Ruby Back to Basics

Introduction

Every day, we control how our life flows. This morning, before you got out of bed, you probably pushed the blanket off. You walked into the bathroom to brush your teeth. Then you sat down at the computer to read this README. When you're hungry, you'll grab a snack. As humans, we use logic to control how our days flow without much thought to the process behind it. Computers need a bit more guidance. Logic and conditional statements in Ruby allow you to control how your program works. Loops allow you to tell a program to repeat a portion of code until a certain condition is met.

Learning Goals

• Review Conditionals and Logic
  • If Statements
  • Ternary Operators and Statement Modifiers
  • Case Statements
• Review Looping
  • While Loops
  • Until Loops

Review Conditionals and Logic

The term 'conditionals' refers to a set of expressions: if statements, ternary operators, statement modifiers, and case statements. Think of a real life scenario where you would need to make a decision based on certain conditions. For example, if it's raining outside, you take a rain jacket or umbrella. In the context of web technology, if you're logged in, you can see content specific to a registered user, if not (or else), you may see a marketing page or log in screen.

In order for conditional statements to function, we always need a logical statement that evaluates to TRUE or FALSE. It is either TRUE or FALSE that it is raining outside, TRUE or FALSE that you are logged in to a website. These logical statements can be comparisons:

0 < 1
# => true
5 > 10
# => false

Or methods:

[].empty?
# => true
[1, 2, 3].include?(4)
# => false

Its also possible to use values to derive the 'trueness' or 'falseness'. In Ruby, anything that is something is considered 'true', sometimes referred to as 'truthy'. This includes empty hashes and arrays and any number or integer, including 0. The only values that evaluate to false on their own are false and nil.

Conditionals

If Statements

If statements allow us to check if a certain logic condition is true, and if it is, perform a certain action contained within the code block.

money = 20
if money > 10
  going_out_to_lunch = true
end

Else Statements

What happens when we want to check more than one condition? That's where else comes in.

happy_hour = true
if happy_hour
  drinks = market_price * .5
else
  drinks = market_price
end

Elsif

We all know that with lunch options, we like options. Two choices just aren't going to cut it. How do we set up more options?

if vegan && gluten_free
  mood = "thrilled"
elsif vegetarian && gluten_free  
  mood = "satisfied"
elsif gluten_free  
  mood = "content"
else
  mood = "disappointed"
end

Ternary Operators

Ternary operators are a shorthand way of expressing an if/else statement on one line. This helps shorten the amount of code written. Below, we've refactored a previous example:

happy_hour = true ? market_price * .5 : market_price

The ternary operator is most useful in cases where the conditions are simple, and there are no more than 2 possible outcomes.

Statement Modifiers

Statement modifiers are a conditional statement that comes at the end of a statement. It will execute the line of code if the conditional evaluates accordingly:

hot = true
print "Keep oven at 350 degrees" if hot

Unless

The unless statement is structured like an if statement, but in reverse: The code block will execute if the conditional evaluates to false rather than true:

food = "hot"
unless food == "cold"
   puts "Keep refrigerated or on ice"
end

In the above, the text 'Keep refrigerated or on ice' will be displayed because food is not equal to 'cold.' If there is an else statement, it will execute if the conditional evaluates to true:

food = "hot"
unless food == "hot"
   puts "Keep refrigerated or on ice"
 else
   puts "Keep oven at 350 degrees"
end

This time, 'Keep over at 350 degrees' will be displayed. The unless statement works as a statement modifier as well, allowing us to write the things like the following:

hot = true
print "Keep oven at 350 degrees" if hot
print "Keep refrigerated or on ice" unless hot

Case Statements

Case statements are similar to if statements in that they allow you to control the flow of your program, and allow you to test multiple conditions, but they are used to test multiple conditions against one value. For example:

age =  14
case age
when 0 .. 2
   school = "daycare age"
when 3 .. 4
   school = "pre-school age"
when 5 .. 9
   puts "elementary/primary school age"
when 10 .. 13
   puts "middle school age"
when 14 .. 18
   puts "high school age"
when 19 .. 25
   puts "college age"
else
   puts "legal adult"
end

The age condition set in this example is being tested against multiple cases that would determine which school age group a 14 year old might fall into. This will produce the following result:

high school age

Review Looping

Loops are a way to tell our program to perform a certain part of the code a set number of times until a condition is met. This condition can be hard coded into the method, or more commonly set by the program itself.

loop

The code is executed in a loop block:

loop do
  puts "This code will loop forever."
end

The above code is not ideal and will loop continuously. In order to prevent this, you can set conditions and/or use the break keyword to exit loops based on certain conditions:

i = 0
loop do
  puts "This code will loop forever unless the loop breaks"
  i += 1
  break if i > 4
end

This loop will run 5 times before the statement modifier if i > 4 evaluates to true and break is executed.

while

Using the comparison operators in our loops works, but it is not as clean as a process as we'd like, and there are much better ways to write them. The while loop is often a better choice. With the while loop, as long as a condition evaluates to true, the loop will continue to fire. The following is valid, syntactically correct code:

i = rand(20)
while i < 20
   puts i
   i += 1
end

We could have written this using a standard loop with a break included, but while takes care of that for us.The above code will generate a random number between 0 and 20 and increment as long as i is less than 14. If i is assigned the value 14 to start, it will increment inside the while loop until it reaches 20. In the terminal, the numbers 14 through 19 will be displayed.

Why 19 and not 20? On the very last loop iteration, i is incremented to 20, which causes i < 20 to evaluate to false, breaking the loop before puts i is executed again.

Until Loops

Until loops are similar to while loops, but instead are the inverse. Until loops will keep executing until a specific condition is true. Another way to think about this concept is "loop while conditional is not true". It loops until the condition evaluates to true.

i = 0
until i == 20
   puts i
   i += 1
end

Conclusion

Conditionals, logic and loops are essential tools for every Ruby programmer. They are the most common methods for making our programs dynamic and useful.

Both in the programming world and real life, decisions are made and conditions must be met. We use if, else, elsif, unless and case statements to give conditional results and use while and until statements to iterate until conditions are met.

Resources

Ruby Operators Stack Overflow - Ternary Operators