Pointers to Pointers and Array of Pointers · The main advantage of arrays of pointers is their...

Chapter 15:

Pointers to Pointers and Array of Pointers

int myVar;

int *ptr;

myVar = 12;

ptr = &myVar;

int myVar;

int *ptr;

int **ptr_to_ptr;

myVar = 12;

ptr = &myVar;

ptr_to_ptr = &ptr;

int myVar;

int *ptr;

int **ptr_to_ptr;

myVar = 12;

ptr = &myVar;

ptr_to_ptr = &ptr;

“The two worlds” become “The three worlds”!

& is the “escalator up” * is the “escalator down”

ptr_to_ptr

ptr

myVar

Your turn:

Important application of p2p: Two-dimensional arrays

int a[3][4];

Important application of p2p: Two-dimensional arrays

int a[3][4];

What is common to all elements in a row?

Two-dimensional arrays

int a[3][4];

a[0]

a[1]

a[2]

Each row is an array!

Two-dimensional arrays

int a[3][4];

a[0]

a[1]

a[2]

This makes the matrix a an array of arrays

a

a[0]

a[0][1]

a[1] a[2]

a[1][3] a[2][0]

a[2][3] a[2][1]

ptr_to_ptr

ptr

myVar

Example:

Source: http://www.tutorialspoint.com/cprogramming/c_multi_dimensional_arrays.htm

Source: http://www.tutorialspoint.com/cprogramming/c_multi_dimensional_arrays.htm

Output:

A pointer is a pointer is a pointer

Using sizeof with multi-dimensional arrays

EOL 0.5

QUIZ:

What is a 2D array in C?

QUIZ:

What is a 2D array in C?

A: An array of arrays!

QUIZ:

What is the name of the 2D array?

QUIZ:

What is the name of the 2D array?

A: A pointer to a pointer!

Pointer arithmetic with multi-dim arrays

The elements of multi are one-dimensional arrays of 4 integers

As with one-dim arrays, the name of a two-dim array is a constant

pointer

How can we create a copy of this pointer-to-pointer that we can

change?

Read from right to left (while starting inside the parenthesis!)

int (* p2p)[4];

p2p is a pointer to an array of 4 integers

Read from right to left (while starting inside the parenthesis!)

int (* p2p)[4];

p2p = multi;

p2p++;

If the array multi starts at address 1000 (decimal), what is p2p at the

end of the code below?

int (* p2p)[4];

p2p = multi;

p2p++;

Your turn:

Your turn:

Declare a pointer ppf to an array of 17 floats.

• What is the data type for num, ptr, and ptr2ptr?

• What is printed?

Source: http://www.c4learn.com/c-programming/c-double-pointer/

Can we do ptr2ptr = &&num ?

Source: http://www.c4learn.com/c-programming/c-double-pointer/

Your turn:

• Declare an array a of 3x5 characters.

• Copy the array name into a user-defined pointer-to-pointer b.

• Use b and pointer arithmetic to display the last element on the second row of a.

Not in text: How to declare a 2D array w/malloc()

Zooming in on next slide

Here we’re only allocating memory for the first-level array!

When passing a 2D array to a function, we’re passing a p2p

Like 1D arrays, 2D arrays are passed by reference, not by value!

• Listing 15.4 in our text gives two different examples of passing 2D arrays to functions, but they’re less general:

• The number of columns (4) is hard-coded in the functions’ headers.

• Read and understand Listing 15.4, but use the method in the previous slide for reference!

3D array example

0 1 2

0

1

Using p2p to create a new data structure: jagged arrays

Reference: R. Reese, Understanding and Using C Pointers p.102, O’Reilly, 2013.

When are jagged arrays really useful?

Reference: R. Reese, Understanding and Using C Pointers p.102, O’Reilly, 2013.

Example: jagged array of characters

Passing a jagged array to a function

The main advantage of arrays of pointers is their flexibility!

Example: Efficient sorting by reordering of pointers! (Listing 15.7)

To do for next time:

• Read and take notes: Up to Listing 15.7

• Exercises 1, 2, 3

• Enter in the IDE the program from Listing 15.7 and test it by entering only three strings of your choice