Pointers

1. Pointers

1.1. Definition

Pointers store the address of variables or a memory location.

1.2. To Use Pointers in C, We Must Understand These Two Unary Operators (* and &)

• To declare a pointer variable, we must use the unary operator * (asterisk) before its name.

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#include<stdio.h>

int main()
{
    int x = 10;
    int *prt;

    /* The & operator before x is used to get
     * the address of x and then this address
     * is assigned to ptr. */
    ptr = &x;

    return(0);
}

• To access the address of a variable in a pointer, we use the unary operator & (ampersand) that returns the address of the variable it is pointing to. For example, &x gives us the address of the variable x.

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#include<stdio.h>

int main()
{
    int x;

    /* Prints the address of x */
    printf("%p", &x);
    return(0);
}

• To access the value stored at the address, we use the unary operator * (asterisk), which returns the value of the variable located at the address specified by its operand.

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#include<stdio.h>

int main()
{
    int x = 10;
    int *ptr = &x;

    printf("Value of x = %d\n", *ptr);
    printf("Address of x = %p\n", ptr);

    *ptr = 20;
    printf("After doing *ptr = 20, now *ptr is %d\n", *ptr);
    printf("After doing *ptr = 20, now x is %d\n", x);

    return(0);
}

Please refer to the picture below to gain a better understanding of what this code is doing.

1.3. Pointer Expressions and Pointer Arithmetic

A limited set of arithmetic operations can be performed on pointers. A pointer may be:

• incremented (++)
• decremented (–)
• An integer may be added to a pointer (+ or +=)
• An integer may be subtracted from a pointer (- or -=)

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#include<stdio.h>

int main()
{
    int v[3] = {10, 20, 30};

    int *ptr;
    ptr = v;

    for (int i = 0; i < 3; i++) {
        printf("Value of *ptr = %d\n", *ptr);
        printf("Value of ptr = %p\n", ptr);
        ptr++; /* Increments pointer ptr by 1 (the size of an integer = 4) */
    }
}

1.4. Array Name as Pointers

An array name acts like a pointer constant. The value of this pointer constant is the address of the first element. For example, if we have an array named v, then v and &v[0] can be used interchangeably.

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#include<stdio.h>

int main()
{
    int v[3] = {10, 20, 30};
    int *ptr;
    ptr = v;  /* The same as ptr = &v[0]; */

    printf("The elements of the array are: %d - %d - %d\n", ptr[0], ptr[1], ptr[2]);
}

1.5. Pointers and Multidimensional Arrays

A pointer notation for the two-dimensional numeric arrays, consider the following declaration

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int nums[2][3]  =  { {16, 18, 20}, {25, 26, 27} };

In general, nums[i][j] is equivalent to *(*(nums+i)+j)