C traps and defects Chapter 3 semantic "traps" 3.8 operators &, |, and!

Arithmetic &&、|| and ! 
C There are two kinds of logical operators in language , Sometimes it can be exchanged ： Bitwise operators &、| and ~, And logical operators &&、|| and !.
If the programmer replaces the corresponding operator in the other class with an operator in one class , He might be surprised ： After the exchange, the program seems to be able to “ normal ” Work , But in fact, this is just a coincidence . 
Bitwise operators &、| and ~ Operands are treated as a binary sequence of bits , Align each bit separately for operation .
& The operator represents if and only if a bit of the binary representation of two operands is 1 when , In the binary representation of the final result, this bit is 1.
Logical operators &&、|| and ! The way to handle operands is ： Or think of it as “ really ”, Or think of it as “ false ”, Instead of 0 Regard as “ really ”.
These operators when the result is “ really ” When to return to 1, When the result is “ false ” When to return to 0, They can only return 0 or 1. Except for the second ,
Operator && And operators || When the value of the left operand can determine the final result , The right operand is not evaluated at all .

Query a specific element in the table  
i = 0;
while (i < tabsize && tab[i] != x) {
    i++;
}
The intention is ： If i be equal to tabsize Then the cycle ends , This indicates that the element to be found is not found in the table ; And in other cases , here i Is the index of the element you are looking for . Notice that asymmetric boundaries are used in this loop . 
i = 0;
while (i < tabsize & tab[i] != x) {
    i++;
}
This loop statement is also possible “ normal ” Work , But just because of two very lucky reasons .
first “ A fluke ” yes ,while The expression in & Both sides of the operator are comparison operations , The comparison result is “ really ” Is equal to 1, For “ false ” When is equal to the 0. as long as x and y All values of are limited to 0 or 1,x&y And x&&y Always get the same result . However , If either of the two comparison operators is divided 1 Other than 0 representative “ really ”, Then this cycle will not work properly .
the second “ A fluke ” yes , For the next element after the end of the array （ It doesn't really exist ）, As long as the program does not change the value of this element , Just read its value , Generally, there will be no harm . Operator & And operators && Different , Operator & Both operands must be evaluated .

For the next element after the end of the array , It's legal to take its address . This is in the section , We try to actually read the value of this element , The result of this approach is undefined , And there are very few C The compiler can detect this problem .