JuniorTheoryCommon

What happens when this program is compiled and run?

SWIFT

let text = "12a"
let number = Int(text)!
print(number + 1)

The program crashes

Expecting a compile error: the compiler accepts Int(text)!, since whether the text is a number becomes known only at run time, where ! stops the program.
Expecting 13: Int("12a") does not read the leading digits, it rejects the whole text and gives nil, and ! on nil crashes.

How would you change the second line so the program keeps running when the text is not a number?
When is force unwrapping with ! acceptable?

JuniorTheoryCommon

SWIFT

let text = "12a"
let number = Int(text)!
print(number + 1)

The program crashes

Expecting a compile error: the compiler accepts Int(text)!, since whether the text is a number becomes known only at run time, where ! stops the program.
Expecting 13: Int("12a") does not read the leading digits, it rejects the whole text and gives nil, and ! on nil crashes.

How would you change the second line so the program keeps running when the text is not a number?
When is force unwrapping with ! acceptable?