var, let KeywordsUnderstand the difference between var and let. Review the cannot assign to value error.
This page was last reviewed on Jan 13, 2024.
Var. In Swift 5.8 we use var and let for locals. The constant "let" can never be changed. But for loops and objects like arrays, var is needed.
Swift error. If our program gives us a "cannot assign to value" error, we should consider changing a let to a var. This will often fix the compilation issue.
Var example. Let us use the var keyword in a simple example. Here we bind the identifier "color" to the string "blue." We then bind "color" to green with a reassignment.
Next After this example, we try to rewrite the code with let. We use 2 let constants instead of 1 var.
// Create a variable and assign a string literal to it. var color = "blue" print(color) // Change the assigned value. color = "green" print(color)
blue green
Let error. Now we cause some trouble. We create a constant with the value "blue." But then we try to change its binding to point to "green." This fails and a compile-time error is caused.
// This part works. let color = "blue" print(color) // But this part does not compile. // ... A constant cannot be reassigned. color = "green" print(color)
Cannot assign to value: 'color' is a 'let' constant
Separate let constants. Here we fix our "cannot assign to value" error. We use two "let" constants. This style of code may lead to better, more durable programs.
Tip When possible, this style of code is ideal. It means no mistaken assignments to color1 or color2 can occur.
// For constants, we must use separate bindings. let color1 = "orange" print(color1) let color2 = "mauve" print(color2)
orange mauve
Var argument. A func cannot modify its arguments in Swift (by default). With the var keyword, we specify that an argument can be modified by the method's statements.
func incrementId(id var: Int) -> Int { // Print and increment the var argument. print(id) id += 1 print(id) return id } var id = 10 // Call incrementId with argument of 10. // ... Print the returned value. print(incrementId(id: id))
10 11 11
Mutating operator error. Here the incrementId func tries to increment its argument Int. But the argument is not marked with var, so this causes a "mutating operator" error.
func incrementId(id: Int) { // This causes an error. id += 1 } var id = 55 incrementId(id: id)
Cannot pass immutable value to mutating operator: 'id' is a 'let' constant
Typically, the let-keyword is best when it can be applied. If a variable is mutated in a program, the var keyword can be used. But immutable things offer advantages and are often preferred.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Jan 13, 2024 (edit link).
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