By: Steven Whitaker
Re-posted from: https://glcs.hashnode.dev/variables-and-functions
Variables and functionsare the building blocksof any programmer’s code.Variables allow computations to be reused,and functions help keep code organized.
In this post,we will cover some of the basicsof variables and functionsin Julia,a relatively new,free, and open-source programming language.In particular,we will discuss what a variable is,what sorts of datacan be assigned to a variable,andhow to define and use functions.
Variables
A variable is a labelused to refer to an object.
julia> a = 11In the above code snippet,we assigned a value of 1to a variable called a.Now we can use a in other expressions,and the value of a (1 in this case)will be used.
julia> a + 23We can also reassign variables.
julia> a = 44julia> a = "Hello""Hello"julia> a = [1, 2, 3]3-element Vector{Int64}: 1 2 3We can even use Unicode characters!We can write many math symbolsby typing the corresponding LaTeX commandand then pressing <tab>.Here,we assign (a Julia constant equal to \( \pi \)and typed with \pi<tab>)to (typed with \theta<tab>).
julia> = = 3.1415926535897...Variables Are Labels
One important thing to remember about variablesis that they are labels for data,not the data itself.Let’s illustrate what that means.At this point,a refers to a Vector.We will create another variable, b,and assign it the value of a.
julia> b = a3-element Vector{Int64}: 1 2 3Now let’s change one of the elements of b.
julia> b[1] = 100; b3-element Vector{Int64}: 100 2 3We didn’t change a,so it should be the same as before, right?Nope!
julia> a3-element Vector{Int64}: 100 2 3What happened?Remember, a is just a labelfor some data (a Vector).When we created b,we created a new labelfor the same data.Both a and b referto the same data,so modifying one modifies the other.
If you want bto have the same values as abut refer to different data,use copy.
julia> b = copy(a)3-element Vector{Int64}: 100 2 3julia> b[1] = 1; b3-element Vector{Int64}: 1 2 3julia> a3-element Vector{Int64}: 100 2 3
Now that we knowhow to create variables,let’s learn aboutsome basic types of datawe can assign to variables.
Basic Types
Julia has many basic data types.
integer = 9000floating_point = 3.14boolean = trueimaginary = 1 + 2imrational = 4//3char = 'x'str = "a string"array = [1.0, 2.0]Integers and floating-point numberscan be expressedwith different numbers of bits.
Int64, Int32, Int16 # and moreFloat64, Float32, Float16 # and moreBy default,integer numbers(technically, literals)are of type Int64 on 64-bit computersor of type Int32 on 32-bit computers.(Note that Int is shorthand for Int64 or Int32for 64-bit or 32-bit computers, respectively.Therefore, all integer literals are of type Int.)
On the other hand,floating-point numbers (literals)of the form 3.14 or 2.3e5are of type Float64 on all computers,while those of the form 2.3f5are of type Float32.
To use different numbers of bits,just use the appropriate constructor.
julia> Int16(20)20julia> Float16(1.2)Float16(1.2)Basic Operations
Now we will coversome basic operations.This is by no means an exhaustive list;check out the Julia documentationfor more details.
# Mathaddition = 1 + 2.0subtraction = 1 - 1multiplication = 3 * 4//3division = 6 / 4integer_division = 6 4 # Type \div<tab>power = 2^7# Booleannot = !falseand = true && notor = not || and# Comparisonequality = addition == 1greater = division > integer_divisionchained = addition < subtraction <= power# Stringsstring_concatenation = "hi " * "there"string_interpolation = "1 - 1 = $subtraction"string_indexing = string_interpolation[5]substring = string_concatenation[4:end]parsing = parse(Int, string_indexing)# Arraysa = [1, 2, 3]b = [4, 5, 6]concat_vert = [a; b]concat_horiz = [a b]vector_indexing = b[2]vector_range_indexing = b[1:2]matrix_indexing = concat_horiz[2:3,1]elementwise1 = a .+ 1elementwise2 = a .- b# Displayingprint(addition)println(string_concatenation)@show not@info "some variables" power aFunction Basics
Some of the basic operations we saw above,e.g., parse and print,were functions.As demonstrated above,functions are calledusing the following familiar syntax:
func() # For no inputsfunc(arg1) # For one inputfunc(arg1, arg2) # For two inputs# etc.Note that just writing the function name(i.e., without parentheses)is valid syntax, but it is not a function call.In this case,the function name is treated essentially like a variable,meaning, for example, it can be used as an inputto another function.
For example,one way to compute the sumof the absolute valueof an array of numbersis as follows:
julia> sum(abs, [-1, 0, 1])2Here,the function abs is not being called (by us)but is used as an input to the function sumas if it were a variable.
Function Vectorization
Often,we have a functionthat operates on a single inputthat we want to applyto every element of an array.Julia provides a convenient syntaxto do so:just add a dot (.).For example,the following takes the absolute valueof every array element:
julia> abs.([-1, 0, 1])3-element Vector{Int64} 1 0 1There is also a function, map,that does the same thingin this example:
julia> map(abs, [-1, 0, 1])3-element Vector{Int64} 1 0 1(Note, however,that map and the dot syntaxare not always interchangeable.)
Defining Functions
When writing Julia code,it is convenientto place code inside of functions.There are two main syntaxesfor creating a function.
- Using the
functionkeyword:function myfunc(x) return x + 1end - Using the assignment form:
myfunc2(x, y) = x + y
Optional Arguments
Sometimes we want a functionto have optional inputs.The syntax for specifying optional arguments is
function myfunc3(required, optional = "hello") println(required, optional)endHere,optional is optionaland has a default value of "hello"if not provided by the caller.
julia> myfunc3("say ")say hellojulia> myfunc3("see you ", "later")see you laterKeyword Arguments
Another way to specify optional argumentsis to use keyword arguments.The syntax is almost the sameas regular optional arguments,except we use a semicolon (;) instead of a comma (,).
function myfunc4(x; y = 3) return x * yendHere,y is optional,but to specify itwe need to use the keyword y.
julia> myfunc4(2)6julia> myfunc4(2; y = 10)20julia> myfunc4(2, 10)ERROR: MethodError: no method matching myfunc4(::Int64, ::Int64)When calling myfunc4we can also use a commawhen specifying y.
julia> myfunc4(2, y = 1)2Returning Multiple Values
Sometimes we need a functionto return multiple values.The way to do this in Juliais to return a Tuple.Here’s an example:
function plusminus1(x) return (x + 1, x - 1)endThen multiple variables can be assigned at once.
julia> (plus1, minus1) = plusminus1(1)(2, 0)julia> plus12julia> minus10Note that taking the outputof a function with multiple return valuesand assigning it to a single variablewill assign that variable the whole Tuple of outputs.The following code illustrates thisand shows how to return just one output:
julia> both = plusminus1(1);julia> both(2, 0)julia> (one,) = plusminus1(1);julia> one2(Note, however, that in this last casethe second output is still computed;it is just immediately discarded,so there are no savings in computation.)
Vectorizing a Function with Multiple Return Values
Vectorizing a function with multiple return valuesrequires a bit more work.For this example,we will use the sincos functionthat computes the sine and cosine simultaneously.We can still use the dot syntax,but we might be tempted to try the following:
julia> (s, c) = sincos.([0, /2, ]);julia> s(0.0, 1.0)julia> c(1.0, 6.123233995736766e-17)Here, s has the value of sincos(0),not the value of sin.([0, /2, ])like we might have expected.
Instead, we can do the following:
julia> sc = sincos.([0, /2, ])3-element Vector{Tuple{Float64, Float64}}: (0.0, 1.0) (1.0, 6.123233995736766e-17) (1.2246467991473532e-16, -1.0)julia> s = first.(sc)3-element Vector{Float64}: 0.0 1.0 1.2246467991473532e-16julia> c = last.(sc)3-element Vector{Float64}: 1.0 6.123233995736766e-17 -1.0(Note that instead of using first or last,we could write it this way:output_i = getindex.(sc, i).This way also works for functionsthat return more than two values.)
Summary
In this post,we learned about what a variable isand some basic data types.We also learned abouthow to define and use functions.
There is a lot more we could coverabout these topics,so if you want to learn more,check out the links below,or write a comment belowletting us know what additional concepts or topicsyou would like to see!
Understand variables and functions in Julia?Move on to thenext post to learn how to master the Julia REPL!Or,feel free to take a lookat our other Julia tutorial posts!
