C ++逻辑运算符Q

Question

This question might seem very noobish but my question is this: are these two statements logically the same?

int a;
int b;
int c;


if (!a && !b && !c)
//do something



if (!(a || b || c))
//do something

Answer 1

A truth table is useful to understanding logics.

#include <iostream>

using std::cout;
using std::endl;

int main(void) {
    int a;
    int b;
    int c;
    bool differ = false;

    cout << "a b c x y\n";
    for (a = 0; a <= 1; a++) {
        for (b = 0; b <= 1; b++) {
            for (c = 0; c <= 1; c++) {
                bool x = (!a && !b && !c);
                bool y =  (!(a || b || c));
                differ = differ || (x != y);
                cout << a << " " << b << " " << c << " " << x << " " << y << "\n";
            }
        }
    }
    if (differ) {
        cout << "they differ" << endl;
    } else {
        cout << "they are the same" << endl;
    }
    return 0;
}

Actually they are the same thanks to De Morgan's laws:

  !a && !b && !c
= !(a || b) && !c
= !((a || b) || c)
= !(a || b || c)

( = here is not the C++ assignment operator)

Answer 2

No. On your first statement, all conditions must be met. && operator means All should be true to make the result true otherwise it would be false. On your second statement, If one or more condition is true, then the condition would be done.

Sorry for a very brief explanation.