how to use inverse in C

[how to use ~ operator ]

I have a structure say Alpha. I know the value of element inside Alpha (say a) which can be 0 or 1 - I want the other element of same structure to take inverse value of Alpha.a. For example:

if Alpha.a = 1
then Alpha.b = 0

and vice versa

I have tried:

Alpha.b = ~ (Alpha.a)

But unfortunately it doesnt work - when Alpha.a is 1, A开发者_运维百科lpha.b gets set to 254

Any ideas?

Thanks and regards,

SamPrat

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In C, true is represented by 1, and false by 0. However, in a comparison, any non-false value is treated is true.

The ! operator does boolean inversion, so !0 is 1 and !1 is 0.

The ~ operator, however, does bitwise inversion, where every bit in the value is replaced with its inverse. So ~0 is 0xffffffff (-1). ~1 is 0xfffffffe (-2). (And both -1 and -2 are considered as true, which is probably what's confusing you.)

What you want is !, instead of ~.

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Use XOR operator:

Alpha.b = Alpha.a ^ 1;

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The ~ operator negates each individual bit. For example, assume that Alpha.a is an unsigned char. Then ~1 would read, in binary as, ~00000001, and the result would be 11111110 (again, in binary), which is the same as 254 in decimal and 0xFE in hex.

As others have suggested, use !Alpha.a or Alpha.a ^ 1.

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A nice cross-platform cross language solution to this common problem is:

Alpha.b = 1 - Alpha.a;

---

You can't use ~ as this will turn 00000000 into 11111111 rather than 00000001 as I think you're expecting.

If you have bools you can use:

Alpha.b = !(Alpha.a)

but if not you may have to use if / else logic:

if (Alpha.a == 0)
{
    Alpha.b = 1;
}
else
{
    Alpha.b = 0;
}

---

What about a postoperational bitmask?

using unsigned chars looking for bit0:

b = 0x01u & ( ~a );

or even

b = a ^ 0x01u;

or for "Boolean-Thinkers" ( be aware TRUE and FALSE may differ from 0/1 ! if you want it "real boolean" you should use TRUE and FALSE if defined.)

b = (1 != a)?(0u):(1u);

Cheers

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You want to use another operator. Specifically !

Alpha.b = !Alpha.a

Since the values are zero or one, it is much simplier.