Loops in Ruby

if: conditionally doing something once

Consider the following program, which utilizes an if statement:

numbers = Array.new

if numbers.length < 10
  new_number = rand(100)
  numbers.push(new_number)
end

len = numbers.length

p numbers
p len

Click here for a REPL to try it.

What do you expect the output of the program to be when you click run? Try to interpret the program yourself before you ask Ruby to.

Okay, now you can click “run ▶”. Did you guess right?

We start off with a blank array, numbers. If its length is less than 10 (this is true, since length is currently 0), we push a new random number into it.

Once Ruby reaches the if’s end, it proceeds to the next line and continues to execute the rest of the code (whether the if’s condition was true or not).

At the end of the day, numbers has one element in it and len is 1.

while: conditionally doing something multiple times

Now, consider almost identical code, but with the if keyword swapped for a new keyword — while:

numbers = Array.new

while numbers.length < 10
  new_number = rand(100)
  numbers.push(new_number)
end

len = numbers.length

p numbers
p len

Click here for a REPL to try it.

while works almost exactly like if — it evaluates the expression next to it, and if the expression is truthy, it executes the code on the lines between it and it’s end; if not, it ignores the code on the lines between it and it’s end.

There is one key difference: if the condition next to a while is truthy, after we reach it’s end, the execution of the program jumps back up to the while statement.

Then the condition is evaluated again. If it is still true, then the code inside the while statement is executed again. And then the execution of the program jumps back up to the while statement again. Etc.

So in this case,

• The first time we reach the end, we jump back up to the while.
• Evaluate a.length < 10 again — still true, since 1 < 10.
• So we push in another random number, and jump back up.
• 2 < 10? Yep, so we do it again.
• We push in another random number, and jump back up.
• 3 < 10? Yep, so we do it again.
• 4 < 10? Yep, so we do it again.
• Etc.
• 10 < 10? Nope, so now proceed to the line after the end and continue.
• And len ends up being 10.

What we’ve seen here is our very first loop; code that is executed multiple times. It could be an arbitrary number of times, perhaps even an infinite number of times if we aren’t careful.

Blocks

Fundamentally, all looping is implemented with while; but, this being Ruby, there are all sorts of convenience methods on top to make it as easy as possible to create loops for various contexts. For example, let’s say I wanted to print:

"1 Mississippi"
"2 Mississippi"
"3 Mississippi"
# etc
"10 Mississippi"

exactly 10 times. I could do it using while like this; try interpreting the following code before you click “run”:

mississipis = 1

while mississipis <= 10
  p mississipis.to_s + " Mississippi"

  mississipis = mississipis + 1
end

Click here for a REPL to try it.

Does the code make sense to you?¹

Or, rather than while, I could use Integer’s .times method, like this:

mississipis = 1

10.times do
  p mississipis.to_s + " Mississippi"

  mississipis = mississipis + 1
end

Click here for a REPL to try it.

Notice there’s a new keyword here: do. This is because the .times method, in order to do its job of executing some code 10 times, needs a special argument — the code to execute.

In order to pass a method some lines of code as an argument, we need to wrap the lines of code within the do and end keywords, creating what’s called a block of code.

So, given a block of code, the 10.times method will execute it for us exactly 10 times; this saves us the trouble of writing a condition for while.

Block variables

But the .times method will save us even more trouble than that; we can stop worrying about creating and incrementing the counter variable, mississipis, too. The .times method will create a block variable and assign values to it for us automatically, but we have to choose a name for it using some new syntax after the do: the vertical bars, | |, or “pipes”. It looks like this:

10.times do |mississipis|
  p mississipis.to_s + " Mississippi"
end

Click here for a REPL to try it.

Try running it. Here’s what’s going on:

• We created a block of code with do/end and gave it to .times.
• We chose a name for a block variable, mississipis, with the | | after the do.
• Behind the scenes, the .times method did mississipis = 0 before the first iteration.
• The .times method executed the block of code the first time.
• Behind the scenes, the .times method did mississipis = 1 before the second iteration.
• The .times method executed the block of code the second time.
• Etc.

Why does .times start by assigning 0 to its block variable during the first iteration, rather than 1? Well, that’s just how the author of the .times method made it work.

Fortunately, Ruby provides lots of other looping convenience methods that we can take advantage of instead, and each one assigns different values to its block variable.

In the REPL above, replace 10.times with each of the following and play around with the arguments to get a sense of how each method works:

5.upto(10)
99.downto(90)
1.step(10, 3)
10.step(1, -4)

• Loops in Ruby
  • if: conditionally doing something once
  • while: conditionally doing something multiple times
  • Blocks
    • Block variables