c: Style: altering variables in place

Question

Suppose we have something along these lines

int f(int n);

....

do{
    int a = b;
    int b = f(a);
}

Is there any risk to saying

do{
    int b = f(b);
}

instead, assuming a is not used elsewhere? Would it be stylistically preferable to do the former?

Answer 1

It is important to understand that you're not "altering" any variables here, just changing the meaning of a name.

The important concept is called the point of declaration.

Let's look at your example:

int b = 42;
{ 
   int a = b; 
   int b = 23 * a;
   printf("%d\n", b);
}
printf("%d\n", b); 

or

int b = 42;
{ 
   int b = 23 * b;
   printf("%d\n", b);
}
printf("%d\n", b); 

The name b references two objects, depending on where you are in the code, let's call them b1 and b2. This is unsurprising in case 1:

int b1 = 42;
{ 
   int a = b1; 
   int b2 = 23 * a;
   printf("%d\n", b2);
} 
printf("%d\n", b1); 

Since in C the point of declaration is before the initializer, the second example resolves differently:

int b1 = 42;
{ 
   int b2 = 23 * b2; 
   printf("%d\n", b2);
} 
printf("%d\n", b1); 

Note that here there's no link between b1 and b2, you're initializing b2 with its own (undefined) value. A diligent compiler will warn you about this, like

x.c:7:13: warning: 'b' is used uninitialized in this function [-Wuninitialized]