Julia Fundamentals

Estimated time: 12 minutes

Overview

Questions

• What basic data types can I work with in Julia?
• How can I create a new variable in Julia?
• How do I use a function?
• Can I change the value associated with a variable after I create it?

Objectives

• Assign values to variables.

Variables

---

Any Julia REPL or script can be used as a calculator:

JULIA

3 + 5 * 4

OUTPUT

23

This is great, but not very interesting. To do anything useful with data, we need to assign its value to a variable. In Julia, we assign a value to a variable using the equals sign =. For example, we can track the weight of a patient who weighs 60 kilograms by assigning the value 60 to a variable weight_kg:

JULIA

weight_kg = 60

Now, whenever we use weight_kg, Julia will substitute the value we assigned to it. In simple terms, a variable is a name for a value.

In Julia, variable names:

• can include letters, digits, and underscores
• cannot start with a digit
• are case sensitive

This means that:

• weight0 is valid, but 0weight is not
• weight and Weight refer to different variables

Types of Data

---

You probably know that computers work with sequences of bits. Bits can have either one of two states, which is commonly denoted using the numbers 0 and 1. A data type specifies how a given sequence of bits is to be interpreted.

Julia supports various data types. Common ones include:

• Integer numbers
• Floating point numbers
• Strings

For example, weight_kg = 60 creates an integer variable. If we want to represent a fractional number, we can use a floating point number:

JULIA

weight_kg = 60.3

To store text, we create a string by using double quotes:

JULIA

patient_id = "001"

Using Variables in Julia

---

Once we’ve assigned variables to values, we can use them in calculations:

JULIA

weight_lb = 2.2 * weight_kg

OUTPUT

132.66

Or modify strings:

JULIA

patient_id = "inflam_" * patient_id

OUTPUT

"inflam_001"

Built-in Julia Functions

---

Functions are called with parentheses. You can include variables or values inside them. Julia provides many built-in functions. To display a value, we use print or println, which adds a newline at the end of the output:

JULIA

println(weight_lb)
println(patient_id)

OUTPUT

132.66
inflam_001

To display multiple values in Julia, we can pass them to println separated by commas.

JULIA

println(patient_id, " weight in kilograms: ", weight_kg)

This prints the value of patient_id, followed by the string " weight in kilograms: ", and then the value of weight_kg, all in one line.

In Julia, every value has a specific data type (e.g., integer, floating-point number, string). To check the type of a value or variable, use the typeof function:

JULIA

typeof(60.3)
typeof(patient_id)

OUTPUT

Float64
String

In this example:

• 60.3 is interpreted as a 64-bit floating-point number (specifically, a Float64).
• patient_id contains a sequence of characters, so its type is String.

Understanding data types is important because they determine how values behave in operations, and some functions may only work with certain types.

You can also use typeof to explore the structure of more complex objects like arrays or dictionaries:

JULIA

typeof([1, 2, 3])      # Array of integers
typeof(["a", "b", "c"]) # Array of strings

OUTPUT

Vector{Int64}
Vector{String}

We can even do math directly when passing arguments to println:

JULIA

println("weight in pounds: ", 2.2 * weight_kg)

OUTPUT

weight in pounds: 132.66

The above doesn’t change weight_kg:

JULIA

println(weight_kg)

To change the value of the weight_kg variable, we have to assign weight_kg to a new value

JULIA

weight_kg = 65.0
println("weight in kilograms is now: ", weight_kg)

OUTPUT

weight in kilograms is now: 65.0

Julia vectors

---

We create a vector by putting values inside square brackets and separating the values with commas:

JULIA

odds = [1, 3, 5, 7]
println("odds are: ", odds)

OUTPUT

odds are: [1, 3, 5, 7]

We can access elements of a vector using indices — numbered positions of elements in the vector. These positions are numbered starting at 1 in Julia, so the first element has an index of 1.

JULIA

println("first element: ", odds[1])
println("last element: ", odds[4])
println("\"end\" keyword element: ", odds[end])

OUTPUT

first element: 1
last element: 7
"end" keyword element: 7

Callout

Ch-Ch-Ch-Ch-Changes

Data which can be modified in place is called mutable, while data which cannot be modified is called immutable. Strings and numbers are immutable. This does not mean that variables with string or number values are constants, but when we want to change the value of a string or number variable, we can only replace the old value with a completely new value.

Vectors and other collections, on the other hand, are mutable: we can modify them after they have been created. We can change individual elements, append new elements, or reorder the whole vector. For some operations, like sorting, we can choose whether to use a function that modifies the data in-place or a function that returns a modified copy and leaves the original unchanged.

Be careful when modifying data in-place. If two variables refer to the same vector, and you modify the vector value, it will change for both variables!

JULIA

mild_salsa = ["peppers", "onions", "cilantro", "tomatoes"]
hot_salsa = mild_salsa
hot_salsa[1] = "hot peppers"
println("Ingredients in mild salsa: ", mild_salsa)
println("Ingredients in hot salsa: ", hot_salsa)

OUTPUT

Ingredients in mild salsa: ["hot peppers", "onions", "cilantro", "tomatoes"]
Ingredients in hot salsa: ["hot peppers", "onions", "cilantro", "tomatoes"]

If you want variables with mutable values to be independent, you must make a copy of the value when you assign it.

JULIA

mild_salsa = ["peppers", "onions", "cilantro", "tomatoes"]
hot_salsa = copy(mild_salsa)  # <-- makes a *copy* of the vector
hot_salsa[1] = "hot peppers"
println("Ingredients in mild salsa: ", mild_salsa)
println("Ingredients in hot salsa: ", hot_salsa)

OUTPUT

Ingredients in mild salsa: ["peppers", "onions", "cilantro", "tomatoes"]
Ingredients in hot salsa: ["hot peppers", "onions", "cilantro", "tomatoes"]

Because of pitfalls like this, code which modifies data in place can be more difficult to understand. However, it is often far more efficient to modify a large data structure in place than to create a modified copy for every small change. You should consider both of these aspects when writing your code.

Callout

Heterogeneous Vectors

Vectors in Julia can also contain elements of different types.

JULIA

sample_ages = Any[10, 12.5, "Unknown"]

This gives us flexibility, but comes at a small performance cost compared to vectors where all elements have the same type. When possible, you should use vectors with a consistent element type for efficiency.

In Julia, functions that modify their arguments in place follow a naming convention: their name ends with an exclamation mark !.

For example:

reverse!(odds) reverses the vector in place

reverse(odds) returns a new, reversed copy while leaving odds unchanged

This convention makes it easy to tell at a glance whether a function will mutate its input or not. There are many ways to change the contents of vectors besides assigning new values to individual elements.

JULIA

push!(odds, 11)
println("odds after adding a value: ", odds)

OUTPUT

odds after adding a value: [1, 3, 5, 7, 11]

JULIA

removed_element = popfirst!(odds)
println("odds after removing the first element: ", odds)
println("removed_element: ", removed_element)

OUTPUT

odds after removing the first element: [3, 5, 7, 11]
removed_element: 1

JULIA

reverse!(odds)
println("odds after reversing in place: ", odds)

OUTPUT

odds after reversing in place: [11, 7, 5, 3]

Julia also provides non-mutating versions of many functions. These return a modified copy of the data and leave the original unchanged:

JULIA

odds_copy = reverse(odds)
println("odds after non-mutating reverse: ", odds)
println("copy after reverse: ", odds_copy)

OUTPUT

odds after non-mutating reverse: [11, 7, 5, 3]
copy after reverse: [3, 5, 7, 11]

As we saw earlier, when we modify a vector in-place, multiple variables can refer to the same vector, leading to unintended changes:

JULIA

odds = [3, 5, 7]
primes = odds
push!(primes, 2)
println("primes: ", primes)
println("odds: ", odds)

OUTPUT

primes: [3, 5, 7, 2]
odds: [3, 5, 7, 2]

This happens because primes and odds point to the same vector in memory. If we want to make an independent copy of a vector, we can use the copy function:

JULIA

odds = [3, 5, 7]
primes = copy(odds)
push!(primes, 2)
println("primes: ", primes)
println("odds: ", odds)

OUTPUT

primes: [3, 5, 7, 2]
odds: [3, 5, 7]

Subsets of vectors and strings can be accessed using ranges:

JULIA

binomial_name = "Drosophila melanogaster"
group = binomial_name[1:10]
println("group: ", group)

species = binomial_name[11:23]
println("species: ", species)

chromosomes = ["X", "Y", "2", "3", "4"]
autosomes = chromosomes[3:5]
println("autosomes: ", autosomes)

last = chromosomes[end]
println("last: ", last)

OUTPUT

group: Drosophila
species: melanogaster
autosomes: ["2", "3", "4"]
last: 4

Challenge

Check Your Understanding

What values do the variables mass, speed and age have?

JULIA

mass = 50.0
age = 56
speed = "fast"
println("very " * speed)
mass = mass * 2.0
age_new = age - 20

1. mass == 50.0, speed == "fast", age == 56
2. mass == 100.0, speed == "very fast", age == 56
3. mass == 100.0, speed == "fast", age == 56
4. mass == 100.0, speed == "fast", age == 36

Show me the solution

1. mass indeed gets reassigned at mass = mass * 2.0.
2. println("very " * speed) prints “very fast” as output, but does not alter speed itself.
3. Thats the correct solution
4. age_new = age - 20 binds the result of age - 20 to a new variable and does not change age itself.

Challenge

Sorting Out References

Julia allows multiple assignments in one line. What will this print?

JULIA

first, second = "Hello", "World!"
println(first," ", second)

Show me the solution

OUTPUT

Hello World!

(Note: println prints without space by default. We insert a space by adding a string with just one space character " ".)

Challenge

Seeing Data Types

What are the types of the following?

JULIA

planet = "Earth"
apples = 5
distance = 10.5

Show me the solution

JULIA

typeof(planet)
typeof(apples)
typeof(distance)

OUTPUT

String
Int64
Float64

Key Points

• Basic data types in Julia include integers, strings, and floating-point numbers.
• Use variable = value to assign a name to a value.
• Use println(value) to display output.
• Julia provides many built-in functions, such as typeof.
• Basic data structures include vectors, dictionaries and tuples.
• Vectors can contain any Julia object, including other vectors (i.e., a vector of vectors).
• Vectors are indexed and sliced with square brackets (e.g., vec[1] and vec[2:9]), in the same way as strings and arrays.
• Vectors are mutable (i.e., their values can be changed in place).