Variables and constants

A student’s name may stay fixed while a corrected score changes. Swift separates these intentions: let declares a binding you cannot reassign, while var allows reassignment. Choosing let when a value does not need to change makes that constraint visible to readers and the compiler.

The equals sign assigns a value; it does not ask whether two values are equal. Swift infers types from initial values when it can. A variable may change its value but still keeps its type. Reassigning an Int variable to text is a type error, not a conversion.

Save this complete program as main.swift and run it with the Swift toolchain. No separate input file is needed.

Swift · complete program

let student = "Diya"
var score = 72
score += 8
print(student, score)

Expected output

Diya 80

Follow the execution

The first score is 72. The compound assignment adds eight and stores 80 back in score. print reads the current value. The student binding is never reassigned, so let expresses the intended stability.

Try this program in the Swift compiler. Paste the complete source above.

Your task: What declaration must change if the program needs to correct a score declared with let?

Swift · complete program

let student = "Diya"
var score = 72
score += 8
print(student, score)

Worked answer

Declare that score with var before assigning a new value. Do not change every binding to var; keep names and other unchanged values constant where appropriate.

A let binding to a class instance prevents rebinding the reference, not every possible mutation of the object. We will distinguish that from value-type mutation when studying structures and classes.

Declare that score with var before assigning a new value. Do not change every binding to var; keep names and other unchanged values constant where appropriate. A let binding to a class instance prevents rebinding the reference, not every possible mutation of the object.

Further reading: official Swift language guide.