, self]):

> `applymap(fun, seq)` -->
>   fun.apply(self, seq0), fun.apply(self, seq1), ...

chain(p, q[, ...]):

> `chain(p, q, ...)` --> p0, p1, ... plast, q0, q1, ...

count(n=0):

> `count(n=0)` --> n, n + 1, n + 2, ...

cycle(p):

> `cycle(p)` --> p0, p1, ... plast, p0, p1, ...

dropwhile(pred, seq):

> `dropwhile(pred, seq)` --> seq[n], seq[n + 1], starting when
>   pred(seq[n]) fails

every(iterable, func):

> Return `true` if `func(item)` is `true` for every item in `iterable`.

exhaust(iterable):

> Exhausts an iterable without saving the results anywhere, like list(iterable) when you don't care what the output is.

forEach(iterable, func[, self]):

> Call `func` for each item in `iterable`, and don't save the results.

groupby(iterable[, keyfunc]):

> Make an iterator that returns consecutive keys and groups from the iterable. The key is a function computing a key value for each element. If not specified or is None, key defaults to an identity function and returns the element unchanged. Generally, the iterable needs to already be sorted on the same key function.

> The returned group is itself an iterator that shares the underlying iterable with groupby(). Because the source is shared, when the groupby object is advanced, the previous group is no longer visible. So, if that data is needed later, it should be stored as an array:

>     var groups = [];
>     var uniquekeys = [];
>     forEach(groupby(data, keyfunc), function (key_group) {
>         groups.push(list(key_group[1]));
>         uniquekeys.push(key_group[0]);
>     });

> As a convenience, groupby_as_array() is provided to suit the above use case.

groupby_as_array(iterable[, keyfunc]):

> Perform the same task as groupby(), except return an array of arrays instead of an iterator of iterators.

iextend(lst, iterable):

> Just like list(iterable), except it pushes results on `lst` rather than creating a new one.

ifilter(pred, seq):

> `ifilter(pred, seq)` --> elements of seq where `pred(elem)` is `true`

ifilterfalse(pred, seq):

> `ifilterfalse(pred, seq)` --> elements of seq where `pred(elem)` is
>   `false`

imap(fun, p, q[, ...]):

> `imap(fun, p, q, ...)` --> fun(p0, q0, ...), fun(p1, q1, ...), ...

islice(seq, [start,] stop[, step]):

> `islice(seq, [start,] stop[, step])` --> elements from
>   seq[start:stop:step] (in Python slice syntax)

iter(iterable[, sentinel]):

> Convert the given argument to an iterator (object implementing `.next()`).

> 1. If `iterable` is an iterator (implements `.next()`), then it will be returned as-is.
> 2. If `iterable` is an iterator factory (implements `.iter()`), then the result of `iterable.iter()` will be returned.
> 3. Otherwise, the iterator factory MochiKit.Base.AdapterRegistry is used to find a match.
> 4. If no factory is found, it will throw `TypeError`

> Built-in iterator factories are present for Array-like objects, and objects that implement the `iterateNext` protocol (e.g. the result of Mozilla's `document.evaluate`).

> When used directly, using an iterator should look like this:

>     var it = iter(iterable);
>     try {
>         while (var o = it.next()) {
>             // use o
>         }
>     } catch (e) {
>         if (e != StopIteration) {
>             throw e;
>         }
>         // pass
>     }

> This is ugly, so you should use the higher order functions to work with iterators whenever possible.

izip(p, q[, ...]):

> `izip(p, q, ...)` --> [p0, q0, ...], [p1, q1, ...], ...

list(iterable):

> Convert `iterable` to a new `Array`

next(iterator):

> Return `iterator.next()`

range([start,] stop[, step]):

> Return an iterator containing an arithmetic progression of integers.

> `range(i, j)` returns iter([i, i + 1, i + 2, ..., j - 1])

> `start` (!) defaults to `0`. When `step` is given, it specifies the increment (or decrement). The end point is omitted!

> For example, `range(4)` returns iter([0, 1, 2, 3]). This iterates over exactly the valid indexes for an array of 4 elements.

reduce(fn, iterable[, initial]):

> Apply `fn(a, b)` cumulatively to the items of an iterable from left to right, so as to reduce the iterable to a single value.

> For example:

>     reduce(function (a, b) { return x + y; }, [1, 2, 3, 4, 5])

> calculates:

>     ((((1 + 2) + 3) + 4) + 5).

> If initial is given, it is placed before the items of the sequence in the calculation, and serves as a default when the sequence is empty.

> Note that the above example could be written more clearly as:

>     reduce(operator.add, [1, 2, 3, 4, 5])

> Or even simpler:

>     sum([1, 2, 3, 4, 5])

registerIteratorFactory(name, check, iterfactory[, override]):

> Register an iterator factory for use with the iter function.

> `check` is a `function(a)` that returns `true` if `a` can be converted into an iterator with `iterfactory`.

> `iterfactory` is a `function(a)` that returns an object with a `.next()` method that returns the next value in the sequence.

> `iterfactory` is guaranteed to only be called if `check(a)` returns a true value.

> If `override` is `true`, then it will be made the highest precedence iterator factory. Otherwise, the lowest.

repeat(elem[, n]):

> `repeat(elem, [,n])` --> elem, elem, elem, ... endlessly or up to n times

reversed(iterable):

> Return a reversed array from iterable.

some(iterable, func):

> Return `true` if `func(item)` is `true` for at least one item in `iterable`.

sorted(iterable[, cmp]):

> Return a sorted array from iterable.

sum(iterable, start=0):

> Returns the sum of a sequence of numbers plus the value of parameter `start` (with a default of 0). When the sequence is empty, returns start.

> Equivalent to:

>     reduce(operator.add, iterable, start);

takewhile(pred, seq):

> `takewhile(pred, seq)` --> seq[0], seq[1], ... until pred(seq[n]) fails

tee(iterable, n=2):

> `tee(it, n=2)` --> [it1, it2, it3, ... itn] splits one iterator into n

# See Also

[1] | The iteration protocol is described in PEP 234 - Iterators: http://www.python.org/peps/pep-0234.html

[2] | Python's itertools module: http://docs.python.org/lib/module-itertools.html

[3] | Iteration in JavaScript: http://bob.pythonmac.org/archives/2005/07/06/iteration-in-javascript/

# Authors

- Bob Ippolito <bob@redivi.com>

# Copyright

Copyright 2005 Bob Ippolito <bob@redivi.com>. This program is dual-licensed free software; you can redistribute it and/or modify it under the terms of the MIT License or the Academic Free License v2.1.