Control Flow

Primitive Types

Nil

The simplest 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.

Loops

This section introduces the basics of repeated execution of code.

The basic feature is the while clause. Its structured quite similar to an if clause: The keyword while designates the beginning and is followed by an expression serving as the loop condition. All subsequent expressions are part of the loop until the closing keyword end. The loop continues to repeat itself as long as the return value of the condition is truthy.

Let’s try a simple program for counting from 1 to 10:

counter = 0
while counter < 10
  counter += 1
  puts "Counter: #{counter}"
end

The code between while and end is executed 10 times. It prints the current counter value and increases it by one. After the 10th iteration, the value of counter is 10, thus counter < 10 fails and the loop breaks.

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

counter = 0
until counter >= 10
  counter += 1
  puts "Counter: #{counter}"
end

Tip

You can find more details on these expressions in the language specification: while and until.

Infinite loops

When working with loops, it’s important to care about the loop condition being falsey at some point. Otherwise, it would continue forever or until you stop the program externally (for example Ctrl+C, kill, pull the plug or when armageddon arrives).

In this example, not incrementing the counter it would be the same as writing:

while true
  puts "Counter: #{counter}"
end

Or if the condition was counter > 0, it would match for all values: they only increase from 1. This would not technically be infinite, as it will fail with a math error when the counter reaches the maximum value of a 32-bit integer. But conceptually that’s similar to an infinite loop. Such logic errors can be easy to miss and so it’s very important to pay attention when writing the loop condition and also taking care of meeting said breaking case. A good practice for index variables (such as counter in our example) is to increment them at the beginning of the loop. That makes it harder to forget to update them.

Tip

Fortunately, there are many features in the language that relieve the burden of writing loops manually and also take care of ensuring valid breaking conditions. A few of them will be introduced in following lessons.

In some cases, the intention is to really have an endless loop. An example would be a server that always repeats waiting for a connection, or a command processor waiting for user input. Then it should be obvious of course, and not hidden in a complex, never-failing loop condition. The most plain way to express that is while true. The condition true is always truthy, so the loop repeats endlessly.

while true
  puts "Hi, what's your name? (hit Enter when done)"
  # `gets` returns input from the console
  name = gets
  puts "Nice to meet you, #{name}."
  puts "Now, let's repeat."
end

Note

This example is not an interactive playground by choice because the playground can’t handle non self-terminating programs, and processing user input. It would just time out and print an error. You can compile and run this code with a local compiler, though.

To stop the program, hit Ctrl+C. This sends a signal to the process asking it to exit.

Skipping and Breaking

It can be useful to skip some iterations in between, or stop the iteration entirely on some condition.

The keyword next inside a loop body skips to the next iteration, ignoring any expressions left in the current iteration. If the loop condition isn’t met, the loop finishes and the body won’t execute another time.

counter = 0
while counter < 10
  counter += 1
  if counter % 3 == 0
    next
  end
  puts "Counter: #{counter}"
end

This example could’ve easily be written without next by placing the puts expression in a conditional instead. The worth of next becomes apparent when there are many more expressions in the method body to be skipped.

Loop conditions can be difficult to calculate, for example because they require multiple steps or depend on input that needs to be determined. In such situations, it’s not very practical to write all the logic in the loop condition. The keyword break can be used anywhere in a loop body and serves as an additional option to break from a loop regardless of its loop condition. Control flow immediately continues after the end of the loop.

counter = 0
while true
  counter += 1
  puts "Counter: #{counter}"
  if counter >= 10
    break
  end
end
puts "done"