@thi.ng/iterators

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About

Clojure inspired, composable ES6 iterators & generators.

Most of the provided functionality here is also available in the form of more composable & efficient transducers via @thi.ng/transducers.

Status

DEPRECATED - superseded by other package(s)

Search or submit any issues for this package

Related packages

@thi.ng/transducers - Lightweight transducer implementations for ES6 / TypeScript

Installation

yarn add @thi.ng/iterators

ES module import:

<script type="module" src="https://cdn.skypack.dev/@thi.ng/iterators"></script>

Skypack documentation

For Node.js REPL:

const iterators = await import("@thi.ng/iterators");

Package sizes (brotli'd, pre-treeshake): ESM: 2.46 KB

API

Generated API docs

In alphabetical order:

butLast

Signature: butLast<T>(input: Iterable<T>) => IterableIterator<T>

Yields iterator of all but the last value of input.

import { butLast, range } from "@thi.ng/iterators";

[...butLast([])]
// []
[...butLast([1])]
// []
[...butLast([1,2])]
// [ 1 ]
[...butLast([1,2,3])]
// [ 1, 2 ]
[...butLast("hello")]
// [ "h", "e", "l", "l" ]
[...butLast(range(10))]
// [ 0, 1, 2, 3, 4, 5, 6, 7, 8 ]

cached

Signature: cached<T>(input: Iterable<T>) => CachedIterableIterator<T>

Consumes and lazily caches values of a finite input and returns a no-arg function, which when called return new iterator. These iterator instances always start from the beginning of the cache and allows for separate states and sharing of cached results among arbitrary number of iterators. The original input is only consumed when attempting to read beyond current cache boundary.

import { cached, range } from "@thi.ng/iterators";

const c = cached(range(10));
const a = c();
const b = c();
a.next();
// { done: false, value: 0 } -> from original
b.next();
// { done: false, value: 0 } -> from cache
b.next();
// { done: false, value: 1 } -> from original
a.next();
// { done: false, value: 1 } -> from cache

const angles = cached(range(0, 360, 45));
[...angles()]
// [ 0, 45, 90, 135, 180, 225, 270, 315 ]

import { juxt, map, zip } from "@thi.ng/iterators";

zip(
    angles(),
    map(
        juxt(Math.sin, Math.cos),
        map(
            (x)=> x * 180 / Math.PI,
            angles()
        )
    )
);
// { "0":   [ 0, 1 ],
//   "45":  [ 0.8060754911159176, -0.5918127259718502 ],
//   "90":  [ -0.9540914674728181, -0.2995153947555356 ],
//   "135": [ 0.3232114532680857, 0.9463267704531728 ],
//   "180": [ 0.5715301650260188, -0.8205810566088714 ],
//   "225": [ -0.9996891031455815, 0.02493385353255689 ],
//   "270": [ 0.6117273014893283, 0.7910687129526641 ],
//   "315": [ 0.27563309945482667, -0.9612629164203338 ] }

concat

Signature: concat<T>(...inputs: Iterable<T>[]) => IterableIterator<T>

Produces iterator yielding lazy concatenation of given iterables. For practical purposes none but the last input should be infinite. Any null or undefined input arguments are skipped in the output.

import { concat } from "@thi.ng/iterators";

[...concat([1, 2, 3], [10, 20, 30], [100, 200, 300])]
// [ 1, 2, 3, 10, 20, 30, 100, 200, 300 ]

[...concat.apply(null, ["abc", null, [1, 2, 3]])]
// [ "a", "b", "c", 1, 2, 3 ]

constantly

Signature: constantly<T>(x: T) => (...args: any[]) => T

Helper function returning a new fn which takes any number of args and always returns x.

import {
    constantly, filter, map, reduce, repeatedly, takeWhile
} from "@thi.ng/iterators";

// define an iterable of unknown size
iter = takeWhile(x => x < 0.98, repeatedly(()=> Math.random()));

// use map & reduce to determine size of iterable:
// `constantly` as mapping fn maps all values to 1
// then reduce is used to sum
reduce((a, b)=> a + b, 0, map(constantly(1), iter))
// 241 (varying)

// some complex data transformation
times10 = (flt, coll) => [...map(x => x * 10, filter(flt, coll))];

// some user selectable config
filterModes = {
    odd: x => (x % 2) == 1,
    even: x => (x % 2) == 0,
    all: constantly(true)
};

// use `constantly` as pluggable bypass filter predicate fn
times10(filterModes[???], [1, 2, 3]);
// [10, 30] (odd)
// [20] (even)
// [10, 20, 30] (all)

consume

Signature: consume(input: Iterator<any>, n?: number) => void

Helper function. Consumes & discards items from iterator (possibly for side effects) and optionally only up to the given number of steps.

cycle

Signature: cycle<T>(input: Iterable<T>) => IterableIterator<T>

Produces iterator which lazily caches & infinitely repeats sequence of input. Important: Input MUST be finite, use take to truncate input or output if necessary.

import { cycle, range, take } from "@thi.ng/iterators";

[...take(10, cycle(range(3)))]
// [0, 1, 2, 0, 1, 2, 0, 1, 2, 0]

dedupe

Signature: dedupe<T>(input: Iterable<T>) => IterableIterator<T>

Produces iterator which discards successive duplicate values from input. Important: Uses === for equality checks.

import { dedupe } from "@thi.ng/iterators";

[...dedupe([1, 2, 2, 3, 4, 4, 4, 3])]
// [1, 2, 3, 4, 3]

dedupeWith

Signature: dedupeWith<T>(equiv: (a:T, b: T) => boolean, input: Iterable<T>) => IterableIterator<T>

Like dedupe, but uses given function equiv to determine equivalence of successive values.

import { dedupeWith } from "@thi.ng/iterators";

var coll = [{ a: 1 }, { a: 1, b: 2 }, { a: 2, b: 2 }, { a: 2, b: 2 }, { a: 3 }];
var eq = (a, b) => a.a === b.a;

[...dedupeWith(eq, coll)]
// [ { a: 1 }, { a: 2, b: 2 }, { a: 3 } ]

dense

Signature: dense<T>(input: Iterable<T>) => IterableIterator<T>

Yields iterator of all non-null and non-undefined values of input (e.g. a sparse array).

import { dense } from "@thi.ng/iterators";

var a = []
a[10] = 1;
a[20] = 2;

[...dense(a)]
// [1, 2]

drop

Signature: drop<T>(n: number, input: Iterable<T>) => IterableIterator<T>

Consumes & discards up to n items from input and returns remaining (possibly exhausted) iterator.

import { drop, range, take } from "@thi.ng/iterators";

[...drop(5, range(10))]
// [5, 6, 7, 8, 9]

[...drop(5, range(3))]
// []

[...take(5, drop(5, range()))]
// [ 5, 6, 7, 8, 9 ]

dropNth

Signature: dropNth<T>(n: number, input: Iterable<T>) => IterableIterator<T>

Produces iterator which drops every nth item from input.

import { dropNth, range } from "@thi.ng/iterators";

[...dropNth(2, range(10))]
// [0, 2, 4, 6, 8]
[...dropNth(3, range(10))]
// [ 0, 1, 3, 4, 6, 7, 9 ]

dropWhile

Signature: dropWhile<T>(pred: (x: T) => boolean, input: Iterable<T>) => IterableIterator<T>

Consumes input and calls pred for each item. Discards all items whilst pred returns true, then returns remaining (possibly exhausted) iterator.

import { dropWhile, range } from "@thi.ng/iterators";

[...dropWhile((x) => x < 5, range(10))]
// [5, 6, 7, 8, 9]

every

Signature: every<T>(pred: (x: T) => boolean, input: Iterable<T>) => boolean

Consumes input and calls pred for each item. When pred returns not true, process stops and returns false itself. When all items pass the predicate, the function returns true.

If input is empty/exhausted prior to execution, every will return false.

import { every, iterator } from "@thi.ng/iterators";

var nums = iterator([2, 4, 6, 8, 10]);

every((x) => (x % 2) === 0, nums);
// true, all passed
nums.next()
// { value: undefined, done: true }

nums = iterator([2, 3, 4]);
every((x) => (x % 2) === 0, nums);
// false, `every` stopped at `3`
nums.next()
// { value: 4, done: false }

every((x) => true, [])
// false (empty input)

filter

Signature: filter<T>(pred: (x: T) => boolean, input: Iterable<T>) => IterableIterator<T>

Consumes input and calls pred for each item. Yields iterator of items for which pred returned true.

import { filter, range } from "@thi.ng/iterators";

const multOf3 = (x: number) => (x % 3) === 0;

[...filter(multOf3, range(10))];
// [ 0, 3, 6, 9 ]

flatten

Signature: flatten(input: Iterable<any>, flatObjects = true) => IterableIterator<any>

Produces iterator which recursively flattens input (using flattenWith). Important: Recursion only applies to iterable types (excluding strings) and objects (enabled by default, using objectIterator, see below).

import { flatten } from "@thi.ng/iterators";

[...flatten([1, [2, 3], [4, [5, ["abc"]]]])]
// [ 1, 2, 3, 4, 5, "abc" ]

[...flatten([{ a: 1 }, { a: 23, b: 42, c: [1, 2, 3] }])]
// ["a", 1, "a", 23, "b", 42, "c", 1, 2, 3]

// don't flatten objects
[...flatten([{ a: 1 }, [1, 2, 3], { a: 23, b: 42, c: [1, 2, 3] }], false)]
// [ { a: 1 }, 1, 2, 3, { a: 23, b: 42, c: [ 1, 2, 3 ] } ]

flattenWith

Signature: flattenWith(tx: (x: any) => any, input: Iterable<any>) => IterableIterator<any>

Produces iterator which selectively & recursively flattens input. The first arg tx is a transformation fn called for each non-null/undefined value taken from the input. It's used to check and possibly obtain an iteratable value for further flattening. The transformer must return undefined if the value can't or shouldn't be flattened. If a value is returned it MUST be iterable.

The default transformer used by flatten is:

import { flattenWith, map, maybeIterator, maybeObjectIterator } from "@thi.ng/iterators";

const defaultTx = x =>
    (typeof x !== "string" && (maybeIterator(x) || maybeObjectIterator(x))) ||
    undefined;

// transformer allowing any iterable and strings
// if `x` is a string, return its numeric charcode sequence for flattening
const tx = x => typeof x == "string" ? map(x => x.charCodeAt(0), x) : maybeIterator(x);

[...flattenWith(tx, ["ROOT", undefined, ["CHILD_1", null, ["CHILD_2"]]])]
// [ 82, 79, 79, 84, undefined, 67, 72, 73, 76, 68, 95, 49, null, 67, 72, 73, 76, 68, 95, 50 ]

fnil

Signature: fnil(fn: (...args: any[]) => any, ...ctors: (() => any)[]) => (...args: any[]) => any

Takes a function fn and up to 3 ctor functions. Produces a new function that calls fn, replacing a null or undefined arg with the value obtained by calling its related positional ctor fn (e.g. the first ctor is used to supply first arg, etc.).

The function fn can take any number of arguments, however only the first 3 are being potentially patched, how many depends on the number of ctor fns supplied.

import { fnil, reduce } from "@thi.ng/iterators";

hello = (greet, name) => `${greet}, ${name}!`;

helloEN = fnil(hello, () => "Hi", () => "user");
helloDE = fnil(hello, () => "Hallo", () => `Teilnehmer #${(Math.random()*100)|0}`);

helloEN(); // "Hi, user!"
helleEN(null,"toxi"); // "Hi, toxi!"
helloEN("Howdy","toxi"); // "Howdy, toxi!"

helloDE(); // "Hallo, Teilnehmer #42!"

inc = x => x + 1
// build new fn which calls ctor to supply 0 if arg is null or undefined
adder = fnil(inc, () => 0);

adder();
// 1 => returns 0 (from ctor fn) + 1
adder(null);
// 1 => same as above
adder(10);
// 11 => arg was given, so 10 + 1

// generic object prop updater using given fn `f`
updateWith = f => (obj, id) => { return obj[id] = f(obj[id]), obj; }

// accumulate letter frequencies of string into object
// fnil is used here to avoid `NaN` each time an yet unknown letter is encountered
reduce(updateWith(adder), {}, "abracadabra");
// { a: 5, b: 2, r: 2, c: 1, d: 1 }

fork

Signature: fork<T>(input: Iterable<T>, cacheLimit = 16) => () => IterableIterator<T>

Similar to cached, this function allows multiple consumers of a single input, however is only using a sliding window of N cached values (cached stores the entire input).

fork returns a no-arg function, which returns a new forked iterator when called. There's no limit to the number of active forks.

Important: The use case for fork is synchronized consumption at similar speeds (up to cacheLimit divergence). The cache is shared by all forks. If one of the forks consumes the input faster than the given cacheLimit, the other forks will lose intermediate values. If in doubt, increase the cache limit to a higher value (default 16). The cache uses @thi.ng/dcons to avoid unnecessary copying during window sliding.

import { fork, map, partition, reduce, repeatedly } from "@thi.ng/iterators";

// stream of random numbers, as sliding partitions of 5 values
src = partition(5, 1, repeatedly(()=> (Math.random() * 100) | 0, 10));

// setup forking, only caching single value
f = fork(src, 1);

// create 4 forks (by calling f()), each with their own transformer:
raw = f();
// simple moving average
sma = map((part)=> reduce((a, b) => a + b, 0, part) / part.length, f());
// minimum
min = map((part)=> reduce((a, b) => Math.min(a, b), 100, part), f());
// maximum
max = map((part)=> reduce((a, b) => Math.max(a, b), -1, part), f());

// consume the forks in synchronized manner
for(let part of raw) {
    console.log(`part: ${part}, avg: ${sma.next().value}, min: ${min.next().value}, ${max.next().value}`);
}

// part: 81,29,46,94,38, avg: 57.6, min: 29, 94
// part: 29,46,94,38,93, avg: 60, min: 29, 94
// part: 46,94,38,93,67, avg: 67.6, min: 38, 94
// part: 94,38,93,67,33, avg: 65, min: 33, 94
// part: 38,93,67,33,59, avg: 58, min: 33, 93
// part: 93,67,33,59,76, avg: 65.6, min: 33, 93

frequencies

Signature: frequencies<T>(input: Iterable<T>, key?: (v: T) => any): IterableIterator<FrequencyPair<T>[]>

Consumes input, applies key fn (if given) to each item and yields iterator of 2-element tuples, each [key, freq] (where freq is the number of times the item occurred). Important: The input MUST be finite. Implementation uses JSON.stringify to determine key equality. If no key fn is given, the original values will be used as key.

import { frequencies, filter } from "@thi.ng/iterators";

// without key fn
[...frequencies([[1,2], [2,3], [1,2], [2,4]])]
// [ [[1, 2], 2],
//   [[2, 3], 1],
//   [[2, 4], 1] ]

// with key fn
[...frequencies([1, 2, 3, 4, 5, 9, 3], (x) => x & ~1)]
// [ [ 0, 1 ], [ 2, 3 ], [ 4, 2 ], [ 8, 1 ] ]

// letter frequencies
var isLetter = (x) => /[a-z]/i.test(x);
[...frequencies(filter(isLetter, "hello world!"))].sort((a, b) => b[1] - a[1])
// [ ["l", 3], ["o", 2], ["h", 1], ["e", 1], ["w", 1], ["r", 1], ["d", 1] ]

groupBy

Signature: groupBy<T>(key: (v) => any, input: Iterable<T>) => { [id: string]: T[] }

Consumes input, applies key fn to each item and returns object of items grouped by result of key fn. Important: The input MUST be finite. Implementation uses JSON.stringify to determine key equality.

import { groupBy } from "@thi.ng/iterators";

// group into multiples of 2
groupBy((x) => x & ~1, [1, 2, 3, 4, 5, 9, 3])
// { '0': [ 1 ], '2': [ 2, 3, 3 ], '4': [ 4, 5 ], '8': [ 9 ] }

groupBy((x) => x.toUpperCase(), "AbRaCadaBra")
// { '"A"': [ 'A', 'a', 'a', 'a', 'a' ],
//   '"B"': [ 'b', 'B' ],
//   '"R"': [ 'R', 'r' ],
//   '"C"': [ 'C' ],
//   '"D"': [ 'd' ] }

identity

Signature: identity<T>(x: T) => T

Helper function. Simply returns its argument.

import { identity, every, fnil } from "@thi.ng/iterators";

identity() // undefined
identity(null) // null
identity(42) // 42

tests = [true, true, undefined, true]

// all tests succeeded?
every(identity, tests);
// false

// mark missing test results as success
every(fnil(identity, () => true), tests);
// true

indexed

Signature: indexed<T>(input: Iterable<T>) => IterableIterator<[number, T]>

Yields iterator producing [index, value] pairs of input.

import { indexed } from "@thi.ng/iterators";
[...indexed("hello")]
// [ [ 0, "h" ], [ 1, "e" ], [ 2, "l" ], [ 3, "l" ], [ 4, "o" ] ]

interleave

Signature: interleave(...inputs: Iterable<any>[]) => IterableIterator<any>

Takes an arbitrary number of iterators and yields iterator of interleaved items from each input. Terminates as soon as one of the inputs is exhausted.

import { interleave, range } from "@thi.ng/iterators";

[...interleave(range(), range(100, 200), range(200, 205))]
// [ 0, 100, 200, 1, 101, 201, 2, 102, 202, 3, 103, 203, 4, 104, 204 ]

interpose

Signature: interpose(x: any, input: Iterable<any>) => IterableIterator<any>

Produces an iterator which injects x between each item from input iter.

import { interpose, range } from "@thi.ng/iterators";

[...interpose("/", range(5))]
// [ 0, "/", 1, "/", 2, "/", 3, "/", 4 ]

iterate

Signature: iterate<T>(fn: (x: T) => T, seed: T) => IterableIterator<T>

Produces an iterator of the infinite results of iteratively applying fn to seed.

Important: Use take to truncate sequence.

import { iterate, take } from "@thi.ng/iterators";

[...take(10, iterate((x) => x * 2, 1))]
// [ 1, 2, 4, 8, 16, 32, 64, 128, 256, 512 ]

iterator

Signature: iterator<T>(x: Iterable<T>) => Iterator<T>

Syntax sugar. Returns x[Symbol.iterator](). Most functions in this module call this automatically for each input.

juxt

Signature: juxt<T>(...fns: ((x: T) => any)[]) => (x: T) => any[]

Takes arbitrary number of functions and returns new function, which takes single argument x and produces array of result values of applying each input function to x (juxtoposition of the given transformation functions).

import { juxt, map, range } from "@thi.ng/iterators";

var kernel = juxt(
    (x) => x - 1,
    (x) => x,
    (x) => x + 1
);
kernel(1)
// [0, 1, 2]

[...map(kernel, range(3))]
// [ [-1, 0, 1], [0, 1, 2], [1, 2, 3] ]

last

Signature: last<T>(input: Iterable<T>) => T

Consumes a finite iterator and returns last item.

Important: Never attempt to use with an infinite input!

import { last, range, take } from "@thi.ng/iterators";

last(range(10))
// 9

// last item of truncated infinite input
last(take(10, range()))
// 9

map

Signature: map<T>(fn: (...args: any[]) => T, ...inputs: Iterable<any>[]) => IterableIterator<T>

Consumes an arbitrary number of iterators and applies fn to each of their values. Produces iterator of results. Iteration stops as soon as one of the inputs is exhausted. The mapping fn should accept as many arguments as there are inputs to map. Provides a fast path for single input mapping.

import { map, range } from "@thi.ng/iterators";

[...map((x)=> x*10, range(10))]
// [0, 10, 20, 30, 40, 50, 60, 70, 80, 90]

[...map((x, y, z) => [x, y, z], range(5), range(0, 100, 10), range(0, 1000, 100))]
// [ [0, 0, 0], [1, 10, 100], [2, 20, 200], [3, 30, 300], [4, 40, 400] ]

mapcat

Signature: mapcat<T>(fn: (...args: any[]) => Iterable<T>, ...inputs: Iterable<any>[]) => IterableIterator<T>

Like map, but expects mapping fn to return an iterable result and produces iterator which yields the flat concatenation of results (only the first level of nesting is removed). null or undefined values returned by the mapping fn are skipped in the output.

import { mapcat, range, repeat } from "@thi.ng/iterators";

[...mapcat((x) => repeat(x, 3), "hello")]
// [ "h", "h", "h", "e", "e", "e", "l", "l", "l", "l", "l", "l", "o", "o", "o" ]

[...mapcat((x) => x < 5 ? repeat(x,x) : null, range(10))]
// [ 1, 2, 2, 3, 3, 3, 4, 4, 4, 4 ]

mapIndexed

Signature: mapIndexed<T>(fn: (i: number, ...args: any[]) => T[], ...inputs: Iterable<any>[]) => IterableIterator<T>

Like map, but too passes a monotonically increasing index as first argument to mapping fn.

import { mapIndexed } from "@thi.ng/iterators";

[...mapIndexed((i, a, b) => [i, a, b], "hello", "there")]
// [ [0, "h", "t"],
//   [1, "e", "h"],
//   [2, "l", "e"],
//   [3, "l", "r"],
//   [4, "o", "e"] ]

maybeIterator

Signature: maybeIterator(x: any) => any

Helper function, returning arg's iterator (if present) or else undefined.

maybeObjectIterator

Signature: maybeObjectIterator(x: any) => any

Helper function, checks if x is object-like (but no generator) and returns objectIterator(x) or else undefined.

objectIterator

Signature: objectIterator(x: any) => IterableIterator<any[]>

Produces iterator of an object"s key/value pairs.

import { objectIterator } from "@thi.ng/iterators";

[...objectIterator({a: 23, b: 42, c: [1, 2, 3]})]
// [ ["a", 23], ["b", 42], ["c", [1, 2, 3]] ]

partition

Signature: partition<T>(n: number, step: number, input: Iterable<T>, all = false) => IterableIterator<T[]>

Produces iterator of fixed size partitions/chunks of input values. Produces overlapping partitions, if step < partition size n. Unless the optional all flag is enabled, returns only completely filled partitions. If all = true, the last partition produced may have less than n items (though never empty).

import { partition, range } from "@thi.ng/iterators";

[...partition(3, 3, range(10))]
// [ [0, 1, 2], [3, 4, 5], [6, 7, 8] ]

[...partition(3, 3, range(10), true)]
// [ [0, 1, 2], [3, 4, 5], [6, 7, 8], [9] ]

[...partition(3, 1, range(10))]
// [ [0, 1, 2], [1, 2, 3], [2, 3, 4], [3, 4, 5],
//   [4, 5, 6], [5, 6, 7], [6, 7, 8], [7, 8, 9] ]

[...partition(3, 5, range(10))]
// [ [0, 1, 2], [5, 6, 7] ]

partitionBy

Signature: partitionBy<T>(fn: (x: T) => any, input: Iterable<T>) => IterableIterator<T[]>

Produces iterator of partitions/chunks of input values. Applies fn to each item and starts new partition each time fn returns new result.

import { partitionBy, range } from "@thi.ng/iterators";

[...partitionBy((x) => x / 5 | 0, range(11))]
// [ [0, 1, 2, 3, 4], [5, 6, 7, 8, 9], [10] ]

randomSample

Signature: randomSample<T>(prob: number, input: Iterable<T>) => IterableIterator<T>

Produces iterator which consumes input and yields values with given probability (0 .. 1 range).

import { randomSample, range } from "@thi.ng/iterators";

[...randomSample(0.1, range(100))]
// [ 10, 13, 16, 21, 22, 24, 32, 35, 37, 81, 93 ]

range

Signature: range(from?: number, to?: number, step?: number) => IterableIterator<number>

Produces iterator of monotonically increasing or decreasing values with optional step value.

If called without arguments, produces values from 0 .. +∞.
If called with 1 arg: 0 ...n (exclusive)
If called with 2 arg: from ... to (exclusive)

If from > to and no step is given, a step of -1 is used.

import { take, range } from "@thi.ng/iterators";

[...take(5, range())]
// [0, 1, 2, 3, 4]

[...range(5)]
// [0, 1, 2, 3, 4]

[...range(100, 105)]
// [100, 101, 102, 103, 104]

[...range(5,0)]
// [5, 4, 3, 2, 1]

[...range(0, 50, 10)]
// [0, 10, 20, 30, 40]

[...range(50, -1, -10)]
// [50, 40, 30, 20, 10, 0]

reduce

Signature: reduce<A, B>(rfn: (acc: B, x: A) => B | ReducedValue<B>, acc: B, input: Iterable<A>) => B

Consumes and reduces input using reduction function rfn, then returns reduced result value. rfn can abort reduction process by returning a value wrapped using reduced(x). If this is done, then this value is unwrapped and returned as final result.

If input is empty, returns initial accumulator arg.

import { reduce, reduced, range } from "@thi.ng/iterators";

reduce((acc, x) => acc + x, 0, range(10))
// 45

// infinite input with early termination
reduce(
    (acc, x) => {
        acc += x;
        return acc >= 15 ? reduced(acc) : acc;
    },
    0,
    range()
)
// 15

reductions

Signature: reductions<A, B>(rfn: (acc: B, x: A) => B | ReducedValue<B>, acc: B, input: Iterable<A>) => IterableIterator<B[]>

Like reduce, but yields an iterator of all intermediate reduction results. Always yields at least initial accumulator arg, even if input is empty.

Thus, the result is the equivalent of an exclusive scan operation (with the exception of possible early bail out via reduced).

import { reductions, reduced, range } from "@thi.ng/iterators";

[...reductions((acc, x) => acc + x, 0, range(10))]
// [ 0, 1, 3, 6, 10, 15, 21, 28, 36, 45 ]

// with early termination
[...reductions(
    (acc, x) => {
        acc += x;
        return acc >= 15 ? reduced(acc) : acc
    },
    0,
    range()
)]
// [ 0, 1, 3, 6, 10, 15 ]

reduced

Signature: reduced<T>(x: T) => ReducedValue<T>

For use inside reduction functions only. Wraps result in marker type to cause early termination of reduction (see example above).

repeat

Signature: repeat<T>(x: T, n?: number) => IterableIterator<T>

Produces an iterator of infinite (by default) repetitions of value x. If n is given, produces only that many values.

import { repeat, take } from "@thi.ng/iterators";
[...take(5, repeat(42))]
// [42, 42, 42, 42, 42]

[...repeat(42, 5)]
// [42, 42, 42, 42, 42]

repeatedly

Signature: repeatedly<T>(fn: () => T, n?: number) => IterableIterator<T>

Produces an iterator of infinite (by default) results of calling the no-arg fn repeatedly. If n is given, produces only that many values.

import { repeatedly, take } from "@thi.ng/iterators";

[...take(3, repeatedly(() => Math.random()))]
// [ 0.9620186971807614, 0.8191901643942394, 0.5964328949163533 ]

[...repeatedly(() => Math.random(), 3)]
// [ 0.46381477224416057, 0.22568030685532992, 0.5494769470662977 ]

reverse

Signature: reverse<T>(input: Iterable<T>) => IterableIterator<T>

Yields iterator lazily producing reverse result order of input (must be finite). If input is not already array-like (strings are for this purpose), the function first consumes & caches input as array.

import { iterate, reverse, take } from "@thi.ng/iterators";

[...reverse([1, 2, 3])]
// [3, 2, 1]

[...reverse("hello")]
// [ "o", "l", "l", "e", "h" ]

[...reverse(take(10, iterate(x => x * 2, 1)))]
// [ 512, 256, 128, 64, 32, 16, 8, 4, 2, 1 ]

some

Signature: some<T>(pred: (x: T) => boolean, input: Iterable<T>) => T

Consumes iterator and calls pred for each item. When pred returns true, process stops and returns this first successful item. When none of the items pass the predicate, the function returns undefined.

import { iterator, some } from "@thi.ng/iterators";

var nums = iterator([1, 2, 3]);

some((x) => (x % 2) === 0, nums);
// 2, the 1st value which passed
nums.next()
// { value: 3, done: false }

nums = iterator([1, 2, 3]);
some((x) => x > 3, nums);
// undefined
nums.next()
// { value: undefined, done: true }

take

Signature: take<T>(n: number, input: Iterable<T>) => IterableIterator<T>

Produces iterator of the first n values of input (or less than n, if input is too short...)

import { range, take } from "@thi.ng/iterators";

[...take(3, range())]
// [ 0, 1, 2 ]

takeNth

Signature: takeNth<T>(n: number, input: Iterable<T>) => IterableIterator<T>

Produces an iterator only yielding every nth item from input.

import { range, takeNth } from "@thi.ng/iterators";

[...takeNth(3, range(10))]
// [ 0, 3, 6, 9 ]

takeWhile

Signature: takeWhile<T>(pred: (x: T) => boolean, input: Iterable<T>) => IterableIterator<T>

Produces iterator which calls pred for each item of input and terminates as soon as pred returns false.

Important: Due to lack of look-ahead in the ES6 iterator API, the value failing the given pred will be lost when working with the original iterator after takeWhile.

import { range, takeWhile } from "@thi.ng/iterators";

var input = range(10);
[...takeWhile((x)=> x < 5, input)]
// [ 0, 1, 2, 3, 4 ]

[...input]
// note: `5` is missing (the value which failed takeWhile)
// [ 6, 7, 8, 9 ]

takeLast

Signature: takeLast<T>(n: number, input: Iterable<T>) => IterableIterator<T>

Consumes input and produces iterator of the last n values of input (or less than n, if input is too short...)

Important: Never attempt to use with infinite inputs!

import { range, takeLast } from "@thi.ng/iterators";

[...takeLast(5, range(1000))]
// [ 995, 996, 997, 998, 999 ]

[...takeLast(5, range(3))]
// [ 0, 1, 2 ]

walk

Signature: walk(fn: (x: any) => void, input: Iterable<any>, postOrder = false) => void
Signature: walk(fn: (x: any) => void, children: (x: any) => any, input: Iterable<any>, postOrder = false) => void

Recursively walks input and applies fn to each element (for in-place editing or side effects). Only iterable values and objects (but not strings) are traversed further. Traversal is pre-order by default, but can be changed to post-order via last arg.

Note: Object traversal is done via objectIterator() and so will include pairs of [k, v] values.

The optional children fn can be used to select child values of the currently visited value. If this function is given then only its return values will be traversed further (with the same constraint as mentioned above). If the fn returns null or undefined, no children will be visited.

import { walk } from "@thi.ng/iterators";

// dummy SVG document
let doc = {
    tag: "svg",
    content: [
        {
            tag: "g",
            content: [
                { tag: "rect" },
                { tag: "circle" }
            ]
        },
        { tag: "circle", attr: { fill: "red" } }
    ]
};

// transformer fn for circle tags
// injects/overrides certain attribs
let circleTX = x => {
    if (x.tag === "circle") {
        x.attr = x.attr || {};
        x.attr.x = Math.random()*100;
        x.attr.y = Math.random()*100;
        x.attr.r = 5;
    }
};

// transform doc
walk(circleTX, (x) => x.content, doc);

doc.content[0].content[1]
// { tag: "circle", attr: { x: 83.9269, y: 31.129, r: 5 } }
doc.content[1]
// { tag: "circle", attr: { fill: "red", x: 37.963, y: 87.521, r: 5 } }

walkIterator

Signature: walkIterator(input: Iterable<any>, postOrder = false) => IterableIterator<any>
Signature: walkIterator(input: Iterable<any>, children: (x: any) => any, postOrder = false) => IterableIterator<any>

Yields an iterator performing either pre-order (default) or post-order traversal of input. Only iterable values and objects (but not strings) are traversed further.

Note: Object traversal is done via objectIterator() and so will include pairs of [k, v] values.

import { map, walkIterator } from "@thi.ng/iterators";

// pre-order traversal
[...map(JSON.stringify, walkIterator([[[1, [2]], [3, [4]]], [5]], false))]
// [ "[[[1,[2]],[3,[4]]],[5]]",
//   "[[1,[2]],[3,[4]]]",
//   "[1,[2]]",
//   "1",
//   "[2]",
//   "2",
//   "[3,[4]]",
//   "3",
//   "[4]",
//   "4",
//   "[5]",
//   "5" ]

// post-order traversal
[...map(JSON.stringify, walkIterator([[[1, [2]], [3, [4]]], [5]], true))]
// [ "1",
//   "2",
//   "[2]",
//   "[1,[2]]",
//   "3",
//   "4",
//   "[4]",
//   "[3,[4]]",
//   "[[1,[2]],[3,[4]]]",
//   "5",
//   "[5]",
//   "[[[1,[2]],[3,[4]]],[5]]" ]

zip

Signature: zip(keys: Iterable<any>, vals: Iterable<any>, target?: any) => any

Takes an iterator of keys and iterator of values, pairwise combines items from each input and associates them as key-value mappings in a given target object (if target is missing, returns new object). Stops as soon as either input is exhausted.

import { map, range, zip } from "@thi.ng/iterators";

zip("abcdef", range())
// { a: 0, b: 1, c: 2, d: 3, e: 4, f: 5 }

zip(range(5,10), range(100,200), new Uint8Array(16))
// [ 0, 0, 0, 0, 0, 100, 101, 102, 103, 104, 0, 0, 0, 0, 0, 0 ]

var langs=[
    {id: "js", name: "JavaScript"},
    {id: "clj", name: "Clojure"},
    {id: "ts", name: "TypeScript"}
];

zip(map((x)=> x.id, langs), langs)
// { js: { id: "js", name: "JavaScript" },
//   clj: { id: "clj", name: "Clojure" },
//   ts: { id: "ts", name: "TypeScript" } }

Authors

Karsten Schmidt

If this project contributes to an academic publication, please cite it as:

@misc{thing-iterators,
  title = "@thi.ng/iterators",
  author = "Karsten Schmidt",
  note = "https://thi.ng/iterators",
  year = 2017
}

@thi.ng/iterators

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