Thread: pointer question

Hi ,

I am still confused about pointers. I have a simple function ,that does some math calculations, called

void something( int Y, double *Z)
{
Y = Y + Z }

I want to call it in main, but am confused as to how the *Z should be called

I know it can't be called like :

something (3, 4);

it gives an error of:
"passing double to argument 2 of something(int, double *) lacks a cast"

But, I am unclear of how casting works and what its for, and not sure how this "double *Z" argument works.

Thanks for any explanations!!! It is appreciated.

FYB, you forgot a '&':

Code:

void something(int *X, int Z)
{
*X = *X + Z;
}


int x = 1;
int y = 2;
something(&x,y);

Sorry about that,

the function is essentially:

void something(int X, double *Z) {

X = X + (*Z) }


thanks ...I tried what you both suggested and it did work. I am still fairly unfamilar with pointers and trying to figure out how this function works.

I'm sorry for the simple question, but could you explain how the function and the call in main are working, in reference to the double *Z only??

Thanks for your help and explanation!!

am still confused about pointers. I have a simple function ,that does some math calculations, called

Code:

void something( int Y, double *Z) {
  Y = Y + Z 
}

you're passing an int and a double pointer to something(). To access what Z points to, dereference it:

Code:

void something (int Y, double* Z) {
  Y = Y + *Z;
}

The problem here is that Y is a new copy in the something function. Y will disappear when your function exits. To use another int from before this function, use a reference:

Code:

void something (int& y, double* z) {
  y = y + *z;
}

You might as well use a reference for z, too, since there's no reason to prefer a pointer here.

Code:

void something (int& y, double& z) {
  y += z;
}

a += means add what's on the right to the left variable. it's shorter, so people like it over y = y + z.

A pointer is an address. Your function is expecting an address, not a constant like "4". In general, you don't know or care what the address is, so you use the & operator. And, it's safer to let the computer figure it out.

The error message:
Casting is short for typecasting. It means converting one variable type to another. i.e converting a char to an int. In this case, the compiler is telling you that it can't automatically change the constant 4 into a pointer for you.

Compilers often get confused because they don't know what you actually intended. So, sometimes the error messages are misleading.

Thanks for the explanations!!, I understand pointers a bit more

If Mary has the answers to the homework, she's a normal variable.

If Mary's a pointer, then she doesn't have the answers. She has the address of the person with the answers. You can use Mary= to get the address. Or, you can use *Mary= to make her bring you the answer.

One use for pointers...
I would say that pointers are most often used when you want get around the fact that functions can only return one value. If you pass-in pointers to variables, then you can change the actual variables. For example, functions that affect character strings use pointers.