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Learning Julia (5): array comprehension

jemaloqiu profile image jemaloQiu Updated on ・2 min read

We can use [expr(i) for i in somelist] to obtain a list in Python, which is so called List comprehension. In Julia, we can generate arrays in a similar way.

a = [i*i for i in 1:10]
println("a: ", a, ", typeof(a): ", typeof(a))

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Execution output:

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We see that we get an array using the same method as in Python.
Comprehensions can also be written without the enclosing square brackets, producing an object known as a generator (Base.Generator type).

g = (1/n for n=1:100)
println("g: ", g)
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Execution output:
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I tried some more complex array comprehensions and there are some slight differences from Python (ELSE-statement can be ignored but is highly recommended):

a1 = [i*i  for i in 1:10 if i%2==0]          # IF-condition filter at the end 
a2 = [if i%2==0 i*i else 0 end  for i in 1:10 ]   # IF-ELSE statements ahead of FOR-statement; "end" keyword is required, whichis main difference with Python
a3 = [if i%2==0 i*i  end  for i in 1:10 ]    # IF-statement without ELSE-condition definition, which yields 'nothing' in the result, "end" keyword is required
a4 = [i*j for i in 1:3 for j in 4:6]  
println("a1: ", a1) 
println("a2: ", a2) 
println("a3: ", a3) 
println("a4: ", a4) 
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Execution output:
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Same as in Python, map() is also implemented:


# map() with anonymous functions
a5 = map(x -> x*x, 1:5)          
a6 = map(x -> x%2==0 ? x*x : 0, 1:5)
g = (n+1 for n=1:10)
a7 = map(x -> x*x, g)   # takes a generator object
println("a5: ", a5) 
println("a6: ", a6) 
println("a7: ", a7) 
println(" ")

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Execution output:
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I tried map() and reduce() using given functions:

function func(x::Int)
    if x%2 == 0
        return x
    else 
        return 0
    end
end

a = map(func, 2:2:9)   #  supplies a one-argument function 

println("a: ", a)    # => [2, 4, 6, 8]

function foo(x::Int, y::Int)
    return x*10+y
end

b = reduce(foo, a)
println("b: ", b) 

c = mapreduce(func,foo, 2:2:9)
println("c: ", c) 
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Execution output:
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