Control Flow

Primitive Types

Nil

The most simplistic type is Nil. It has only a single value: nil and represents the absence of an actual value.

Remember String#index from last lesson? It returns nil if the substring does not exist in the search string. It has no index, so the index position is absent.

p! "Crystal is awesome".index("aw"),
  "Crystal is awesome".index("xxxx")

Bool

The Bool type has just two possible values: true and false which represent the truth values of logic and Boolean algebra.

p! true, false

Boolean values are particularly useful for managing control flow in a program.

Boolean Algebra

The following example shows operators for implementing boolean algebra with boolean values:

a = true
b = false
p! a && b, # conjunction (AND)
  a || b,  # disjunction (OR)
  !a,      # negation (NOT)
  a != b,  # inequivalence (XOR)
  a == b   # equivalence

You can try flicking the values of a and b to see the operator behaviour for different input values.

Truthiness

Boolean algebra isn’t limited to just boolean types, though. All values have an implicit truthiness: nil, false, and null pointers (just for completeness, we cover that later) are falsey. Any other value (including 0) is truthy.

Let’s replace true and false in the above example with other values, for example "foo" and nil.

a = "foo"
b = nil
p! a && b, # conjunction (AND)
  a || b,  # disjunction (OR)
  !a,      # negation (NOT)
  a != b,  # inequivalence (XOR)
  a == b   # equivalence

The AND and OR operators return the first operand value matching the operator’s truthiness.

p! "foo" && nil,
  "foo" && false,
  false || "foo",
  "bar" || "foo"

The NOT, XOR, and equivalence operators always return a Bool value (true or false).

Control Flow

Controlling the flow of a program means taking different paths based on conditions. Up until now, every program in this tutorial has been a sequential series of expressions. Now this is going to change.

Conditionals

A conditional clause puts a branch of code behind a gate that only opens if the condition is met.

In the most basic form, it consists of a keyword if followed by an expression serving as the condition. The condition is met when the return value of the expression is truthy. All subsequent expressions are part of the branch until it closes with the keyword end.

Per convention, we indent nested branches by two spaces.

The following example prints the message only if it meets the condition to start with Hello.

message = "Hello World"
if message.starts_with?("Hello")
  puts "Hello to you, too!"
end

note Technically, this program still runs in a predefined order. The fixed message always matches and makes the condition truthy. But let’s assume we don’t define the value of the message in the source code. It could just as well come from user input, for example a chat client.

If the message has a value that does not start with Hello, the conditional branch skips, and the program prints nothing.

The condition expression can be more complex. With boolean algebra we can construct a condition that accepts either Hello or Hi:

message = "Hello World"
if message.starts_with?("Hello") || message.starts_with?("Hi")
  puts "Hey there!"
end

Let’s turn the condition around: Only print the message if it does not start with Hello. That’s just a minor deviation from the previous example: We can use the negation operator (!) to turn the condition into the opposite expression.

message = "Hello World"
if !message.starts_with?("Hello")
  puts "I didn't understand that."
end

An alternative is to replace if with the keyword unless which expects just the opposite truthiness. unless x is equivalent to if !x.

message = "Hello World"
unless message.starts_with?("Hello")
  puts "I didn't understand that."
end

Let’s look at an example that uses String#index to find a substring and highlight its location. Remember that it returns nil if it can’t find the substring? In that case, we can’t highlight anything. So we need an if clause with a condition that checks if the index is nil. The .nil? method is perfect for that.

str = "Crystal is awesome"
index = str.index("aw")
if !index.nil?
  puts str
  puts "#{" " * index}^^"
end

The compiler enforces that you handle the nil case. Try to remove the conditional or change the condition to true: a type error shows up and explains that you can’t use a Nil value in that expression. With the proper condition, the compiler knows that index can’t be nil inside the branch and it can be used as a numeric input.

tip A shorter form for if !index.nil? is if index, which is mostly equivalent. It only makes a difference if you wanted to tell apart whether a falsey value is nil or false because the former condition matches for false, while the latter does not.

Else

Let’s refine our program and react in both cases, whether the message meets the condition or not.

We can do this as two separate conditionals with negated conditions:

message = "Hello World"
if message.starts_with?("Hello")
  puts "Hello to you, too!"
end
if !message.starts_with?("Hello")
  puts "I didn't understand that."
end

This works but there are two drawbacks: The condition expression message.starts_with?("Hello") evaluates twice, which is inefficient. Later, if we change the condition in one place (maybe allowing Hi as well), we might forget to change the other one as well.

A conditional can have multiple branches. The alternate branch is indicated by the keyword else. It executes if the condition is not met.

message = "Hello World"
if message.starts_with?("Hello")
  puts "Hello to you, too!"
else
  puts "I didn't understand that."
end

More branches

Our program only reacts to Hello, but we want more interaction. Let’s add a branch to respond to Bye as well. We can have branches for different conditions in the same conditional. It’s like an else with another integrated if. Hence the keyword is elsif:

message = "Bye World"
if message.starts_with?("Hello")
  puts "Hello to you, too!"
elsif message.starts_with?("Bye")
  puts "See you later!"
else
  puts "I didn't understand that."
end

The else branch still only executes if neither of the previous conditions is met. It can always be omitted, though.

Note that the different branches are mutually exclusive and conditions evaluate from top to bottom. In the above example that doesn’t matter because both conditions can’t be truthy at the same time (the message can’t start with both Hello and Bye). However, we can add an alternative condition that is not exclusive to demonstrate this:

message = "Hello Crystal"
if message.starts_with?("Hello")
  puts "Hello to you, too!"
elsif message.includes?("Crystal")
  puts "Shine bright like a crystal."
end
if message.includes?("Crystal")
  puts "Shine bright like a crystal."
elsif message.starts_with?("Hello")
  puts "Hello to you, too!"
end

Both clauses have branches with the same conditions but in a different order and they behave differently. The first matching condition selects which branch executes.