Recall: using pointer arithmetic to access array elements

  • After setting p = &a[0]: 

       *(p+0)   ≡   a[0]
   *(p+1)   ≡   a[1]
   *(p+2)   ≡   a[2]
   and so on

    Schematically:

An alternate way to process array elements using pointer arithmetic

  • Consider the situation after assigning p = &a[0] and *p = a[0]

  • Question:

      • What is the situation after the statement:   p = p + 1; ?

An alternate way to process array elements using pointer arithmetic

  • After the statement p = p + 1;:   p = &a[1] and *p = a[1]

  • Question:

      • What is the situation after the statement:   p = p + 1; ?

An alternate way to process array elements using pointer arithmetic

  • After the statement p = p + 1;:   p = &a[2] and *p = a[2]

  • And so on:

      • We can use p = p + 1; to traverse an array !!

An alternate way to process array elements using pointer arithmetic

  • Consider the traditional way to tarverse an array: 

    int main(int argc, char *argv[])
{   
    int a[10];


    for (int i = 0; i < 10; i++ )
        a[i] = i;

    // Traditional way to traverse an array


    for ( int i = 0; i < 10; i++ )
    {
        printf("&d\n", a[i]);

    }
}

An alternate way to process array elements using pointer arithmetic

  • Using pointer arithmetic, we can also traverse an array using a pointer variable p: 

    int main(int argc, char *argv[])
{   
    int a[10];
    int *p;

    for (int i = 0; i < 10; i++ )
        a[i] = i;

    // Traversing an array using pointer arithmetic

    p = a;
    for ( int i = 0; i < 10; i++ )
    {
        printf("&d\n", *p);
        p = p + 1;   // Move to the next element in array
    }
}

DEMO: demo/C/set2/pointer-arithm3a.c
Note:   The traversal code can be shorted using a short hand operator !!

Review: the post-increment ++ operator

  • Review: post increment operator: 

     p++     1. return value = p (= the "old" value in p)
         2. p = p + 1  (increment p afterwards)

  • Example: 

    int main(int argc, char *argv[])
{
   int x = 4;

   printf("%d\n", x++);    // Prints 4 (new value of x = 5)
   printf("%d\n", x++);    // Prints 5 (new value of x = 6)
   printf("%d\n", x++);    // Prints 6 (new value of x = 7)
   printf("%d\n", x++);    // Prints 7 (new value of x = 8)
}

DEMO: demo/C/set2/post-inc.c

An alternate way to process array elements using pointer arithmetic   - Revisited

  • Using pointer arithmetic, we can also traverse an array using a pointer variable p: 

    int main(int argc, char *argv[])
{   
    int a[10];
    int *p;

    for (int i = 0; i < 10; i++ )
        a[i] = i;

    // Traversing an array using pointer arithmetic

    p = a;
    for ( int i = 0; i < 10; i++ )
    {
        printf("&d\n", *p);  // Use the value in p
        p = p + 1;   // We incremented p after using the value in p
    }
}

Note:   The traversal code can be shorted using a short hand operator !!

An alternate way to process array elements using pointer arithmetic   - Revisited

  • Because we increment p = p + 1 after using *p, we use the post-increment p++ operator: 

    int main(int argc, char *argv[])
{   
    int a[10];
    int *p;

    for (int i = 0; i < 10; i++ )
        a[i] = i;

    // Traversing an array using pointer arithmetic

    p = a;
    for ( int i = 0; i < 10; i++ )
    {
        printf("&d\n", *(p++)); // Use p and afterwards increment p
           
    }
}

We can simplify the *(p++) expression because the ++ operator has a higher priority than the * operator ()

An alternate way to process array elements using pointer arithmetic   - Revisited

  • The expression *(p++) can be simplified to *p++ 

    int main(int argc, char *argv[])
{   
    int a[10];
    int *p;

    for (int i = 0; i < 10; i++ )
        a[i] = i;

    // Traversing an array using pointer arithmetic

    p = a;
    for ( int i = 0; i < 10; i++ )
    {
        printf("&d\n", *p++);   // Use p and afterwards increment p
                   
    }
}

DEMO: demo/C/set2/pointer-arithm3b.c