Consider the following program, which utilizes an
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
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
Now, consider almost identical code, but with the
if keyword swapped for a new keyword —
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
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
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,
end, we jump back up to the
a.length < 10again — still true, since
1 < 10.
2 < 10? Yep, so we do it again.
3 < 10? Yep, so we do it again.
4 < 10? Yep, so we do it again.
10 < 10? Nope, so now proceed to the line after the
lenends up being
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.
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”:
Does the code make sense to you?1
Or, rather than
while, I could use
.times method, like this:
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
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
.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:
Try running it. Here’s what’s going on:
endand gave it to
mississipis, with the
| |after the
mississipis = 0before the first iteration.
.timesmethod executed the block of code the first time.
mississipis = 1before the second iteration.
.timesmethod executed the block of code the second time.
.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)
Test your skills with the classic programmer’s interview question, FizzBuzz. You’ll have to combine multiple skills that you’ve learned so far — loops, conditionals, etc — but you shouldn’t have to Google2 for help. All the knowledge you need is in the readings that you’ve covered already.
However, you should as always ask lots of questions on Piazza! Our rule is: you’re only allowed to get stuck for 15 minutes before you post a question; and then, while you’re waiting for an answer, comment out your old approach and try to come up with something completely different.
If the line
mississipis = missippis + 1 looks a little odd to you, you’re not alone. Remember, this is variable assignment, not equivalence. So the expression on the right side (
mississipis + 1) is evaluated first until there’s just one object (e.g
2) left; and then that object replaces the contents of the variable (
missippis) named on the left. Rinse and repeat. ↩
The answers that you find on the internet will, at this stage, more than likely just confuse you. They will reference advanced techniques, or sometimes will be outdated or just flat-out wrong; and it’s hard for beginners to tell the difference. ↩