[9fans] C question - completely OT, but I'd like to know the answer

[9fans] C question - completely OT, but I'd like to know the answer

[Winston Kodogo]

Sorry to bother the list, but I thought I might get a sensible answer
here from the few remaining people in the world who actually
understand C.

The following bit of code seems to be more or less syntactically OK:

switch (nurdge)
{
    int nigel = 1;
    case 0:
		if(nigel == 1)
			printf("nigel is one.\n");
		else
			printf("nigel is not one.\n");

	default:
		printf("The value of nigel is %d", nigel);
}

Something close to this compiles under my C compiler, and yet the
variable "nigel" is not initialised, and the test inside the first
case test is pretty much certain to print "nigel is not one". Although
my C++ compiler does complain about an uninitialised variable.

I'm puzzled as to why the line "int nigel = 1;" is syntactically OK,
and although it seems to have declared the variable "nigel" - else the
following code would fail to compile - has failed to give it the
initial value of 1, as requested.

Re: [9fans] C question - completely OT, but I'd like to know the answer

[dexen deVries]

On Thursday 08 of September 2011 11:41:05 Winston Kodogo wrote:
> (...)
> I'm puzzled as to why the line "int nigel = 1;" is syntactically OK,
> and although it seems to have declared the variable "nigel" - else the
> following code would fail to compile - has failed to give it the
> initial value of 1, as requested.

consider the `switch' statement a switchboard that jumps to one of the 
`case's.

nigel is not getting initialized because this code path is not executed -- as 
it is not part of any `case' taken by the switch. 

an `automatic' (non-static) variable is initialized by a piece of code, the 
code gets executed whenever flow of control reaches that particular place.

a static variable (`static int inigel = 1') is initialized by static data, 
before any code gets executed -- and only once. static will do what you 
expect.

this is why you can place a static variable outside of any function and it 
still gets initialized.

both kinds have their uses, in different situations.

for reference:

for (int i = 0; i < 8; ++i) {
  int nigel = 1; // this emits some code that gets executed for every loop and 
resets the var to `1'
  printf("nigel: %d,", nigel);
  ++nigel;
}

will print: nigel: 1,nigel: 1,nigel: 1, ...

on the other hand: 

for (int i = 0; i < 8; ++i) {
  static int nigel = 1; // this var is initialized just once before main() 
begins and doesn't reset
  printf("nigel: %d,", nigel);
  ++nigel;
}

will print: nigel: 1,nigel: 2,nigel: 3, ...


-- 
dexen deVries

[[[↓][→]]]

For example, if the first thing in the file is:
   <?kzy irefvba="1.0" rapbqvat="ebg13"?>
an XML parser will recognize that the document is stored in the traditional 
ROT13 encoding.

(( Joe English, http://www.flightlab.com/~joe/sgml/faq-not.txt ))

Re: [9fans] C question - completely OT, but I'd like to know the answer

[dexen deVries]

On Thursday 08 of September 2011 11:41:05 Winston Kodogo wrote:
> (stuff with switch/case)

btw., the semantics of switch/case in C are quite elastic; there's a neat use 
of it:

http://en.wikipedia.org/wiki/Duffs_device

do/while interleaved with switch/case.
 
-- 
dexen deVries

[[[↓][→]]]

For example, if the first thing in the file is:
   <?kzy irefvba="1.0" rapbqvat="ebg13"?>
an XML parser will recognize that the document is stored in the traditional 
ROT13 encoding.

(( Joe English, http://www.flightlab.com/~joe/sgml/faq-not.txt ))