Type Int8 on Swift does not store maximum integer 255

Question

Doing some pranks on the Playground, With Swift 2.2 we can declare constant

let testeIntOitoBits: Int8 = 127 //maximo - deveria ser 255
let testeIntOitoBits2: UInt8 = 255

Why can’t I store 255 in Int8 and Uint8 yes?

Answer 1

You can use this to find out the maximum value of a type:

print(Int8.max) //vai responder 127
print(UInt8.max) //vai responder 255

I put in the Github for future reference.

Int8 is an integer marked type, that is, it has 8 bits and one of them is the signal if the number is positive or negative. Only 7 bits remain to represent the same number. With 7 bits we can only represent 128 different numbers, so it can only go from 0 to 127. In the case of negatives as it does not need to represent 0, it goes from -1 to -128. Totaling 256 different numbers (-128 to 127). If you have Int8.min will receive a -128.

Already the UInt8 is an unmarked integer (the U means unsigned) and you can use the 8 bits to represent the number, so you can represent from 0 to 255. Obviously you can’t represent negative numbers.

Avoid using unmarked types. Use only where you really need to and when you understand all the implications of yours. I reinforce this recommendation: it is rare to be useful to use an unmarked type. You will find that you need it, but it will probably be a wrong decision. When you’re sure you need to make sure you’ve understood all the differences of working with arithmetic in a type like this.