Pointer Arithmetics

You previously learned what is a pointer and how to manipulate pointers. In this tutorial you will be learning the arithmetic operations on pointers. There are multiple arithmetic operations that can be applied on C pointers: ++, --, -, +

Incrementing a Pointer with (++)

Just like any variable the ++ operation increases the value of that variable. In our case here the variable is a pointer hence when we increase its value we are increasing the address in the memory that pointer points to. Let's combine this operation with an array in our example:

    #include <stdio.h>

    int main()
    {
        int intarray[5] = {10,20,30,40,50};
    
        int i;
        for(i = 0; i < 5; i++)
            printf("intarray[%d] has value %d - and address @ %x\n", i, intarray[i], &intarray[i]);
    
        int *intpointer = &intarray[3]; //point to the 4th element in the array
        printf("address: %x - has value %d\n", intpointer, *intpointer); //print the address of the 4th element
    
        intpointer++; //now increase the pointer's address so it points to the 5th elemnt in the array
        printf("address: %x - has value %d\n", intpointer, *intpointer); //print the address of the 5th element
    
        return 0;
    }


                        

Decreasing Pointer with (--)

Just like in our previous example we increased the pointer's pointed-to address by one using the ++ operator, we can decrease the address pointed-to by one using the decrement operator (--).

    #include <stdio.h>

    int main()
    {
        int intarray[5] = {10,20,30,40,50};
    
        int i;
        for(i = 0; i < 5; i++)
            printf("intarray[%d] has value %d - and address @ %x\n", i, intarray[i], &intarray[i]);
    
        int *intpointer = &intarray[4]; //point to the 5th element in the array
        printf("address: %x - has value %d\n", intpointer, *intpointer); //print the address of the 5th element
    
        intpointer--; //now decrease the point's address so it points to the 4th element in the array
        printf("address: %x - has value %d\n", intpointer, *intpointer); //print the address of the 4th element
    
        return 0;
    }


                        

Adding Pointers with (+)

We previously increased a pointer's pointed-to address by one. We can also increase it by an integer value such:

    #include <stdio.h>

    int main()
    {
        int intarray[5] = {10,20,30,40,50};
    
        int i;
        for(i = 0; i < 5; i++)
            printf("intarray[%d] has value: %d - and address @ %x\n", i, intarray[i], &intarray[i]);
    
        int *intpointer = &intarray[1]; //point to the 2nd element in the array
        printf("address: %x - has value %d\n", intpointer, *intpointer); //print the address of the 2nd element
    
        intpointer += 2; //now shift by two the point's address so it points to the 4th element in the array
        printf("address: %x - has value %d\n", intpointer, *intpointer); //print the addres of the 4th element
    
        return 0;
    }


                        

Note how in the output the address shifted by 8 steps in the memory. You might be wondering why? The answer is simple: Because our pointer is an int-pointer and the size of an int variable is 4 bytes the memory is shift-able by 4 blocks. In our code we shifted by 2 (added +2) to the initial address so that makes them 2 x 4 byte = 8.

Subtracting Pointers with (-)

Similarly we can subtract:

    #include <stdio.h>

    int main()
    {
        int intarray[5] = {10,20,30,40,50};
    
        int i;
        for(i = 0; i < 5; i++)
            printf("intarray[%d] has value: %d - and address @ %x\n", i, intarray[i], &intarray[i]);
    
        int *intpointer = &intarray[4]; //point to the 5th element in the array
        printf("address: %x - has value %d\n", intpointer, *intpointer); //print the address of the 5th element
    
        intpointer -= 2; //now shift by two the point's address so it points to the 3rd element in the array
        printf("address: %x - has value %d\n", intpointer, *intpointer); //print the address of the 3rd element
    
        return 0;
    }


                        

again the address is shifted by blocks of 4bytes (in case of int).

Other Operations

There are more operations such as comparison >, <, ==. The idea is very similar of comparing variables, but in this case we are comparing memory address.