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Tutorials > Julia Programming: a Hands-on Tutorial

For Loops

by Martin D. Maas, Ph.D
@MartinDMaas

Last updated: 2021-11-06

Organizing our code with simple loops, nested for loops, and parallel for loops.

Loops are one of the most basic ways to structure our computations

Simple For Loop

The most basic way to structure computations is by using a for loop. In Julia, the basic syntax of a for loop is

for iterator in range
    execute_statements(iterator)
end

As an example, let’s evaluate the sum of the first 100,000 terms of the quadratic series.

x=0
for k in 1:100000
    x = x + (1/k)^2
end

We get the following output

1.6449240667982423

Nested For Loops

It is very common to have nested for loops, and we would normally code them using something like:

for i in 1:3
    for j in 1:3
        print("i=", i, " j=", j, "\n")
    end
end

This can get hard to read when we have more than two nested loops. A nice syntax we can rely on in this case is the following:

for i in 1:3, j in 1:3
    print("i=", i, " j=", j, "\n")
end

Additionally, we can resort to the even nicer:

for i ∈ 1:3, j ∈ 1:3
    print("i=", i, " j=", j, "\n")
end

Break and Continue Statement

The use of break and continue statements is somewhat contrived, and could be considered bad programming practice by some people. However, I believe break and continue statements are harmless when used carefully, and could even be preferable in some cases.

Break Statement Example

The Break statement is used to exit a for-loop before it has completed all its iterations, usually after checking that a certain condition is met.

For example, in a numerical algorithm, we could be iterating until a certain number of iterations is met, but we could interrupt the computations if some convergence criteria is met.

Let’s compute the sum of a series to include a break statement as follows:

x=0
for k in 1:100000
    term = (1/k)^2
    x = x + term
    if (abs(term) < 1e-10) break end
end

While its possible to rewrite the above code with the sole purpose of avoiding the use of the break statement, we might need to write something like this:

x=0
iter = 0
while ( iter == 0 || abs(term) < 1e-10) && (iter < 100000)
    term = (1/k)^2
    x = x + term
    iter = iter + 1
end

Continue Ctatement Example

The Continue statement is used to exit the present iteration of a for-loop, and to continue with the next iteration immediately.

A reasonable use-case for the continue statement is as some sort of precondition-test for a bad argument passed to a loop.

For example, let’s say we have a function that sums the inverses of a set of random numbers. Logically, we would like to avoid summing 1/0, as that would result in Inf and could lead to errors.

One way to avoid this undesired input is to use a continue statement.

numbers = randn(100)
sum = 0
for k in numbers
    if (k==0) continue end
    sum = sum + 1/k
end

One alternative, without the continue statement, could be as follows:

numbers = randn(100)
sum = 0
for k in numbers
    if (k != 0)
        sum = sum + 1/k
    end
end

In some cases, when we have to check for multiple conditions, the presence of multiple nested-if statements could turn out quite annoying, and the continue statement could be preferred.

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