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C Basics

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References and Dereferences

Pointers are built around two operators:

  • The address-of operator &.

  • The dereference operator *.

Address-of Operator

The address-of operator, represented by &, allows us to directly interact with our computer's RAM. Using & lets you obtain the actual memory address of an object.

Main.c

Main.c

copy
#include <stdio.h>

int main()
{
int x = 100;

printf("Value of variable: %d\n", x);

printf("Address of variable into RAM: %p\n", &x); // using reference operator `&` for getting address

return 0;
}

              
123456789101112

            
#include <stdio.h>

int main()
{
    int x = 100;

    printf("Value of variable: %d\n", x);

    printf("Address of variable into RAM: %p\n", &x); // using reference operator `&` for getting address

    return 0;
}

Note

%p is the format specifier used for addresses (pointer).

Addresses are typically expressed in hexadecimal notation.

Think of the & operator as identifying your home's address using your name.

Dereference Operator

Conversely, the * operator gives you the resident's name when given their address. So, how can we employ this operator if we're not directly dealing with addresses? If you have an expression like &x, which returns the address of the x variable, applying the * operator to it (*&x) gives you the value of the variable stored at that address.

Note

Essentially, *&x is the same as x.

Main.c

Main.c

copy
#include <stdio.h>

int main()
{
int x = 100;

printf("Value of variable: %d\n", x);

printf("Address of variable in RAM: %p\n", &x); // using reference operator `&`

printf("Dereferencing address of variable: %d", *(&x)); // using dereference operator `*`

return 0;
}

              
1234567891011121314

            
#include <stdio.h>

int main()
{
    int x = 100;

    printf("Value of variable: %d\n", x);

    printf("Address of variable in RAM: %p\n", &x); // using reference operator `&`

    printf("Dereferencing address of variable: %d", *(&x)); // using dereference operator `*`

    return 0;
}

Note

Don't mix up the dereference operator (*x) with the multiplication operator (x*y).

Opgave

Swipe to start coding

  • Create an integer array of 5 elements and populate it.
  • Retrieve the address of the third element.
  • Increment the address of the third element (i.e., address + 1).
  • Attempt to dereference the address obtained in the previous step.

Løsning

#include <stdio.h>

int main()
{
int array[] = { 1, 2,3, 4, 5 };

printf("Value of third element %d\n", array[2]);

printf("Address of third element %p\n", &(array[2]));

printf("(address-of-third-element) + 1: %d\n", *((&array[2]) + 1));

return 0;
}

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Sektion 6. Kapitel 2
single

single

#include <stdio.h>

int main()
{
int array[] = { ___, ___, ___, ___, ___ };

printf("Value of third element %d\n", array[2]);

printf("Address of third element %p\n", ___(array[2]));

printf("(address-of-third-element) + 1: %d\n", ___((___array[2]) + 1));

return 0;
}

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