Mutating functions and multiple dispatch

Introduction to Julia

James Fulton

Climate informatics researcher

Mutating functions

Some functions modify inputs

Starting with the array:

x = [1,2,3]

push!(x, 4)
println(x)

[1, 2, 3, 4]

append!(x, [4,5,6])
println(x)

[1, 2, 3, 4, 5, 6]

pop!(x)
println(x)

[1, 2]
Introduction to Julia

Non-mutating functions

Starting with the array:

x = [3,1,2]

l = length(x)
println(x)

[3, 1, 2]

x_sorted = sort(x)
println(x)

[3, 1, 2]

x_type = typeof(x)
println(x)

[3, 1, 2]
Introduction to Julia

Mutating and non-mutating functions

Mutating functions change their inputs

  • pop!()
  • push!()
  • append!()
  • ...

Non-mutating functions do not change their inputs

  • sort()
  • println()
  • typeof()
  • string()
  • length()
  • ...
Introduction to Julia

Writing a mutating function

function modify_array!(x)
    x[1] = 0
end


# Try to mutate y
y = [1,2,3,4,5]
modify_array!(y)


# y has changed
println(y)

[0, 2, 3, 4, 5]

function modify_array!(x)
    x = [0,2,3,4,5]
end


# Try to mutate y
y = [1,2,3,4,5]
modify_array!(y)


# y has changed
println(y)

[1, 2, 3, 4, 5]
Introduction to Julia

Writing a mutating function

function modify_array!(x)
    x[1] = 0
end


# Try to mutate y
y = [1,2,3,4,5]
modify_array!(y)


println(x)

ERROR: UndefVarError: x not defined

function modify_array!(x)
    x = [0,2,3,4,5]
end


# Try to mutate y
y = [1,2,3,4,5]
modify_array!(y)


println(x)

ERROR: UndefVarError: x not defined
Introduction to Julia

Writing a mutating function

function setarray2zero!(x)
    x .= 0
end


y = [1,2,3,4,5]
setarray2zero!(y)


println(y)

[0, 0, 0, 0, 0]
Introduction to Julia

Writing a mutating function

function modify_array!(x)
    x .= x .- 1
end

y = [1,2,3,4,5]

modify_array!(y)

println(y)

[0, 1, 2, 3, 4]
Introduction to Julia

Multiple dispatch

function double(x)
    return x*2
end









println(double(2)) # Works on integers

4

println(double(10.0)) # Works on floats

20.0

println(double("yo")) # Not on strings

ERROR: MethodError: ...
Introduction to Julia

Multiple dispatch

function double(x)
    return x*2
end


function double(x::String)
    return x*x
end





println(double(2)) # Works on integers

4

println(double(10.0)) # Works on floats

20.0

println(double("yo")) # Works on strings

yoyo
Introduction to Julia

Multiple dispatch

function double(x)
    return x*2
end

function double(x::String)
    return x*x
end


function double(x::Bool)
    return x
end

println(double(2)) # Works on integers

4

println(double(10.0)) # Works on floats

20.0

println(double("yo")) # Works on strings

yoyo
Introduction to Julia

Multiple dispatch





function double(x::String)
    return x*x
end

function double(x::Bool)
    return x
end

println(double(2)) # Not on integers

ERROR: MethodError: ...

println(double(10.0)) # Not on floats

ERROR: MethodError: ...

println(double("yo")) # Works on strings

yoyo
Introduction to Julia

Multiple dispatch

function double(x)
    return x*2
end
Introduction to Julia

Let's practice!

Introduction to Julia

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