逻辑运算符

Question

I have come across an answer and I can't seem to figure out why it's right.

So there are two int variables x and y and they represent 0x66 and 0x39 respectively.

The question asked what is the result value based on the expression.

x && y is apparently 0x01 (1)
x || y is 1
!x || !y is 0
x && ~y is 1

From what I was thinking, I thought as long as an argument was not zero it was considered true. So as long as x and y were some non-zero value then && operator would produce a 1, and of course as long as one of them is true the || operator would produce a 1.

So why is the third question 0? Is the ! different from the bitwise ~ operator? So originally x is 0101 0101 in binary since it's non zero it is true in the logical sense, but the ! of it would be false or would it do the one's complement of the number so its 1010 1010?

Answer 1

A boolean result is always true or false, and in C true is represented by 1 and false by 0 .

The logical not operator ! gives a boolean result, ie 1 or 0 . So if an expression is "true" (ie non-zero) then applying ! on that expression will make it false, ie 0 .

In your example you have !x || !y !x || !y . First !x is evaluated, and it evaluates to false, leading to !y being evaluated, and it also evaluates to false, so the while expression becomes false, ie 0 .

Answer 2

Yes, ! is different from the bitwise not ~ . It's a logical not. It yields either 0 or 1.

Answer 3

~(0x01) will 0x10 and !(0x01) will 0x00

'~' is a bitwise operator. '!' is a logical operator.

Answer 4

From what I was thinking, I thought as long as an argument was not zero it was considered true. So as long as x and y were some non-zero value then && operator would produce a 1, and of course as long as one of them is true the || operator would produce a 1.

This is all correct.

So why is the third question 0? Is the ! different from the bitwise ~ operator?

Yes. It is logical NOT. Just like the other logical operators && and || , it only cares about if a variable has a non-zero value or not.

!0 will yield 1 .

!non_zero will yield 0 .

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Note that all logical operators in C yield type int rather than bool , as an ugly, backwards-compatibility remain from the time before C had a boolean type. In C++ and other C-like languages, logical operators always yield type bool .

Similarly, true and false in C are actually just macros that expand to the integers 1 and 0 . In other languages they are keywords and boolean constants.