Tutorial Details
- Topic: Ruby
- Difficulty: Easy
- Estimated Completion Time: 30 minutes
Ruby is a one of the most popular languages used on the web. We’ve started a new screencast series here on Nettuts+ that will introduce you to Ruby, as well as the great frameworks and tools that go along with Ruby development. In this chapter, we’ll be looking at how conditional statements and loops work in Ruby.
Catch Up
- Part 1: Installing Ruby and Getting Started
- Part 2: Variables, Datatypes, and Files
- Part 3: Working with Classes
- Part 4: Conditionals and Loops
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Conditional #1: If Statement
The if statement is one of the first types of branching you learn when programming. You can guess what it means: if this is true, do one thing; if it’s not, do something else. In Ruby, these are pretty easy to write:
name = "Andrew" if name == "Andrew" puts "Hello Andrew" end if name == "Andrew" puts "Hello Andrew" else puts "Hello someone else" end
After the keyword if, code in your conditional statement. After that comes the code to execute if the condition returns true. You close the statement with the end keyword. If you’d like, you can squeeze an else statement in there, which will predictably execute if the condition is false.
elsif
It’s not hard to check for multiple conditions. Just put as many elsif statements as you’d like between the if and else statements. Yes, that’s elsif, a marriage of else and if.
order = { :size => "medium" }
def make_medium_cofee
puts "making medium statement"
end
#assume other functions
if order[:size] == "small"
make_small_coffee;
elsif order[:size] == "medium"
make_medium_coffee;
elsif order[:size] == "large"
make_large_coffee;
else
make_coffee;
end
As I mentioned, you can have as many elsif conditions and their corresponding code blocks as you want.
Statements vs. Expressions
Most programming languages make a distinction between statements and expressions. A statements is a code construct that doens’t evaluate to a certain value. An expression is a code construct does return a value. For example, calling a function is an expression, because it returns a value. However, an if statement is exactly that, a statement, because it does not return a value. This means that you can’t do this in your JavaScript:
message = if (someTruthyValue) {
"this is true";
} else {
"this is false";
}
Obviously, you can’t do this because the if statement does not return a value that you can assign to message.
However, you can do this with Ruby, because statements are actually expressions, meaning they return a value. So we can do this
message = if order[:size] == "small"
"making a small"
elsif order[:size] == "medium"
"making a medium"
else
"making coffee"
end
Whichever block of code is executed will become the value of message.
If as a Modifier
If you don’t have any elsif or else clauses, and your if statement has only one line, you can use it as a modifier to a “normal” line.
puts "making coffee" if customer.would_like_coffee?
Conditional #2: Unless Statement
In most programming languages, we want to reverse the return of the conditional expression, we have to negate it, usually with the bang (!) operator.
engine_on = true if !engine_on # meaning "if not engine_on" puts "servicing engine" #should not be put, because "not engine_on" is false end
However, Ruby has a really nice unless operator, that keeps us from having to do that, while giving us much more readable code:
unless engine_on # "unless engine_on" is better than "if not engine_on" "servicing engine" end
Just like if, you can use unless as a modifier:
puts "servicing engine" unless engine_on
Conditional #3: Case / When Statement
If you’ve got a lot of options to work through, using if/elsif/else might be somewhat wordy. Try the case statement.
hour = 15 case when hour < 12 puts "Good Morning" when hour > 12 && hour < 17 puts "Good Afternoon" else puts "Good Evening" end
It’s kinda-sorta-maybe like a switch/case statement in JavaScript (or other languages), except that there’s no one variable you’re evaluating. Inside the case/end keywords, you can put as many when statements as you’d like. Follow that when by the conditional expression, and then the lines of code go after it. Just like the if statement, the case statement is really an expression, so you can assign it to an expression and capture a returned value.
hour = 15
message = case
when hour < 12
"Good Morning"
when hour > 12 && hour < 17
"Good Afternoon"
else
"Good Evening"
end
puts message
Breaking Up Expressions
If you’re familiar with JavaScript, you’ll know that the blocks of code in an if statement are surrounded by curly braces. We don’t do this in Ruby, so it may seem like Ruby is dependant on the whitespace. Nothing could be further from the truth (take that, pythonistas :)).
If we want write your statements as one-liners, have to separate the different parts of the statements … but how? Well, you can use semi-colons:
if name == "Andrew"; some_code; else; some_code; end
If you don’t like the look of that (which I don’t), you can put the keyword then between the conditional expressions and the line of code.
if name == "Andrew" then sode_code; end
This also works for a case statement.
case
when x > 20; puts "<20"
when x < 20 then puts "<20"
end
Loop #1: While Loop
So, those are conditional statements (I mean, expressions). How about loops? Let’s look at while loops first.
A while loop will continue to execute until the condition stated is false:
arr = ["John", "George", "Paul", "Ringo"]
i = 0
while arr[i]
puts arr[i]
i += 1
end
Here we’re looping over an array; when arr[i] returns false (meaning there are no items left in the array), the loop will stop executing. Inside the loop, we print out the current item in the array, and them add one to our increment variable.
You can also use while as a modifier
arr = ["John", "George", "Paul", "Ringo"] i = -1 puts arr[i += 1] while arr[i]
Loop #2: Until Loop
Just like unless is the opposite of if, until is the opposite of while. It will continue to loop until the condition is true:
days_left = 7;
until days_left == 0
puts "there are still #{days_left} in the week"
days_left -= 1
end
And of course, it’s a modifier, too.
days_left = 8
puts "there are still #{days_left -= 1} in the week" until days_left == 1
Loop #3: For Loop
Yes, Ruby has a for loop. No, it’s not like the for loop in other languages. It acts like a foreach loop, for looping over the values in an array or hash:
arr = ["John", "George", "Paul", "Ringo"]
for item in arr
puts item
end
If you’re looping over a hash, you can use two variable names, one for the key and one for the value:
joe = { :name => "Joe", :age => 30, :job => "plumber" }
for key, val in joe
puts "#{key} is #{val}"
end
That’s it!
I hope you’re enjoying our Ruby for Newbies Screencast series. If there’s something you’d like to see, let me know in the comments! (And if you’re not watching the videos, you should be. There’s a screencast-only bonus at the end of each one.)
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By Andrew Burgess
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Great as usual! Keep em coming!
Loving the Ruby stuff
Hi,
First of all thanks for this great series. I am starting to learn ruby from here. I just have two question about this post. you haven’t explained what is this
order = { :size => “medium” }
I guess it is some thing like object literal in javascript.
second thing can you please shed some more light on what is a modifier and what is their usage.
Regards
Mohit Thakral
Thanks!
You’re right; it’s pretty similar to an object literal in JS; it’s a hash; I went over them in Chapter 2.
A modifier is just a conditional or loop that “modifies” a single line of code, as shown in the examples. They’re useful when your loop or condition has only one line of code inside it. Instead of adding an extra two lines for the condition/loop, just use it as a modifier. As an added bonus, it usually reads more like English.
Let me know if that helps!
arkadaşlar tutsplusa girip türkçe bilen kaç kişiyiz reply yapın lütfen
Are this and your previous video going to be available to download for plus members? I like to keep these around in case I need to go back and take a look at something really quickly.
Loving the series so far!
This is fantastic. Great job with the series so far. Which way is the most efficient way to print something based on a condition?
There is obviously the obvious one, if -> puts, but I was hoping for more. In PHP I’m quite used to using short-hand coding as such: 67) ? “33% chance of this being the output”; ?>
I would love to see a Rails from Scratch like CI from Scratch, would be awesome.
Probably because they’re not technically loops, but you didn’t mention a couple of the most widely used forms of iteration. These seem to be used much more than for or until loops
5.times { |i| i * 2 }
some_collection.each {
|item| item.do_something
}
You’re right; I wanted to cover just the raw forms of loopings in this lesson. We’ll talk about iterations in a chapter or two :)
Good job, I recently learned ruby on rails and I even built two or three apps with it, but as I was trying to learn Rails, I guess I kinda lost my way and never actually got a firm understanding of the language itself which made me think of “ruby” as “ruby on rails”, never as a language on its own, I’m hoping I could fix that soon by reading more about the language, but you sure are helping out!.
keep it up!
Really enjoying this series. Thanks Andrew!
Just a quick question…
How is case in ruby any better than if? Aren’t these equivalent?
hour = 15
message = case
when hour 12 && hour < 17
"Good Afternoon"
else
"Good Evening"
end
puts message
hour = 15
message = if hour 12 && hour < 17
"Good Afternoon"
else
"Good Evening"
end
puts message
Am I missing something?
Guess I should have listened a few seconds more :) There really isn’t much difference. I suppose it’s what you’re more comfortable with. Perhaps like python, ruby doesn’t really have a switch/case statement either.
BTW, so much for those CODE tags working. :)
It’s mostlyfor readability and really comes into play when there are more than 2 or 3 options, and more complicated test conditions (to avoid repeating).
For example, see below. Writing this with if then and multiple elseif statements would be a lot more cumbersome and harder to read.
grade = ((grade_fraction * 24) + weighting_factor * 300)
letter_grade = case score
when 0..9 then “U”
when 20..29 then “F”
when 30..39 then “E”
when 40..49 then “D”
when 50..59 then “C”
when 60..69 then “B”
when 70..100 then “A”
else “Invalid Score”
end
puts letter_grade
sorry line 2 should have “case grade” (not score).
nice tutorial.. keep going bro.
I’m pretty sure perl has an unless control structure. I don’t think the unless control structure to too obscure.
I have copied every line of code from this verbatim. I have even copied and pasted the code from above, and I cannot get past the undefined local variable “make_medium_coffee” error. I am on a Windows platform, and everything has worked to this point. Please advise.
A little expansion on the hour loop here
<pre name="code" class="html">
def the_time( hour )
case hour
when hour < 12
puts "Good Morning"
when 12..17
puts "Good Afternoon"
when hour > 17
puts "Good Evening"
end
end
puts the_time(hour)
</html>
Extending the hour loop into a method that accepts an argument (hour)
try that again
<pre class="js">
def the_time( hour )
case hour
when hour &amp;lt; 12
puts &amp;quot;Good Morning&amp;quot;
when 12..17
puts &amp;quot;Good Afternoon&amp;quot;
when hour &amp;gt; 17
puts &amp;quot;Good Evening&amp;quot;
end
end
puts the_time(hour)
</pre>
You guys should allow ruby code as an option for the pre tags :/
We had to define the function before we called it, do they have function prototypes in ruby?
Really digging these tuts! Is their a resource you would suggest (screen cast preferably) that will go over object oriented programing. Just want to make sure I am straight with classes, objects, methods and functions. Took a couple classes at school but was never taught in a way that made the lightbulb go off.
Hey andrew!
i got something awkward in my code
screenshots of my code
this one is works right, but it shouldn’t http://d.ekinertac.com/AF7U
this is the other one it looks awkward http://d.ekinertac.com/xc2b
i got “nil” for shortened the while loop
Conditional #1 : elsif – You don’t need to use semi-colons to call a function in ruby, right?
Sorry didn’t watch till the end…