Conditionals

Introduction

Conditionals are how we tell computers "if this.. do that! otherwise.. do this!" There a are a couple key words to learn:

if
else
elsif
unless

Let's learn how to use these tools to create powerful code! This is where things get fun :)

`if`

The syntax for using if blocks in ruby looks like this:

if _______ #this thing is truthy
  # do something!
end

For example, we could say:

if 1 == 1
  puts "yup! 1 equals 1"
end

if statements become even more powerful when we write methods that can do different things.

def exclaim_even_or_odd(n)
  if n.odd?
    return "this is odd!"
  end
  "this is even"
end

`else`

The else statement is an optional add-on to the if statement. Let's say we wanted to change our method above to puts rather than return...

def exclaim_even_or_odd(n)
  if n.odd?
    puts "this is odd!"
  end
  puts "this is even"
end

This won't work as we intended, because "this is even" will always be printed! We could use an if .. else statement to accomplish this..

def exclaim_even_or_odd(n)
  if n.odd?
    puts "this is odd!"
  else
    puts "this is even"
  end
end

`elsif`

In ruby, we have the ability to chain an infinite number of if statements together using elsif. For example..

def do_i_know_you?(name)
  if name == "Pierre"
    return "I know this guy!"

  elsif name == "Kofi"
    return "We are friends!"

  elsif name == "Fatima"
    return "We go wayyy back"

  else
    return "Sorry, I don't know you!"
  end
end

`unless`

Finally, ruby gives us one more conditional to use: unless. unless is semantically identical to if !..

unless x # ==> if !x
if x     # ==> unless !x

We can make if ! statements read more like English if we refactor them to use unless. For example:

if !(x == 5)
  # do something...
end

unless x == 5
  # do something
end

Both conditional statements above are perfectly valid! But read them both aloud and think about which seems more intuitive to work with.

One-liners

In ruby , we can write short conditionals on one line using if and unless.

if x == 10
  puts "yay 10!"
end

Can be refactored into...

puts "yay 10!" if x == 10

We can make one-liner unless statements too:

puts "yay not 10!" unless x == 10

This is very common practice in ruby. Generally, if you can make the conditional fit on one line, put it on one line. Note that you cannot use one-liners when writing if .. else statements.

Readings