Julia 1.0 Programming Complete Reference Guide

Comprehensions

Array comprehensions provide a very powerful way to construct arrays. It is similar to the previously discussed array literal notation, but instead of passing in the actual values, we use a computation over an iterable object.

An example will make it clear:

julia> [x += 1 for x = 1:5] 
10-element Array{Int64,1}: 
  2 
  3 
  4 
  5 
  6

This can be read as—for each element x within the range 1 to 5, compute x+1 and put the resultant value in the array.

Just like with the plain array literals, we can constrain the type:

julia> Float64[x+=1 for x = 1:5] 
5-element Array{Float64,1}: 
 2.0 
 3.0 
 4.0 
 5.0 
 6.0 
 

Similarly, we can create multi-dimensional arrays:

julia> [x += y for x = 1:5, y = 11:15] 5×5 Array{Int64,2}: 12 13 14 15 16 13 14 15 16 17 14 ...