Automating Repetition with Loops

Last updated on 2026-01-27 | Edit this page

Overview

Questions

  • How can I do the same operations on many different values?

Objectives

  • Explain what a for loop does.
  • Correctly write for loops to repeat simple calculations.
  • Explain what a while loop does.
  • Trace changes to a loop variable as the loop runs.
  • Trace changes to other variables as they are updated by a for loop.

In the episode “Visualizing Tabular Data”, we wrote Julia code that plots values of interest from our first inflammation dataset (inflammation-01.csv), which revealed some suspicious features in it.

Line graphs showing average, maximum and minimum inflammation across all patients over a 40-day period.

We now have a dozen datasets, and potentially more on the way if Dr. Maverick keeps up their surprisingly fast clinical trial rate. We would like to create plots for all of our datasets without having to copy-paste code for each file.

To do that, we need to teach the computer how to repeat actions automatically — this is where loops come in.

An example of a task that can be solved with a loop is accessing the numbers stored in a vector. We could do this by printing each number on its own.

JULIA 

odds = [1, 3, 5, 7]

In Julia, an array (1D arrays are called vectors, 2D arrays are called matrices) is an ordered collection of elements, and each element has a unique number associated with it — its index.

For example, the first number in odds is accessed via odds[1]. One way to print each number is to write four separate println statements:

JULIA 

println(odds[1])
println(odds[2])
println(odds[3])
println(odds[4])

OUTPUT 

1
3
5
7

However, this approach has three major drawbacks:

  1. Not scalable – if the array has hundreds of elements, writing one line per element is unmanageable.
  2. Difficult to maintain – if we want to decorate each element an asterisk or any other character, we’d have to change every line.
  3. Fragile – if the array is longer or shorter than expected, we either miss elements or get an error.

Example with a shorter array:

JULIA 

odds = [1, 3, 5]
println(odds[1])
println(odds[2])
println(odds[3])
println(odds[4])

OUTPUT 

1
3
5

ERROR 

ERROR: BoundsError: attempt to access 3-element Vector{Int64} at index [4]

Here’s a better approach: a for loop

JULIA 

odds = [1, 3, 5, 7]

for num in odds
    println(num)
end

OUTPUT 

1
3
5
7

This is shorter — definitely shorter than writing a println for every number in a long list — and more robust as well:

JULIA 

odds = [1, 3, 5, 7, 9, 11]

for num in odds
    println(num)
end

OUTPUT 

1
3
5
7
9
11

In a for loop, the loop variable (like num in the example) is just a name we give to each element of the collection as we go through it.

  • num is the loop variable.

  • During the first iteration, num = 1; during the second, num = 3; and during the third, num = 5, etc.

  • You can choose any valid variable name instead of num

In Julia, the general form of a for loop is:

JULIA 

for variable in collection
    # do things using variable, such as println
end

Here’s another loop that repeatedly updates a variable:

JULIA 

count = 0
people = ["Curie", "Darwin", "Turing"]

for person in people
    count += 1
end

println("There are ", count, " names in the vector.")

OUTPUT 

There are 3 names in the vector.

It’s worth tracing the execution of this little program step by step. Since there are three names in people, the statement inside the loop will be executed three times.

  • First iteration: count is 0 (set on line 1) and person is "Curie". count = count + 1 updates count to 1.

  • Second iteration: person is "Darwin" and count is 1, so count becomes 2.

  • Third iteration: person is "Turing" and count becomes 3.

Since there are no more elements in people, the loop finishes. Finally, the println statement shows the result.

Note that in Julia, the loop variable does not overwrite a variable with the same name outside the loop. The loop variable is local to the loop, so it only exists inside the loop body.

For example:

JULIA 

person = "Rosalind"

for person in ["Curie", "Darwin", "Turing"]
    println(person)
end

println("after the loop, name is ", person)

OUTPUT 

Curie
Darwin
Turing
after the loop, name is Rosalind

Note also that finding the length of an object is such a common operation that Julia has a built-in function for it called length:

JULIA 

println(length([0, 1, 2, 3]))

OUTPUT 

4

length is much faster than any function we could write ourselves, and much easier to read than writing a loop to count elements. It also works on many different kinds of collections in Julia, so we should always use it when we can.

While Loops

Sometimes, we want to repeat an action until a certain condition is met, rather than looping over a collection. For this, Julia provides a “while loop”.

The general form is:

JULIA 

while condition
    # do something
end

Example:

JULIA 

count = 0

while count < 5
    println("count is ", count)
    count += 1
end

OUTPUT 

count is 0
count is 1
count is 2
count is 3
count is 4

The loop checks the condition count < 5 before each iteration. As long as the condition is true, the loop body runs. Once count reaches 5, the condition is false and the loop stops.

ou can use while loops when the number of iterations is not known in advance. But be careful!: if the condition never becomes false, the loop will run forever (an infinite loop).

!!!WARNING!!! Example of a potential infinite loop:

JULIA 

x = 0
while x < 3
    println(x)
end

This will print 0 endlessly because x never changes.

Challenge

Understanding the loops

Given the following loop:

JULIA 

word = "oxygen"
for letter in word
    println(letter)
end

How many times is the body of the loop executed?

  • 3 times
  • 4 times
  • 5 times
  • 6 times

The body of the loop is executed 6 times, once for each character in "oxygen".

Challenge

Computing Powers With Loops

Exponentiation is built into Julia:

JULIA 

println(5 ^ 3)

OUTPUT 

125

Write a loop that calculates the same result as 5 ^ 3 using multiplication (and without exponentiation).

JULIA 

result = 1
for i in 1:3
    result = result * 5
end
println(result)

OUTPUT 

125
Challenge

Summing a vector

Write a loop that calculates the sum of elements in a vector by adding each element and printing the final value, so [124, 402, 36] prints 562.

JULIA 

numbers = [124, 402, 36] 
sum = 0
for num in numbers
    sum = sum + num
end
println(sum)

OUTPUT 

562

A shorter way to reach the goal would be:

JULIA 

numbers = [124, 402, 36] 
sum = 0
for num in numbers
    sum += num
end
println(sum)

OUTPUT 

562
Key Points
  • Use for variable to process the elements of a collection (like a vector) one at a time.
  • The body of a for loop must be placed inside for ... end.
  • The body of a while loop must be placed inside while ... end.
  • Use length(thing) to determine the length of a collection (vector, array, string, etc.).