Change find() to return the same index type as pairs() (#24774)

This does not change anything for AbstractVector, Tuple and general iterables,
which continue to use linear indices. For other AbstractArrays, return
CartesianIndexes (rather than linear indices). For AbstractDict, AbstractString
and NamedTuple, return keys (previously not supported at all).

Relying on collect() to choose the return element type allows supporting
any definition of pairs(), including that for Dict, which creates a standard
Generator for which eltype() returns Any.

NEWS.md

@@ -362,13 +362,19 @@ This section lists changes that do not have deprecation warnings.
     trait; see its documentation for details. Types which support subtraction (operator
     `-`) must now implement `widen` for hashing to work inside heterogeneous arrays.
 
-  * `AbstractSet` objects are now considered equal by `==` and `isequal` if all of their
+  * `findn(x::AbstractVector)` now returns a 1-tuple with the vector of indices, to be
+    consistent with higher order arrays ([#25365]).
+
+  * `find` now returns the same type of indices as `keys`/`pairs` for `AbstractArray`,
+    `AbstractDict`, `AbstractString`, `Tuple` and `NamedTuple` objects ([#24774]).
+    In particular, this means that it returns `CartesianIndex` objects for matrices
+    and higher-dimensional arrays instead of linear indices as was previously the case.
+    Use `Int[LinearIndices(size(a))[i] for i in find(f, a)]` to compute linear indices.
+
+ * `AbstractSet` objects are now considered equal by `==` and `isequal` if all of their
     elements are equal ([#25368]). This has required changing the hashing algorithm
     for `BitSet`.
 
-  * `findn(x::AbstractVector)` now return a 1-tuple with the vector of indices, to be
-    consistent with higher order arrays ([#25365]).
-
   * the default behavior of `titlecase` is changed in two ways ([#23393]):
     + characters not starting a word are converted to lowercase;
       a new keyword argument `strict` is added which
@@ -377,7 +383,6 @@ This section lists changes that do not have deprecation warnings.
       to get the old behavior (only "space" characters are considered as
       word separators), use the keyword `wordsep=isspace`.
 
-
 Library improvements
 --------------------
 
@@ -1155,6 +1160,7 @@ Command-line option changes
 [#24713]: https://github.com/JuliaLang/julia/issues/24713
 [#24714]: https://github.com/JuliaLang/julia/issues/24714
 [#24715]: https://github.com/JuliaLang/julia/issues/24715
+[#24774]: https://github.com/JuliaLang/julia/issues/24774
 [#24781]: https://github.com/JuliaLang/julia/issues/24781
 [#24785]: https://github.com/JuliaLang/julia/issues/24785
 [#24786]: https://github.com/JuliaLang/julia/issues/24786

base/array.jl

@@ -1720,48 +1720,60 @@ findlast(testf::Function, A) = findprev(testf, A, endof(A))
 """
     find(f::Function, A)
 
-Return a vector `I` of the linear indices of `A` where `f(A[I])` returns `true`.
+Return a vector `I` of the indices or keys of `A` where `f(A[I])` returns `true`.
 If there are no such elements of `A`, return an empty array.
 
+Indices or keys are of the same type as those returned by [`keys(A)`](@ref)
+and [`pairs(A)`](@ref) for `AbstractArray`, `AbstractDict`, `AbstractString`
+`Tuple` and `NamedTuple` objects, and are linear indices starting at `1`
+for other iterables.
+
 # Examples
 ```jldoctest
+julia> x = [1, 3, 4]
+3-element Array{Int64,1}:
+ 1
+ 3
+ 4
+
+julia> find(isodd, x)
+2-element Array{Int64,1}:
+ 1
+ 2
+
 julia> A = [1 2 0; 3 4 0]
 2×3 Array{Int64,2}:
  1  2  0
  3  4  0
-
 julia> find(isodd, A)
-2-element Array{Int64,1}:
- 1
- 2
+2-element Array{CartesianIndex{2},1}:
+ CartesianIndex(1, 1)
+ CartesianIndex(2, 1)
 
 julia> find(!iszero, A)
-4-element Array{Int64,1}:
- 1
- 2
- 3
- 4
+4-element Array{CartesianIndex{2},1}:
+ CartesianIndex(1, 1)
+ CartesianIndex(2, 1)
+ CartesianIndex(1, 2)
+ CartesianIndex(2, 2)
+
+julia> d = Dict(:A => 10, :B => -1, :C => 0)
+Dict{Symbol,Int64} with 3 entries:
+  :A => 10
+  :B => -1
+  :C => 0
+
+julia> find(x -> x >= 0, d)
+2-element Array{Symbol,1}:
+ :A
+ :C
 
-julia> find(isodd, [2, 4])
-0-element Array{Int64,1}
 ```
 """
-function find(testf::Function, A)
-    # use a dynamic-length array to store the indices, then copy to a non-padded
-    # array for the return
-    tmpI = Vector{Int}()
-    inds = _index_remapper(A)
-    for (i,a) = enumerate(A)
-        if testf(a)
-            push!(tmpI, inds[i])
-        end
-    end
-    I = Vector{Int}(uninitialized, length(tmpI))
-    copyto!(I, tmpI)
-    return I
-end
-_index_remapper(A::AbstractArray) = linearindices(A)
-_index_remapper(iter) = OneTo(typemax(Int))  # safe for objects that don't implement length
+find(testf::Function, A) = collect(first(p) for p in _pairs(A) if testf(last(p)))
+
+_pairs(A::Union{AbstractArray, AbstractDict, AbstractString, Tuple, NamedTuple}) = pairs(A)
+_pairs(iter) = zip(OneTo(typemax(Int)), iter)  # safe for objects that don't implement length
 
 """
     find(A)
@@ -1786,22 +1798,10 @@ julia> find(falses(3))
 ```
 """
 function find(A)
-    nnzA = count(t -> t != 0, A)
-    I = Vector{Int}(uninitialized, nnzA)
-    cnt = 1
-    inds = _index_remapper(A)
-    warned = false
-    for (i,a) in enumerate(A)
-        if !warned && !(a isa Bool)
-            depwarn("In the future `find(A)` will only work on boolean collections. Use `find(x->x!=0, A)` instead.", :find)
-            warned = true
-        end
-        if a != 0
-            I[cnt] = inds[i]
-            cnt += 1
-        end
+    if !(eltype(A) === Bool) && !all(x -> x isa Bool, A)
+        depwarn("In the future `find(A)` will only work on boolean collections. Use `find(x->x!=0, A)` instead.", :find)
     end
-    return I
+    collect(first(p) for p in _pairs(A) if last(p) != 0)
 end
 
 find(x::Bool) = x ? [1] : Vector{Int}()

base/sparse/sparsematrix.jl

@@ -1276,7 +1276,7 @@ function find(p::Function, S::SparseMatrixCSC)
     end
     sz = size(S)
     I, J = _findn(p, S)
-    return Base._sub2ind(sz, I, J)
+    return CartesianIndex.(I, J)
 end
 find(p::Base.OccursIn, x::SparseMatrixCSC) =
     invoke(find, Tuple{Base.OccursIn, AbstractArray}, p, x)

test/arrayops.jl

@@ -457,6 +457,12 @@ end
     @test findnext(equalto(0x00), [0x00, 0x01, 0x00], 2) == 3
     @test findprev(equalto(0x00), [0x00, 0x01, 0x00], 2) == 1
 end
+@testset "find with Matrix" begin
+    A = [1 2 0; 3 4 0]
+    @test find(isodd, A) == [CartesianIndex(1, 1), CartesianIndex(2, 1)]
+    @test find(!iszero, A) == [CartesianIndex(1, 1), CartesianIndex(2, 1),
+                               CartesianIndex(1, 2), CartesianIndex(2, 2)]
+end
 @testset "find with general iterables" begin
     s = "julia"
     @test find(c -> c == 'l', s) == [3]

test/dict.jl

@@ -757,3 +757,10 @@ end
     end
     @test map(string, keys(d)) == Set(["1","3"])
 end
+
+@testset "find" begin
+    @test @inferred find(equalto(1), Dict(:a=>1, :b=>2)) == [:a]
+    @test @inferred sort(find(equalto(1), Dict(:a=>1, :b=>1))) == [:a, :b]
+    @test @inferred isempty(find(equalto(1), Dict()))
+    @test @inferred isempty(find(equalto(1), Dict(:a=>2, :b=>3)))
+end

test/namedtuple.jl

@@ -208,3 +208,8 @@ abstr_nt_22194_3()
 @test Base.structdiff((a=1, b=2, z=20), NamedTuple{(:b,)}) == (a=1, z=20)
 @test typeof(Base.structdiff(NamedTuple{(:a, :b), Tuple{Int32, Union{Int32, Nothing}}}((1, Int32(2))),
                              (a=0,))) === NamedTuple{(:b,), Tuple{Union{Int32, Nothing}}}
+
+@test @inferred find(equalto(1), (a=1, b=2)) == [:a]
+@test @inferred find(equalto(1), (a=1, b=1)) == [:a, :b]
+@test @inferred isempty(find(equalto(1), NamedTuple()))
+@test @inferred isempty(find(equalto(1), (a=2, b=3)))

test/strings/search.jl

@@ -324,3 +324,7 @@ end
 @test findnext(equalto('('), "(⨳(", 2) == 5
 @test findlast(equalto('('), "(⨳(") == 5
 @test findprev(equalto('('), "(⨳(", 2) == 1
+
+@test @inferred find(equalto('a'), "éa") == [3]
+@test @inferred find(equalto('€'), "€€") == [1, 4]
+@test @inferred isempty(find(equalto('é'), ""))

test/tuple.jl

@@ -364,3 +364,10 @@ end
 @testset "issue 24707" begin
     @test eltype(Tuple{Vararg{T}} where T<:Integer) >: Integer
 end
+
+@testset "find" begin
+    @test @inferred find(equalto(1), (1, 2)) == [1]
+    @test @inferred find(equalto(1), (1, 1)) == [1, 2]
+    @test @inferred isempty(find(equalto(1), ()))
+    @test @inferred isempty(find(equalto(1), (2, 3)))
+end