C traps and defects Chapter 2 syntax "traps" 2.6 problems caused by "hanging" else

“ Hang ”else Raised questions
 
if (x == 0) 
    if (y == 0) error();
else {
    z = x + y;
    f(&z);
}
The intention of programmers is that there should be two main situations ：x be equal to 0 as well as x It's not equal to 0. about x be equal to 0, Unless y Also equal to 0（ The function is called error）, Otherwise, the program will not do any processing ; about x It's not equal to 0 The circumstances of , The program will first x And y The sum of is assigned to z, And then to z Call the function with the address of as a parameter f.

However , What this code actually does is far from the programmer's intention . The reason lies in C There are such rules in language , namely else Always match the nearest unmatched... In the same pair of parentheses if coordination . 
The above program is actually implemented by logic to adjust the code indentation , As follows ： 
if (x == 0) {
    if (y == 0) error();
    else {
        z = x + y;
        f(&z);
    }
}
in other words x It's not equal to 0, The program will not do anything . If you want to get the result of the programmer's intention reflected by the code indentation in the original example , It should be written like this ： 
if (x == 0) {
    if (y == 0) {
        error();
    }
} else {
    z = x + y;
    f(&z);
}
The reason lies in the second if Has been parenthesized “ encapsulation ” up .
There are some C Programmers can achieve a similar effect by using macro definitions ：
#define IF {if(
#define THEN ){
#define ELSE }else {
#define FI}}
such , In the previous example C The program can be written as ：
IF x == 0
THEN IF y == 0
    THEN error();
    FI
ELSE z = x + y;
f(&z);
FI 
amount to
{
    if(x == 0)
    {
        {
            if(y == 0)
            {
                error();
            }
        }
    }else 
    {
        z = x + y;
        f(&z);
    }
}

Another way is for everyone if The statement is equipped with a else, So it won't show up “ Hang ”else The problem of , But it will increase the difficulty of reading .