Merge pull request JuliaArrays#43 from piever/pv/docs

Fix JuliaArrays#39 : document undef initializer

Author: Pietro Vertechi

README.md

@@ -7,7 +7,7 @@ This package introduces the type `StructArray` which is an `AbstractArray` whose
 
 `Base.getproperty` or the dot syntax can be used to access columns, whereas rows can be accessed with `getindex`.
 
-The package is largely inspired from the `Columns` type in [IndexedTables](https://github.com/JuliaComputing/IndexedTables.jl)
+The package was largely inspired by the `Columns` type in [IndexedTables](https://github.com/JuliaComputing/IndexedTables.jl) which it now replaces.
 
 ## Example usage to store complex numbers
 
@@ -39,6 +39,39 @@ julia> StructArray([1+im, 3-2im])
  3 - 2im
 ```
 
+### Collection and initialization
+
+One can also create a `StructArrray` from an iterable of structs without creating an intermediate `Array`:
+
+```julia
+julia> StructArray(log(j+2.0*im) for j in 1:10)
+10-element StructArray{Complex{Float64},1,NamedTuple{(:re, :im),Tuple{Array{Float64,1},Array{Float64,1}}}}:
+ 0.8047189562170501 + 1.1071487177940904im
+ 1.0397207708399179 + 0.7853981633974483im
+ 1.2824746787307684 + 0.5880026035475675im
+ 1.4978661367769954 + 0.4636476090008061im
+  1.683647914993237 + 0.3805063771123649im
+ 1.8444397270569681 + 0.3217505543966422im
+  1.985145956776061 + 0.27829965900511133im
+ 2.1097538525880535 + 0.24497866312686414im
+ 2.2213256282451583 + 0.21866894587394195im
+ 2.3221954495706862 + 0.19739555984988078im
+```
+
+Another option is to create an uninitialized `StructArray` and then fill it with data. Just like in normal arrays, this is done with the `undef` syntax:
+
+```julia
+julia> s = StructArray{ComplexF64}(undef, 2, 2)
+2×2 StructArray{Complex{Float64},2,NamedTuple{(:re, :im),Tuple{Array{Float64,2},Array{Float64,2}}}}:
+ 6.91646e-310+6.91646e-310im  6.91646e-310+6.91646e-310im
+ 6.91646e-310+6.91646e-310im  6.91646e-310+6.91646e-310im
+
+julia> rand!(s)
+2×2 StructArray{Complex{Float64},2,NamedTuple{(:re, :im),Tuple{Array{Float64,2},Array{Float64,2}}}}:
+  0.446415+0.671453im  0.0797964+0.675723im
+ 0.0340059+0.420472im   0.907252+0.808263im
+```
+
 ## Example usage to store a data table
 
 ```julia