Conditionals

What `if`¶

An if statement allows you to take different actions depending on which conditions are met. For example:

if city == "Toronto"
   drinking_age = 19
else
   drinking_age = 21
end

This says:

if the city is equal to (==) “Toronto”, then set drinking_age to 19.
Otherwise (else) set drinking_age to 21.

`true` and `false`¶

Ruby has a notion of true and false. This is best illustrated through some example. Start irb and type the following (taking special note of the difference between = and ==):

shell> irb --simple-prompt
>> 3 > 2
=> true
>> 3 < 2
=> false
>> num = 4
=> 4
>> num == 4
=> true
>> city = "Toronto"
=> "Toronto"
>> city == "Toronto"
=> true

The if statements evaluates whether the expression (e.g. ’city == “Toronto”) is true or false and acts accordingly.

Notice the difference between = and ==:

= is an assignment operator, it allows to you assign a value to a variable. It will evaluate to the value assigned.
== is a comparison operator, it will evaluate to true or false, depending on whether both sides are equal or different.

Most common conditionals¶

Here is a list of some of the most common conditionals:

==	equal
!=	not equal to
>	greater than
<	less than
>=	greater than or equal to
<=	less than or equal to

String comparisons¶

How do these operators behave with strings? Well, == is string equality and > and friends are determined by ASCIIbetical order.

What is ASCIIbetical order? The ASCII character table contains all the characters in the keyboard. It lists them in this order:

…012…789…ABC…XYZ…abc…xyz…

Start irb and type these in:

shell> irb --simple-prompt
>> "9" < "D"
=> true
>> "a" < "b"
=> true
>> "h" == "h"
=> true
>> "H" == "h"
=> false
>> "Z" <= "b"
=> true
>> "j" != "r"
=> true

`elsif`¶

elsif allows you to add more than one condition. Take this for example:

if age >= 60
    puts "Senior fare"  
elsif age >= 14
    puts "Adult fare"
elsif age > 2
    puts "Child fare"
else
    puts "Free"
end

Let’s go through this:

If age is 60 or more, we give a senior fare.
If that’s not true, but age is 14 or more, we give the adult fare.
If that’s not true, but age is more than 2 we give the child fare.
Otherwise we ride free.

Ruby goes through this sequence of conditions one by one. The first condition to hold gets executed. You can put as many elsif’s as you like.

Exercises:¶

A Condition Between Cities

Condition Gradebook

Choosing the right nominee


Scott 2008-01-31	Exercise idea: Create a grading rubric for a homework assignment where A >=90, B >=80, etc, etc, and then input fake scores.


ianderson 2008-01-31	What would the purpose of string comparisons be? I could understand a use for comparing numbers, but I am unable to think of a reason why comparing letters in terms of greater than, less than, or equal to would be useful.


beckybee 2008-02-05	I don’t remember if this has been answered in class, but a possibility of why we compare strings is to find out whether the user typed in the correct string? i.e. login names and passwords. I have a question though, say we have a paragraph, in order to find a word would we have to convert it to an array first?

What if¶