If-Else
If Statements are a language construct that can be used to change the behaviour of the program depending on a condition. They consist of a head, a body and an optional else branch.
// without else
if condition expression
if condition { statements... }
// with else:
if condition expression else expression
if condition { statements... } else { statements... }
// parentheses around the condition are optional
The semantics of an If Statements are as follows:
- 1.Evaluate the condition
- 2.If the result has a boolean value of
true, evaluate the 'then' branch and return the result - 3.Otherwise, evaluate the 'else' branch if it exists and return the result.
The following program illustrates this behaviour:
let condition = 1 < 2
if condition
{
print("1 < 2")
}
else
{
print("something went wrong")
}
// prints "1 < 2"
If Statements may also be used in contexts where an expression is expected, e.g. as a method argument:
let condition = true
print(if (condition) 1 else 2)
// prints 1
Note that in simple cases like the above, a Ternary Conditional Operator should be used instead of an If Expression.
print(condition ? 1 : 2)
Using an If Expression without an Else branch in an expression context causes an error diagnostic.
Binding If Statements are a special form of If Statements meant for use with Optional Values. Instead of a condition, their head consists of a variable binding that can have an arbitrary type:
if let name = expression { statements... }
// with else and explicit type
if let name: Type = expression { statements... } else { statements... }
The semantics of a Binding If Statement differ from those of a normal If Statement:
- 1.Evaluate the expression after the
=of the variable. - 2.If the result is not
null, bind it to the variable name and evaluate the 'then' branch. The variable name is available in the scope of the 'then' branch. - 3.If the result was
null, evaluate the 'else' branch. The variable name is not available in this scope.
The example below uses a Binding If Statement to check if certain data is available.
case class Person(firstName: String, middleName: String?, lastName: String)
let person = Person("John", null, "Doe")
if let middleName = person.middleName
{
print("\(person.firstName) \(middleName) \(person.lastName)")
}
else
{
print("\(person.firstName) \(person.lastName)")
}
The program prints "John Doe" when executed. Changing the second line to
let person = Person("John", "Peter", "Doe") causes it to print "John Peter Doe" instead.As with If Statements, Binding If Statements may also be used in expression contexts.
Last modified 4yr ago