Published by Arunprasadh C on 04 May 2022 • Last Updated on 19 May 2022
Decision-making Constructs in Swift
Decision-making constructs are used to control the flow of execution in a program based on some condition. Swift provides the following Decision-making Constructs:
if Statement
The if Statement executes a block of code when the given condition evaluates to true.
Syntax :
if(condition)
{
// block of code
}
Example :
var age = 22
if(age >= 18 && age <= 100)
{
print("You're eligible to vote !")
}
Output :
You're eligible to vote !
if-else Statement
The if-else Statement executes a block of code when the given condition evaluates to true or an alternate block of code when the given condition evaluates to false.
Syntax :
if(condition)
{
// block of code
}
else
{
// alternate block of code
}
Example :
var age = 16
if(age >= 18 && age <= 100)
{
print("You're eligible to vote !")
}
else
{
print("You're not eligible to vote !")
}
Output :
You're not eligible to vote !
if-else if-else Statement
The if-else if-else Statement executes a block of code when the given condition evaluates to true or an alternate block of code when an alternate condition evaluates to true. If none of the conditions are met, the else block code is executed.
Syntax :
if(condition1)
{
// block of code
}
else if(condition2)
{
// alternate code block 1
}
...
else if(conditionN)
{
// alternate code block N-1
}
else
{
// alternate code block N
}
Example :
var age = -9
if(age >= 18 && age <= 100)
{
print("You're eligible to vote !")
}
else if(age < 1 || age > 100)
{
print("Invalid Age !")
}
else
{
print("You're not eligible to vote !")
}
Output :
Invalid Age !
Nested if Statement
The Nested if Statement transfers the control to the inner if statement when the condition provided in the outer if statement is met. The control flow is then handled by the inner if statement.
Syntax :
if(condition1)
{
if(condition2)
{
// block of code
}
else
{
// alternate block of code
}
}
else
{
// alternate code block
}
Example :
var num = 6
if(num >= 0)
{
if(num == 0)
{
print("Zero !")
}
else
{
print("Positive Number !")
}
}
else
{
print("Negative Number !")
}
Output :
Positive Number !
guard Statement
In Swift, we use the guard statement to transfer program control out of scope when certain conditions are not met. The guard statement is similar to the if statement with one major difference. The if statement runs when a certain condition is met. However, the guard statement runs when a certain condition is not met. A control statement is mandatory in guard block.
Syntax :
guard expression else
{
//block of code
//control statement: return, break, continue or throw. or exit() function.
}
You can notice control statements in the above syntax. The return statement is used to return a value and/or transfer the control outside a function block. The break statement is used to terminate a loop. The continue statement is used to skip the current iteration and move on to the next iteration. The throw statement is used to throw an Error.
Example :
var i = 1
while (i <= 20)
{
// guard condition to check the odd number
guard i % 2 != 0 else
{
i += 1
continue
}
print(i, terminator: ", ")
i += 1
}
Output :
1, 3, 5, 7, 9, 11, 13, 15, 17, 19,
The if-let Statement and guard-let Statement
We have seen about if-let Statement in Optionals and Tuples. The guard-let Statement can also be used for the same purpose.
The switch Statement
The switch statement allows us to execute a block of code among many alternatives. The code block of the case which is matched is executed. When there is no match for any case, the default case code block is executed. An explicit break statement is not required for each case (unlike C/C++/Java), as the execution of the switch statement ends as soon as a case matched block is executed. However, if we want to force the control to fall through to other cases (equivalent to not using break in C/C++/Java), we can use the fallthrough statement. Unlike Java, a range/comma separated values can be used in case label.
Syntax :
switch (expression)
{
case value1:
// statements
// optional fallthrough
case value2:
// statements
// optional fallthrough
...
default:
// statements
}
Example 1:
let ageGroup = 22
switch ageGroup
{
case 0...16:
print("Child")
case 17...30:
print("Young Adults")
case 31...45:
print("Middle-aged Adults")
default:
print("Old-aged Adults")
}
Output 1:
Young Adults
Example 2:
let integerToDescribe = 5
var description = "The number \(integerToDescribe) is"
switch integerToDescribe
{
case 2, 3, 5, 7, 11, 13, 17, 19:
description += " a prime number, and also"
fallthrough
default:
description += " an integer."
}
print(description)
Output 2:
The number 5 is a prime number, and also an integer.
Let’s see a complex example showcasing optional tuple, array of tuples, random array value and pattern matching.
Example 3:
let valueRepo: [(Int, String)] = [(5, "Kris"), (6, "Shiv")]
var tupOpt: (Int, String)? = nil
print(type(of: tupOpt))
var shouldContinue = true
while shouldContinue
{
switch tupOpt
{
case .none:
print("Found nil ! Assigning value...")
tupOpt = valueRepo.randomElement()
// fallthrough can't be used here as swift compiler doesn't allow to fallthrough nil case. hence shouldContinue boolean is used
case .some(let (age, name)):
print("\(name) is a \(age) year old boy")
shouldContinue = false
}
}
Output 3:
Optional<(Int, String)>
Found nil ! Assigning value...
Shiv is a 6 year old boy
The Ternary Operator ? :
Swift also has a Ternary Operator ? : like C/C++/Java which acts as a shorthand for if-else construct. First a Bool predicate (Any statement whose result is a boolean value) is given. The predicate gets evaluated. If it evaluates to true, the statement following ? is executed. Else, the statement following : is executed. The ternary operator can also be used for conditionally assigning values to a variable or constant.
Syntax :
var ifWanted = predicate ? statement : alternativeStatement
Example :
var a = 48
a%2 == 0 ? print("EVEN") : print("ODD") // executing statements using ? :
a = a%2 == 0 ? 0 : 1 // assigning values using ? :
print(a)
Output :
EVEN
0
Now, as we have seen about Decision-making Constructs, let’s move on to see about Loops in Swift.