TypeSortedCollections provides the `TypeSortedCollection`

type, which can be used to store type-heterogeneous data in a way that allows operations on the data to be performed in a type-stable manner. It does so by sorting a type-heterogeneous input collection by type upon construction, and storing these elements in a `Tuple`

of concretely typed `Vector`

s, one for each type. TypeSortedCollections provides type stable methods for `map!`

, `foreach`

, `broadcast!`

, and `mapreduce`

that take at least one `TypeSortedCollection`

.

An example:

```
julia> using TypeSortedCollections
julia> f(x::Int64, y::Float64) = x * y
f (generic function with 2 methods)
julia> f(x::Float64, y::Float64) = round(Int64, -x * y)
f (generic function with 2 methods)
julia> xs = Number[1.; 2; 3];
julia> sortedxs = TypeSortedCollection(xs);
julia> ys = [1.; 2.; 3.];
julia> results = Vector{Int64}(length(xs));
julia> map!(f, results, sortedxs, ys)
3-element Array{Int64,1}:
-1
4
9
julia> @allocated map!(f, results, sortedxs, ys)
0
```

`TypeSortedCollection`

s are appropriate when the number of different types in a heterogeneous collection is (much) smaller than the number of elements of the collection. If the number of types is approximately the same as the number of elements, a plain `Tuple`

may be a better choice.

Note that construction of a `TypeSortedCollection`

is of course not type stable, so the intended usage is not to construct `TypeSortedCollection`

s in tight loops.

See also FunctionWrappers.jl for a solution to the related problem of storing and calling multiple callables in a type-stable manner, and Unrolled.jl for a macro-based solution.

By default, `TypeSortedCollection`

s do not preserve iteration order, in the sense that the order in which elements are processed in `map!`

, `foreach`

, `broadcast!`

, and `mapreduce`

will not be the same as if these functions were called on the original type-heterogeneous vector:

```
julia> xs = Number[1.; 2; 3.];
julia> sortedxs = TypeSortedCollection(xs);
julia> foreach(println, sortedxs)
1.0
3.0
2
```

If this is not desired, a `TypeSortedCollection`

that *does* preserve iteration order can be constructed by passing in an additional constructor argument:

```
julia> xs = Number[1.; 2; 3.];
julia> sortedxs = TypeSortedCollection(xs, true);
julia> foreach(println, sortedxs)
1.0
2
3.0
```

The cost of preserving iteration order is that the number of `Vector`

s stored in the `TypeSortedCollection`

becomes equal to the number of contiguous subsequences of the input collection that have the same type, as opposed to simply the number of different types in the input collection. Note that calls to `map!`

and `foreach`

with both `TypeSortedCollection`

and `AbstractVector`

arguments are correctly indexed, regardless of whether iteration order is preserved:

```
julia> xs = Number[1.; 2; 3.];
julia> sortedxs = TypeSortedCollection(xs); # doesn't preserve iteration order
julia> results = similar(xs);
julia> map!(identity, results, sortedxs) # results of applying `identity` end up in the right location
3-element Array{Number,1}:
1.0
2
3.0
```

`TypeSortedCollections`

Consider the following example:

```
julia> xs = Number[Float32(1); 2; 3.; 4.];
julia> ys = Number[1.; 2.; 3; 4];
julia> results = Vector{Float64}(length(xs));
julia> sortedxs = TypeSortedCollection(xs);
julia> sortedys = TypeSortedCollection(ys);
julia> map!(*, results, sortedxs, sortedys) # Error!
```

The error happens because `xs`

and `ys`

don't have the same number of different element types. This problem can be solved by aligning the indices of `sortedys`

with those of `sortedxs`

:

```
julia> sortedys = TypeSortedCollection(ys, TypeSortedCollections.indices(sortedxs));
julia> map!(*, results, sortedxs, sortedys)
4-element Array{Float64,1}:
1.0
4.0
9.0
16.0
```

Broadcasting (in place) is implemented for `AbstractVector`

return types:

```
julia> x = 4;
julia> ys = Number[1.; 2; 3];
julia> sortedys = TypeSortedCollection(ys);
julia> results = similar(ys, Float64);
julia> results .= x .* sortedys
3-element Array{Float64,1}:
4.0
8.0
12.0
julia> @allocated results .= x .* sortedys
0
```

08/10/2017

1 day ago

106 commits