Basic Functions

Introduction to Julia

James Fulton

Climate informatics researcher

What are functions?

Functions you have used:

println()
typeof()
string()
push!()
pop!()
append!()
length()
sort()

x = [2,1,3]


# Takes an array, returns integer
l = length(x)


# Takes an array, returns sorted array
x_sorted = sort(x)


# Takes a value, prints it to console
println(l)

Why use functions?

Allows us to focus on program structure
Can ignore irrelevant details of how a function works

Writing custom functions

# Declare function to convert temperatures
function fahrenheit2celsius(temp)

    # Function body
    return (temp - 32) * 5/9

end


# Use function
println(fahrenheit2celsius(212))

100.0

Writing custom functions

# Declare function to convert temperatures
function fahrenheit2celsius(temp)
    # Function body
    return (temp - 32) * 5/9
end

# Use function many times
println(fahrenheit2celsius(212))
println(fahrenheit2celsius(100))

100.0
37.77

Longer functions

# Declare function to convert temperatures
function fahrenheit2celsius(temp)
    # Function body
    temp_sub = temp - 32
    temp_c = temp_sub * 5/9
    return temp_c
end


t = fahrenheit2celsius(212)
println(t)

100.0

Longer functions

# Declare function to convert temperatures
function fahrenheit2celsius(temp)
    # Function body
    temp_sub = temp - 32       # variable inside function not available outside
    temp_c = temp_sub * 5/9
    return temp_c
end

t = fahrenheit2celsius(212)
println(temp_sub)

ERROR: UndefVarError: temp_sub not defined

Return keyword

function x_or_zero(x)

    if x>0
        return x
    else
        return 0
    end

end


println(x_or_zero(-3))
println(x_or_zero(3))

0
3

Return keyword

# Function with longer body
function check_if_raining(rain_amount)
    is_raining = rain_amount > 0
    if is_raining
        println("Better get your coat")
    else
        println("The sky looks clear")
    end
end


check_if_raining(0.2) # Function returns nothing - only prints

Better get your coat

Multiple arguments

function power(x, y)
    return x^y
end

# Use function to calculate 5*5
println(power(5, 2))

# Use function to calculate 2*2*2*2*2
println(power(2, 5))

Broadcasting functions

function fahrenheit2celsius(temp)
    return (temp - 32) * 5/9
end

temps_f = [212, 32, 100]


# Function not written to work with arrays
temps_c = fahrenheit2celsius(temps_f)

ERROR: MethodError: ...

Broadcasting functions

function fahrenheit2celsius(temp)
    return (temp - 32) * 5/9
end

temps_f = [212, 32, 100]

# Broadcast function with dot syntax
temps_c = fahrenheit2celsius.(temps_f)

println(temps_c)

[100.0, 0.0, 37.77]

Broadcasting functions

x = ["one", 2, 3.0]

# Broadcast using typeof function
println(typeof.(x))

[String, Int64, Float64]

Broadcasting multiple arguments

function power(x, y)
    return x^y
end

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


# Square each element of the array
println(power.(x_arr, 2))

[1, 4, 9, 16, 25]

Broadcasting multiple arguments

function power(x, y)
    return x^y
end

x_arr = [1,2,3,4,5]
y_arr = [1,2,3,4,5]

# Use function on x_arr and y_arr
println(power.(x_arr, y_arr))

[1, 4, 27, 256, 3125]

Let's practice!

Introduction to Julia