Add eachindex() fallback for arbitrary iterables

Especially useful for strings, but also for any iterable
which would use unevenly spaced or non-integer indices.
Fixes #11649.

base/iterator.jl

@@ -2,6 +2,18 @@
 
 isempty(itr) = done(itr, start(itr))
 
+# eachindex
+
+immutable EachIndex{I}
+    itr::I
+end
+eachindex(itr) = EachIndex(itr)
+
+length(e::EachIndex) = length(e.itr)
+start(e::EachIndex) = start(e.itr)
+next(e::EachIndex, state) = (state, next(e.itr, state)[2])
+done(e::EachIndex, state) = done(e.itr, state)
+
 # enumerate
 
 immutable Enumerate{I}

doc/stdlib/arrays.rst

@@ -33,7 +33,9 @@ Basic functions
 
 .. function:: eachindex(A...)
 
-   Creates an iterable object for visiting each index of an AbstractArray ``A`` in an efficient manner. For array types that have opted into fast linear indexing (like ``Array``), this is simply the range ``1:length(A)``. For other array types, this returns a specialized Cartesian range to efficiently index into the array with indices specified for every dimension. Example for a sparse 2-d array::
+   Creates an iterable object for visiting each index of an AbstractArray ``A`` in an efficient manner. For array types that have opted into fast linear indexing (like ``Array``), this is simply the range ``1:length(A)``. For other array types, this returns a specialized Cartesian range to efficiently index into the array with indices specified for every dimension. For other iterables, including strings, this returns an iterator object supporting arbitrary index types (e.g. unevenly spaced or non-integer indices).
+
+   Example for a sparse 2-d array::
 
     julia> A = sprand(2, 3, 0.5)
     2x3 sparse matrix with 4 Float64 entries:

test/arrayops.jl

@@ -812,6 +812,7 @@ let a36 = boo32_64()
 end
 @test isequal([1,2,3], [a for (a,b) in enumerate(2:4)])
 @test isequal([2,3,4], [b for (a,b) in enumerate(2:4)])
+@test isequal([1,2,3], [i for i in eachindex(2:4)])
 
 @test_throws DomainError (10.^[-1])[1] == 0.1
 @test (10.^[-1.])[1] == 0.1

test/strings.jl

@@ -1732,3 +1732,7 @@ d = UTF32String(c)
 c[1] = 'A'
 @test d=="A"
 
+# iteration
+@test [c for c in "ḟøøƀäṙ"] == ['ḟ', 'ø', 'ø', 'ƀ', 'ä', 'ṙ']
+@test [i for i in eachindex("ḟøøƀäṙ")] == [1, 4, 6, 8, 10, 12]
+@test [x for x in enumerate("ḟøøƀäṙ")] == [(1, 'ḟ'), (2, 'ø'), (3, 'ø'), (4, 'ƀ'), (5, 'ä'), (6, 'ṙ')]