Ruby if Examples: elsif, else and unless

Use the if-statement, elsif and else. See the unless statement.

If, else. A fire emits light. It burns out and the light is lost. The fire has 2 states: on and off. In an expression we could test whether it is burning.

In a selection statement we direct the flow of control. In Ruby we use if. With elsif and else we handle alternative branches. An end is required.

If example. After the if-keyword we place an expression. Ruby then checks whether this statement evaluates to true or false. If it evaluates to true, the inner block is entered.

However: If the expression evaluates to false, the inner block of an if-statement is not reached. The statements are not executed.

Ruby program that uses if-statements # Some integer variables. a = 1 b = 2 c = 3 # Test for inequality. if a != b puts "1 != 2" end # Add and then test for equality. if a + b == c puts "1 + 2 == 3" end Output 1 != 2 1 + 2 == 3

Elsif, else. An if-statement can have more parts. It can include an elsif-statement and an else-statement. They are reached only when the initial if-statement evaluates to false.

Here: The statement within the else-block is reached. The if and elseif evaluate to false, so "Z" is printed.

Tip: For performance, putting the condition that matches most often first is fastest. Fewer conditions must be evaluated in total.

Ruby program that uses if, elsif and else # Two integers. a = 1 b = 2 # Use if, elsif, else on the integers. if a > b # Not reached. print "X" elsif a == b # Not reached. print "Y" else # This is printed. print "Z" end Output Z

One-line. Ruby supports one-line if-statements. The "if" is used to modify the preceding statement. These statements are only run if the if-expression evaluates to true.

Here: The first statement has no effect because the variable is not greater than 5. But the second statement takes effect.

Ruby program that uses one-line if-statements # Variable equals 5. i = 5 # Use one-line if-statements. puts "Greater than 5" if i > 5 puts "Not greater than 5" if i <= 5 Output Not greater than 5

Unless. This is a negated if-statement. It has the same functionality as an if-statement that tests for false, not true. But this syntactic sugar is sometimes useful.

Also: There is an else-statement that can follow the unless statement. The syntax is just like an if-block.

Ruby program that uses unless i = 4 # Use an unless-else construct. unless i == 3 puts "UNLESS" else puts "ELSE" end Output UNLESS

Unless, one line. An unless modifier can also be used on one line. This syntax makes programs readable, almost like English: you take a certain action unless a condition is true.

Here: We display the value of the string with puts, unless the value equals Cat. So the value Dog is displayed.

Ruby program that uses unless, one-line animal = "Dog" # Display the animal string unless it equals Cat. puts animal unless animal == "Cat" Output Dog

And, or. Sometimes we need to chain together expressions in an if-statement. We can use the "and" or "&&" operators. Two syntax forms for "or" are also supported.

Short-circuit: These operators short-circuit. And will stop checking after its first false result, "or" after its first true.

Note: The English operators (and, or) are easier to read. But the symbolic ones are more familiar to developers of C-like languages.

Note 2: The English operators have a lower precedence. For consistency the C-like operators may be a better choice.

Ruby program that uses and, or animal1 = "cat" animal2 = "dog" # And operators. if animal1 == "cat" and animal2 == "dog" puts 1 end if animal1 == "cat" && animal2 == "dog" puts 2 end # Or operators. if animal1 == "whale" or animal2 == "dog" puts 3 end if animal1 == "whale" || animal2 == "dog" puts 4 end Output 1 2 3 4

And, or precedence. In the Ruby grammar there is an important difference between "and" and "&&" and the "or" versions. The English words have lower operator precedence.

Note: In my testing, this does not affect many simple expressions. But the change in precedence could affect more complex things.

Program: Consider the 2 if-expressions. The first has "and" which means the part after the "||" is evaluated together.

And: The second if-statement has "&&" which means "top" is evaluated all by itself.

Ruby program that shows precedence change left = 0 top = 2 # Not true because "and" has lower precedence. if top == 2 || left == 0 and top == 3 puts "1" end # True because && has higher precedence. if top == 2 || left == 0 && top == 3 puts "2" end Output 2 Evaluation order: If 1: (top == 2) (left == 0, top == 3) If 2: (top == 2, left == 0) (top == 3)

Ternary. This statement uses a question mark and a ":" after an expression. If the expression evaluates to true, the first result is chosen.

Otherwise: The second value is used as the result. Here, the value equals 10, so the result is 20. The 0 is returned in all other cases.

Ruby program that uses ternary value = 10 # Ternary statement. result = value == 10 ? 20 : 0 puts result Output 20

Equals. In an if-conditional, two equals signs check for equality. Ruby will report a warning if you use just one equals sign. An assignment uses one equals. A conditional, two.

But: The program still compiles. In some languages, this syntax results in a fatal, compile-time error.

Ruby program that causes equals sign warning value = 10 # We should use two = in a conditional. if value = 20 puts true end Warning C:/programs/file.rb:6: warning: found = in conditional, should be == Output true

Assign. Like a method, an if-statement returns a value. This comes from the last statement evaluated. We can assign a variable to the result of an if-statement.

Result: In this program, the variable cat_sound is assigned to "meow" because the size variable is equal to 0.

Ruby program that assigns to if # Set size to zero. size = 0 # Assign cat_sound to result of if-statement. cat_sound = if size == 0 "meow" else "roar" end puts(cat_sound) Output meow

Then. This keyword can be part of an if-statement. But usually in Ruby programs do not use the "then." This keyword may be preferred if makes the code clearer to read.

Note: In programming, clarity is key. But standard forms—using Ruby that other developers also use—is also important.

Ruby program that uses then keyword value = 10 size = 10 # The "then" keyword is optional. if value == size then puts true end # Omit the then. if value == 10 puts 10 end Output true 10

True, false. In an if-statement all values can be evaluated for truth. In this program, we test for truth. We test numbers, empty collections like arrays, and nil.

Numbers: All numbers (including 0) evaluate to true. Other programming languages sometimes treat 0 as false, but not Ruby.

Empty array: An empty Array evaluates also to true. Empty collections are treated the same as ones with elements.

Nil: False and nil evaluate to false—these were the only false values found in this test.

Ruby program that tests for true, false values = [0, 1, -1, true, false, nil,] # Test all the values for truth. values.each do |v| if v puts String(v) + " = true" else puts String(v) + " = false" end end Output 0 = true 1 = true -1 = true true = true false = false = false [] = true

A summary. The if-statement is an imperative branching statement. This means it affects what statements are next executed. Its effect is directly specified by the programmer.

Tip: Thanks to Jason Newell for writing in with an important point about operator precedence.

Some concepts. With an if-statement, we create conditional blocks. The data in our programs influences what operations are taken. An elseif and else provide alternative paths.
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