Oh no. Oh no no no. This is not a fun module. This will not be pleasant. But this will, very much, be necessary and incredibly empowering. Sit down, buckle up, we’re about to see what loops do.
Iteration is one of the most central, most useful, most confusing concepts there is. But without it, there is little we can do that is useful or fun. The one and only redeeming feature of iteration is that if you understand its most basic example, you have actually understood all of the more complicated ways to iterate. The point of this module is to introduce the way Julia thinks about iteration, and to show as many examples as possible.
Let us start with the simplest possible loop:
for i in [1, 2, 3] @info "The value of i is $(i)" end
[ Info: The value of i is 1 [ Info: The value of i is 2 [ Info: The value of i is 3
That’s it. That’s all there is to know about iteration, but in order to move forward we need to study it; we need to study it like a kōan.
Broken down into its most fundamental components, this loop (the structure
that begins at
for and ends at
end) is simple.
It has an opening statement:
for. This means that we will repeat an action
for multiple things. These multiple things are given on the first line:
i in [1, 2, 3]
But what does it means? This means that
i will take all of the values in
[1, 2, 3] in turn. Let’s verbally expand this:
i will be 1, then
will be 2, then
i will be 3. But not everywhere! No,
i will only have
these values inside the loop. So
i has no value, unless it takes one
inside the loop.
And so we need to go deeper: inside the loop. The inside of the loop is
everything under the first line, and above the last line. If it sounds
obvious, great. But it is important to emphasize this, as this is the space in
i exists, in the sense that this is the space where it has a value.
Currently, the inside of our loop has a single instruction: we show a string
that has the value of
i using interpolation as we have seen in a previous
module. But we can have a loop that is arbitratily complex:
for i in [1, 2, 3] if i == 2 @info "The value of i is 2" else @info "The value of i isn't 2 (it's $(i))" end end
[ Info: The value of i isn't 2 (it's 1) [ Info: The value of i is 2 [ Info: The value of i isn't 2 (it's 3)
We have no asked for a full
else statement within our loop. This result
in a more complex output, but also increases what we call the cyclomatic
complexity of our program — there are more “branches” we can
explore. Think of a program as a “chose your own adventure” type book: we now
have a lot more possible endings.
elsestatements and to write functions. We will explore how to write functions in the next section.
We can put whatever we want in a loop. Even, of course, another loop!
for i in [1, 2, 3] @info "i = $(i)" for j in [1, 2, 3] @info " j = $(j) → i×j = $(i*j)" end end
[ Info: i = 1 [ Info: j = 1 → i×j = 1 [ Info: j = 2 → i×j = 2 [ Info: j = 3 → i×j = 3 [ Info: i = 2 [ Info: j = 1 → i×j = 2 [ Info: j = 2 → i×j = 4 [ Info: j = 3 → i×j = 6 [ Info: i = 3 [ Info: j = 1 → i×j = 3 [ Info: j = 2 → i×j = 6 [ Info: j = 3 → i×j = 9
Notice that the value of
i exists within its own loop, but also within the
j’s loop exists within
i’s. Where variables are
usable is called scoping, and understanding scoping is something done through
avid reading of the documentation. For now, it is safer to assume that the
variables are not accessible outside their loop.
Not only are the variables not accessibles, they are not defined. Using a
try statement (it captures error, and we will take a deep dive into it very
soon), we can verify this:
try j catch error @info error end
[ Info: UndefVarError(:j)
The existence of
j (and of
i, as well) is limited to the inside of its
But what are loops good for? Now that we have established what loops are, the next module will deal with very powerful ways to iterate over objects, which is one of the most important things to do.