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OrbWorks Community Forum • View topic - I don't get pointers
Page 1 of 1

PostPosted: Sun Jun 18, 2006 6:36 pm
by christaylor
What am I doing wrong? How come when main passes the two pointers to the printit function it doesn't remember what the setit function set the values of x and y to? It just prints zeros like they're undefined.

Code...

printit (pointer a, pointer b) {
puts(*a + " " + *b + "\n");
}

setit(pointer a, pointer b) {
int x, y;
x = y = 1;
a = &x;
b = &y;
}

main (){
pointer a, b;
setit(a,b);
printit(a,b);
}

...end code

PostPosted: Sun Jun 18, 2006 7:40 pm
by mgreen
You're mixing up pointers and integer values and trying to use locally defined variables that don't exist when you actually use them. C always passes parameters "by value". There are no "reference" parameters. If you want to pass the address of a variable, you have to pass "&v" rather than "v". Also, variables declared locally within a function <u>do not exist</u> when the function returns. Your code might look like (printit is ok):

setit(pointer a, pointer b) {
*a = 1
*b = 1
}
main() {
int x, y;
setit(&x,&y);
printit(&x,&y);
}

PostPosted: Tue Jun 20, 2006 2:25 am
by christaylor
Thanks for your response. So when x an y are declared as ints in main() and passed to the other functions in turn, does their type get changed to type 'pointer'? I'm wondering because you just pass them as parameters and don't have to declare them in the other functions.

PostPosted: Tue Jun 20, 2006 1:35 pm
by mgreen
That's a good question. In PocketC (as opposed to Standard C), values carry their types with them. This means that the interpreter can tell if a value is an int or a string or a float. In the code I posted, notice that I'm passing the "address" of the variables using "&" (in other words - a pointer value). This is compatible with a pointer variable. When I reference the underlying variable using "*", the interpreter can look at the 'tag' associated with the variable and determine the type of the variable.

PostPosted: Thu Jun 22, 2006 4:26 am
by christaylor
Thanks for the good information!