10 November 2015Birkbeck College, U. London1 Introduction to Computer Systems Lecturer: Steve Maybank Department of Computer Science and Information Systems Autumn 2015 Week 7a: Pointers and Linked Lists

Review: Arrays An array is a block of values of the same type. Eg. A 2D array of size 10x November 2015Birkbeck College, U. London2

Review: Array Indexing There is a standard way of referring to the entries in an array. Eg. in Python: A[0], A[9], B[12,1]. In 2D arrays, the order of the indices is row then column. 10 November 2015Birkbeck College, U. London3

Three Dimensional Arrays If C is a 3D array of size 10x10x10, if each entry occupies one memory cell and if C[0,0,0] is stored at x, then C[i, j, k] is stored at x+100*i+10*j+k Thus C[2,5,1] is stored at x+100*2+10*5+1 = x November 2015Birkbeck College, U. London4

10 November 2015Brookshear, Sections 8.2 and 8.75 Pointers  A pointer is a storage area containing the address at which a piece of information is stored.  Example: the program counter in the CPU The programme counter points to memory cell 207 which contains the value 60. programme counter

10 November 2015Brookshear, Section 8.26 Why are Pointers Useful? Each pointer contains a relatively small number of bits. It is easier to move or copy pointers rather than move or copy the data they point to.

10 November 2015Birkbeck College, U. London7 Example: Sorting AZM AZM Array of pointers Data in memory Data in memory Sorted array of pointers

10 November 2015Brookshear, Section 8.28 Example: Alternative List … A Farewell to Arms Hemingway Pointer … For Whom the Bell Tolls Hemingway Pointer … The Sun Also Rises Hemingway Pointer …

10 November 2015Brookshear, Sections 8.1 and 8.39 Lists A list is a collection of data whose entries are arranged sequentially. Example of a list with 4 entries: addressphotodatetel. The entries vary widely in size.

10 November 2015Brookshear, Section Contiguous Storage of a List List entries are stored consecutively in memory Advantage: simple Disadvantage: insertion and deletion are difficult.

10 November 2015Brookshear, Section Linked Lists  Each list entry contains a pointer to the next entry  List entries can be stored in any part of memory.  There is a special head pointer for the first entry and a Nil pointer in the last entry.

10 November 2015Brookshear, Section Example of a Linked List photo86address7date87telNil Head

10 November 2015Brookshear, Section Deletion  Find the preceding list item f1 and the following list item f2. Set f1.pointer=f2 datapointer datapointer datapointer f1 f2 deleted entry new pointer old pointer

10 November 2015Brookshear, Section Insertion  to insert after a list item f1 set newEntry.pointer = f1.pointer f1.pointer = location of newEntry datapointer datapointer datapointer new pointer old pointer f1 newEntrynew pointer

10 November 2015Birkbeck College, U. London15 Pseudocode for Pointers Assign the value 4 to x: f1.x = 4 Point to the next item: f1.next = f2 pointer f1 variable xpointer next

10 November 2015Birkbeck College, U. London16 Printing a Linked List f = head(L); While (f<>nil) print(f.data); f = f.next; EndWhile

10 November 2015BB Ch. 8 Review Problems No. 717 Example 11C 12 13G 14 15E 16 17B 18 19U 20 21F 22 address contents The table represents the contents of some cells in memory, along with the address of each cell. Place addresses in the empty cells such that each cell containing a letter together with the following cell form an entry in a linked list in which the letters appear in alphabetical order. What address should the head pointer contain?

Example of a Tree 10 November 2015Birkbeck College, U. London18 S N VP V NP John hit DN the ball S: sentence NP: noun phrase VP: verb phrase N: noun V: verb D: determiner Colour: data Tree: hidden structure

Binary Tree Each node has the form 10 November 2015Brookshear, Sections 8.3, dataleft pointerright pointer A B C D

Binary Tree Stored in Memory A3339B36nilC D 10 November 2015Birkbeck College, U. London