# IterTools

## Installation

Install this package with `Pkg.add("IterTools")`

# Usage

## distinct(xs)

Iterate through values skipping over those already encountered.

`IterTools.distinct`

— Function.`distinct(xs)`

Iterate through values skipping over those already encountered.

```
julia> for i in distinct([1,1,2,1,2,4,1,2,3,4])
@show i
end
i = 1
i = 2
i = 4
i = 3
```

## groupby(f, xs)

Group consecutive values that share the same result of applying `f`

.

`IterTools.groupby`

— Function.`groupby(f, xs)`

Group consecutive values that share the same result of applying `f`

.

```
julia> for i in groupby(x -> x[1], ["face", "foo", "bar", "book", "baz", "zzz"])
@show i
end
i = ["face", "foo"]
i = ["bar", "book", "baz"]
i = ["zzz"]
```

## imap(f, xs1, [xs2, ...])

Iterate over values of a function applied to successive values from one or more iterators.

`IterTools.imap`

— Function.`imap(f, xs1, [xs2, ...])`

Iterate over values of a function applied to successive values from one or more iterators. Like `Iterators.zip`

, the iterator is done when any of the input iterators have been exhausted.

```
julia> for i in imap(+, [1,2,3], [4,5,6])
@show i
end
i = 5
i = 7
i = 9
```

## iterated(f, x)

Iterate over successive applications of `f`

, as in `x, f(x), f(f(x)), f(f(f(x))), ...`

.

`IterTools.iterated`

— Function.`iterated(f, x)`

Iterate over successive applications of `f`

, as in `x`

, `f(x)`

, `f(f(x))`

, `f(f(f(x)))`

, ...

Use `Base.Iterators.take()`

to obtain the required number of elements.

```
julia> for i in Iterators.take(iterated(x -> 2x, 1), 5)
@show i
end
i = 1
i = 2
i = 4
i = 8
i = 16
julia> for i in Iterators.take(iterated(sqrt, 100), 6)
@show i
end
i = 100
i = 10.0
i = 3.1622776601683795
i = 1.7782794100389228
i = 1.333521432163324
i = 1.1547819846894583
```

## ncycle(xs, n)

Cycles through an iterator `n`

times.

`IterTools.ncycle`

— Function.`ncycle(iter, n)`

Cycle through `iter`

`n`

times.

```
julia> for i in ncycle(1:3, 2)
@show i
end
i = 1
i = 2
i = 3
i = 1
i = 2
i = 3
```

## nth(xs, n)

Return the `n`

th element of `xs`

.

`IterTools.nth`

— Function.`nth(xs, n)`

Return the `n`

th element of `xs`

. This is mostly useful for non-indexable collections.

```
julia> mersenne = Set([3, 7, 31, 127])
Set([7, 31, 3, 127])
julia> nth(mersenne, 3)
3
```

## partition(xs, n, [step])

Group values into `n`

-tuples.

`IterTools.partition`

— Function.`partition(xs, n, [step])`

Group values into `n`

-tuples.

```
julia> for i in partition(1:9, 3)
@show i
end
i = (1, 2, 3)
i = (4, 5, 6)
i = (7, 8, 9)
```

If the `step`

parameter is set, each tuple is separated by `step`

values.

```
julia> for i in partition(1:9, 3, 2)
@show i
end
i = (1, 2, 3)
i = (3, 4, 5)
i = (5, 6, 7)
i = (7, 8, 9)
julia> for i in partition(1:9, 3, 3)
@show i
end
i = (1, 2, 3)
i = (4, 5, 6)
i = (7, 8, 9)
julia> for i in partition(1:9, 2, 3)
@show i
end
i = (1, 2)
i = (4, 5)
i = (7, 8)
```

## ivec(xs)

Iterate over `xs`

but do not preserve shape information.

`IterTools.ivec`

— Function.`ivec(iter)`

Drops all shape from `iter`

while iterating. Like a non-materializing version of `vec`

.

```
julia> m = collect(reshape(1:6, 2, 3))
2×3 Array{Int64,2}:
1 3 5
2 4 6
julia> collect(ivec(m))
6-element Array{Int64,1}:
1
2
3
4
5
6
```

## peekiter(xs)

Peek at the head element of an iterator without updating the state.

`IterTools.peekiter`

— Function.`peekiter(xs)`

Lets you peek at the head element of an iterator without updating the state.

```
julia> it = peekiter(["face", "foo", "bar", "book", "baz", "zzz"])
IterTools.PeekIter{Array{String,1}}(["face", "foo", "bar", "book", "baz", "zzz"])
julia> @show peek(it);
peek(it) = Some("face")
julia> @show peek(it);
peek(it) = Some("face")
julia> x, s = iterate(it)
("face", ("foo", 3))
julia> @show x;
x = "face"
julia> @show peek(it, s);
peek(it, s) = Some("foo")
```

## repeatedly(f, [n])

Call a function `n`

times, or infinitely if `n`

is omitted.

`IterTools.repeatedly`

— Function.```
repeatedly(f)
repeatedly(f, n)
```

Call function `f`

`n`

times, or infinitely if `n`

is omitted.

```
julia> t() = (sleep(0.1); Dates.millisecond(now()))
t (generic function with 1 method)
julia> collect(repeatedly(t, 5))
5-element Array{Any,1}:
993
97
200
303
408
```

## takenth(xs, n)

Iterate through every n'th element of `xs`

`IterTools.takenth`

— Function.`takenth(xs, n)`

Iterate through every `n`

th element of `xs`

.

```
julia> collect(takenth(5:15,3))
3-element Array{Int64,1}:
7
10
13
```

## subsets(xs, [k])

Iterate over every subset of an indexable collection `xs`

, or iterate over every subset of size `k`

from an indexable collection `xs`

.

`IterTools.subsets`

— Function.```
subsets(xs)
subsets(xs, k)
subsets(xs, Val{k}())
```

Iterate over every subset of the indexable collection `xs`

. You can restrict the subsets to a specific size `k`

.

Giving the subset size in the form `Val{k}()`

allows the compiler to produce code optimized for the particular size requested. This leads to performance comparable to hand-written loops if `k`

is small and known at compile time, but may or may not improve performance otherwise.

```
julia> for i in subsets([1, 2, 3])
@show i
end
i = Int64[]
i = [1]
i = [2]
i = [1, 2]
i = [3]
i = [1, 3]
i = [2, 3]
i = [1, 2, 3]
julia> for i in subsets(1:4, 2)
@show i
end
i = [1, 2]
i = [1, 3]
i = [1, 4]
i = [2, 3]
i = [2, 4]
i = [3, 4]
julia> for i in subsets(1:4, Val{2}())
@show i
end
i = (1, 2)
i = (1, 3)
i = (1, 4)
i = (2, 3)
i = (2, 4)
i = (3, 4)
```

## takestrict(xs, n)

Equivalent to `take`

, but will throw an exception if fewer than `n`

items are encountered in `xs`

.

`IterTools.takestrict`

— Function.`takestrict(xs, n::Int)`

Like `take()`

, an iterator that generates at most the first `n`

elements of `xs`

, but throws an exception if fewer than `n`

items are encountered in `xs`

.

```
julia> a = :1:2:11
1:2:11
julia> collect(takestrict(a, 3))
3-element Array{Int64,1}:
1
3
5
```

## takewhile(cond, xs)

Iterates through values from the iterable `xs`

as long as a given predicate `cond`

is true.

`IterTools.takewhile`

— Function.`takewhile(cond, xs)`

An iterator that yields values from the iterator `xs`

as long as the predicate `cond`

is true.

```
julia> collect(takewhile(x-> x^2 < 10, 1:100))
3-element Array{Int64,1}:
1
2
3
```

## flagfirst(xs)

Provide a flag to check if this is the first element.

`IterTools.flagfirst`

— Function.`flagfirst(iter)`

An iterator that yields `(isfirst, x)`

where `isfirst::Bool`

is `true`

for the first element, and `false`

after that, while the `x`

s are elements from `iter`

.

```
julia> collect(flagfirst(1:3))
3-element Array{Tuple{Bool,Int64},1}:
(true, 1)
(false, 2)
(false, 3)
```

## IterTools.@ifsomething

Helper macro for returning from the enclosing block when there are no more elements.

`IterTools.@ifsomething`

— Macro.`IterTools.@ifsomething expr`

If `expr`

evaluates to `nothing`

, equivalent to `return nothing`

, otherwise the macro evaluates to the value of `expr`

. Not exported, useful for implementing iterators.

```
julia> IterTools.@ifsomething iterate(1:2)
(1, 1)
julia> let elt, state = IterTools.@ifsomething iterate(1:2, 2); println("not reached"); end
```