How &a is a pointer to a if it generates the address of a?

Question

I'm new to programming, and at present I am learning about pointers in C.

1. I know that pointers are the variables which contain or hold address of another variable. Today when I was more learning about it with K&R I got confused in a line which says "&a is a pointer to a" in a given function swap(&a,&b) in Page No. 80. How &a is a pointer? It is not a variable, it is the address of the variable. Am I right?
2. I know that arguments can be passed to a function in two ways: call by value and call by reference. The caller's value of the argument is not changed in the first one, but it can be changed in the second one.

My question is I had read that if we want to change the value of the variable we have to pass the pointer to it (i.e. the location of the value we want to modify). What is meant by that? I mean, do we have to pass pointers to the function? And what is the meaning of the statement, "we have to pass the pointers to the location we want to modify".

Answer 1

A pointer is not a variable. A pointer is a value.

A variable, in C, designates a storage location, and a value can be stored in that location. Thus, if you have a variable a declared with int a, then a is a variable in which an integer value can be stored. If you have a variable int *x, then x is a variable in which a pointer to an integer value can be stored.

The address of a storage location can be obtained using the & operator. E.g., &a is the address of the storage location designated by a, or the address of a, and can be stored in (among other things) a variable of the corresponding type. Thus you can have:

int  a = 42;  /* a is a variable of type int,  and has value 42 */
int* x = &a;  /* x is a variable of type int*, and has value &a */

Although analogies in programming are often dangerous, you might think of things like page numbers in a book. The page number of a page is not the same thing as the page, but page numbers can still be written down on pages. E.g., the table of contents page has lots of page numbers written down on it. Thus:

actual_page p = ...; /* a page structure */
page_number n = &p;  /* a page number    */

Answer 2

A pointer is an address. A pointer variable is a variable holding an address.

It is perhaps analogous to the difference between a literal integer 1 and an integer variable int a. One might refer to both a and 1 as integers, just as you might refer to &a and int* p as pointers.

Note also that &a is not an lvalue and cannot be assigned - so it is as you say not a variable, but it is a pointer nonetheless, and can itself be assigned to a pointer variable.

Answer 3

If a is a variable of some type T, then &a is an expression which evaluates to the address of that variable, also known as a pointer to that variable—the terms are interchangeable. &a has the type T*, which is pronounced “pointer to T”.

int x = 4;   // 4 has type int, so we can assign it to an int variable.
int *p = &x; // &x has type int*, so we can assign it to an int* variable.

The reason that you can modify function arguments passed by pointer is that a pointer introduces a sort of alias for a variable. You can alter it from multiple locations, either directly (via the variable) or indirectly (via the pointer).

// modifying x modifies *p.
++x;
printf("%d %d\n", x, *p);

// modifying *p modifies x.
++*p;
printf("%d %d\n", x, *p);

Answer 4

As others have observed, a pointer is a particular kind of value, not a particular kind of variable.

With regard to calling functions:

1. You cannot "pass a variable to a function" in C. If you put a variable name in the argument list of a function call then the value stored in that variable is passed, not the variable itself.

2. Although in a general sense programming languages may provide for passing function arguments either by value or by reference, C provides only pass by value. You can emulate pass by reference in C, however, by passing a pointer (value) to the location where another value is stored. That's what swap(&a, &b) does: you are passing the locations where the values of variables a and b are stored (that is, pointers to those values).

Answer 5

How &a is a pointer to a if it generates the address of a?

The terms pointer and address in C are synonymous.

When you create a variable:

int a = 0; 

It is created in memory at a specific memory location, or address, with sufficient memory to hold 1 int.

&a does not generate the address, it simply provides the address.

The & operator can be used to set the address of a pointer to a specific location, such as this:

int a = 0;//a now exists in memory at a specific location;
int *b = {0}; // b is created in memory as a pointer, and can be assigned a location

b = &a; //b is assigned the location (address) of the variable a  

Stated a little differently, in the previous line, & is referred to as the address-of operator, so:

b = &x; Can be Read: Assign to b (a pointer) the address of a.

Answer 6

1. Pointer contains the address - so synonymous.
2. When you pass by reference, you are passing in the address - to directly modify the value.

Answer 7

A pointer is a type of variable that stores the address to an object.

Basically, the pointer is the address.

Think of it as a piece of paper. When it has a number printed on it, it's an integer (or other numeric type).
A pointer is a piece of paper that says "the data is on the peice of paper in location x", where "location x" is the address of the object.