是什么？ 运算符意味着在变量前面？

Question

I have just a simple question, I'm learning C programming and I know that the. operator means logical NOT? My question is that what does it mean in this case in a do while cycle as a condition? Is it checking somehow if the variable is changed?

I know I should probably find it somehow, but when I try to google '.'s it doesn't want to show what I meant.

#include <stdio.h>

int main() {
    int input, ok=0;
    do {
        scanf("%d", &input);
        if ( /* condition */) {
            //the input is wrong it will scan another one
        }
        else {
            //the input is correct, so the while cycle can stop
            //we don't need more inputs
            ok = 1;
        }
    } while (!ok);

    //...
    //...
    //...

    return 0;
}

Answer 1

!ok is the same as ok == 0 .

Remember that in C, any non-zero scalar value in a Boolean context means "true" while zero means "false". It's a common C idiom to write !foo instead of foo == 0 . It works for pointers as well:

FILE *foo = fopen( "some_file", "r" );
if ( !foo )
{
  fprintf( stderr, "could not open some_file\n" );
  return EXIT_FAILURE; 
}

So, while ( x ) is the same as while ( x != 0 ) , and while ( !x ) is the same as while ( x == 0 ) .

Answer 2

Speaking loosely, the given code executes the contents of the loop repeatedly "while NOT ok", meaning "while OK is zero". In the else block, ok is set to 1, meaning that the loop will not repeat again.

More formally, !ok is an expression that is true when ok is equal to zero, and false when ok is any value other than zero.

Answer 3

In the context of logical operations, anything except zero (*) is true , zero is false .

so !1 is "! true" is false
!0 is "! false" is true

(*) any kind of zero: NULL counts as zero, 0.0 counts as zero

Answer 4

Just think about it, the ok has a Boolean value, which is false at the initialisation.

(In c, 0 has a false value, I assume that you know this.)

In the do while, it is simply negates the logical value, it still means a NOT. If the condition is true, it will loop, otherwise, won't.

If ok has a false value and you'll negate it, it will have a true value, the loop will, umm loop.

I hope you'll get the point, it does not have any other meanings.