Thread: Initialising pointers

Hi, My question is this:

The code

int a=0;
printf("%p", &a);

shows on the screen the memory address of variable a, so why can't I assign this value to another variable -

int a=0;
int x=0;
x=&a;

If &a gives a raw number as it does in the printf command, why can't x take the value of this number? The code works if I use *x=0, but I can't see why it needs that.

Thanks for any help.

ok when saying int x = &a is like saying let the integer point to the pointer of an integer so it's not of the same type but when you say int * x . you are saying that x is a pointer to an integer so when saying *x = &a is like saying let the pointer x point to the integer a . hope that makes it clearer for you .
Form what I've "learned" & and * are the same thing they are both used for pointers -> is also equivelent to *(&) ( I think )

Originally Posted by quzah

Code:

int *x;
int a;

x = &a; /* assign the address of a to the pointer x */

*x = &a; /* dereference x, giving you a, and assign it the address of a */

Clearly the second one is incorrect.

The second one is lacking a cast but it is certainly not incorrect. What is incorrect about setting a variable to hold its own address? I mean, is it useful? Who knows But it is not "wrong". To say

*x = (int)&a;

simply means to store the address of a (as an int) into whatever x is pointing at, in the above example it is pointing at a, thus the end result would be to store the address of a as an integer in a.


Mezzano

Originally Posted by Salem

> The second one is lacking a cast but it is certainly not incorrect
If you want to ride rough-shod all over the type system then go ahead, but you'll get no help from the compiler (or anyone else) if you've used so many casts as to make any kind of diagnostics ineffective.

Why even have the ability to cast at all then, lets just take it out it is obviously useless.

Originally Posted by Salem

> But it is not "wrong"
By that, you mean so long as it works on your current compiler, then it's just fine and dandy I suppose.
Surely you should know by now that the "works for me" defence is no argument on this board, just like other brainless ideas like "void main", "fflush(stdin)" and other atrocities.

No it was more, it works on every compiler I have ever used. Of course if you want to assume the OP was planning on running this on some bizarre system which didn't have pointers being the natural word size of the machine, like ints generally are, then you are right. There is a difference between strict adherence to the standard and reasonable adherence. You are correct if you are in the camp of "I never write a single line of code that doesn't adhere 100% to the ISO C standard", you are also probably not writing any interesting code, even threading isn't covered in the standard so lets not talk about it either, I mean who would ever want to do that.


Mezzano

Hey Mezzano,

Thanks for your help! Your idea really helped me a lot!

mw

quzah,

I'm new to programming in C, and I don't understand the point you were trying to make in your last post about "casting not doing jack" in the example you showed. Could you elaborate on how your code works please?

I understood the part about the pointer size exceeding the variable size, & that would obviously create a problem.

Thanks for your input!

mw

Thanks for the replies.

I am still confused. &a just returns a simple number (which happens to be a memory address) right, so why can't I assign that number to an ordinary integer variable?

Originally Posted by patch1024

int a=0;
int x=0;
x=&a;

If &a gives a raw number as it does in the printf command, why can't x take the value of this number? The code works if I use *x=0, but I can't see why it needs that.

Thanks for any help.

hey, pointers giving you hell as well?? yeah well your not the only one around here.

i myself still don't have a firm grasp of it all. but what i think i do know is that pointers are used because when you have several functions etc, you don't want to keep returning some variable(s) becuase this takes up memory. so you use pointers.

quick run down on pointers:
the '&' ampersand. when couple with a variable like '&b'. this gives us the address of b.

the '*' or otherwise known as dereferencing. if we were to write *a. then the '*' gives us access or allows us to change whatever *a is pointing to.

so if you put these two together like so:
a = &b //this assigns the address of b to a.
then later you call a function you use *a to gain access to b. and b could equal to some other number 4. but once *a changes so does b.

say if you want to add two numbers together and write a main with int x, y and the functionname, then you make it print the answer. if you do it the normal way without pointers, sure you can do it that way if you want. but after the user input, you will have to return the variable which is the sum of x, y. then your going to have to create another variable z. the function returns the value of z back into main, then you can either make it equal something else (in this case 'ans') or you can print it like so: printf("%d", mfunction(x,y)); e.g. sorry if you try to run it, i'm not sure why it's running twice.

Code:

int mfunction(int x, int y)
{
    int z;
    
    printf("enter number x: ");
    scanf(" %d", &x);
    printf("enter number y: ");
    scanf(" %d", &y);
    
    z = x + y;
    
    return z;
}


main()
{
    int x, y, ans;
    
    mfunction(x,y);
    ans = mfunction(x,y);
    printf("%d", ans);
    
    return 0;
}

ok now we'll try with pointers it'll be about the same but with less variables

Code:

#include <stdio.h>

void mfunction(int *x, int *y)
{
      
    printf("enter number x: ");
    scanf(" %d", x);
    printf("enter number y: ");
    scanf(" %d", y);
                
}


main()
{
    int x, y, ans;
    
    mfunction(&x,&y);
    ans = x+y;
    printf("%d", ans);
    
    return 0;
}

hope this helps.. sorry if it's not what you want and if i've said any crap please correct me!