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. If you ask which car occupies a parking spot, and no car is parking there, the answer is nil.

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 however is not only defined for boolean types proper. All values have an implicit truthiness: nil, false and null pointers (just for completeness, we cover that later) are falsey. Any other value 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 to take 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 a most basic form it consists of a keyword if followed by an expression serving as 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.

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.

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 changing 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 different order and they behave differently. The first matching condition selects which branch executes.