Added takewhile

docs/src/index.md

@@ -117,6 +117,14 @@ Equivalent to `take`, but will throw an exception if fewer than `n` items are en
 takestrict
 ```
 
+## takewhile(cond, xs)
+
+Iterates through values from the iterable `xs` as long as a given predicate `cond` is true.
+
+```@docs
+takewhile
+```
+
 ## flagfirst(xs)
 
 Provide a flag to check if this is the first element.

src/IterTools.jl

@@ -25,7 +25,8 @@ export
     peek,
     ncycle,
     ivec,
-    flagfirst
+    flagfirst,
+    takewhile
 
 function has_length(it)
     it_size = IteratorSize(it)
@@ -873,4 +874,36 @@ function iterate(ff::FlagFirst, state = (true, ))
     (isfirst, elt), (isfirst & false, nextstate)
 end
 
+# TakeWhile iterates through values from an iterable as long as a given predicate is true.
+
+struct TakeWhile{I}
+    cond::Function
+    xs::I
+end
+
+"""
+    takewhile(cond, xs)
+
+An iterator that yields values from the iterator `xs` as long as the
+predicate `cond` is true.
+
+```jldoctest
+julia> collect(takewhile(x-> x^2 < 10, 1:100)
+3-element Array{Any,1}:
+ 1
+ 2
+ 3
+"""
+takewhile(cond, xs) = TakeWhile(cond, xs)
+
+function Base.iterate(it::TakeWhile, state=nothing)
+    (val, state) = @ifsomething (state === nothing ? iterate(it.xs) : iterate(it.xs, state))
+    it.cond(val) || return nothing
+    val, state
+end
+
+Base.IteratorSize(it::TakeWhile) = Base.SizeUnknown()
+eltype(::Type{TakeWhile{I}}) where {I} = eltype(I)
+IteratorEltype(::Type{TakeWhile{I}}) where {I} = IteratorEltype(I)
+
 end # module IterTools

test/runtests.jl

@@ -407,5 +407,12 @@ include("testing_macros.jl")
 
         @test collect(flagfirst(Int[])) == Tuple{Bool,Int}[]
     end
+
+    @testset "takewhile" begin
+        @test collect(takewhile(x -> x^2 < 10, 1:10)) == Any[1, 2, 3]
+        @test collect(takewhile(x -> x^2 < 10, Iterators.countfrom(1))) == Any[1, 2, 3]
+        @test collect(takewhile(x -> x^2 < 10, 5:10)) == Any[]
+        @test collect(takewhile(x -> true, 5:10)) == collect(5:10)
+    end
 end
 end