COMP 250. Lecture 4. Array lists. Sept. 15, 2017

Transcription

1 COMP 25 Lecture 4 Arra lists Set. 5, 27

2 Arras in Java int[ ] Ints = new int[5]; Ints[3] = -732; Arra whose eleents have a riitive te 2

3 Ints int[ ] Ints = new int[5]; Ints[3] = -732; 2 3 : :

4 Arras in Java Shae[ ] shaes = new Shae[428]; shaes[293] = new Shae( ); The sbol here corresonds to soe arguents that secif a shae. Arra whose eleents have a reference te 4

5 int[ ] Ints = new int[5]; Ints[3] = -732; Shae[ ] shaes = new Shae[428]; shaes[293] = new Shae( ); shaes Ints 2 3 : : : 293 : 427 5

6 The value of a reference variable is an address which secifies where an object is in the couter eor. We often reresent a reference with an arrow In the C rograing language, ou have access to that value and can aniulate it. In Java, ou have access to it but ou can t use it. Ints shaes 2 3 : : : 298 : 427 6

7 Arras have constant tie access A couter accesses an eleent in an arra in constant tie i.e. constant, indeendent of the length N of the arra.. = a[k] ; // read a[k] =. ; // write You will learn ore about how this works in COMP 26 and

8 Arras versus Arra Lists Arras can be used to ake lists, soeties called arra lists. Java has an ArraList class. 8

9 List An ordered set of eleents,,,,, is the nuber of eleents in the list, often called the size of the list. 9

10 What things do we do with a list? get(i) set(i,e) add(i,e) reove(i) reove(e) clear() iset() size() // Returns the i-th eleent (but doesn't reove it) // Relaces the i-th eleent with e // Inserts eleent e into the i-th osition // Reoves the i-th eleent fro list // Reoves first occurrence of eleent e // fro the list (if it is there) // Eties the list. // Returns true if et, false if not et. // Returns nuber of eleents in the list

11 Lists arra list (toda) singl linked list doubl linked list : next week

12 arra list of int arra list of Shae size = 7 length = size = 7 length = 2

13 Let s assue that the arra is a[ ]. How to ileent various oerations? get(i) { } if (i >= ) & (i < size) return a[i] size = 7 length = 3

14 set(i,e){ // relaces the object at index i } if (i >= ) & (i < size) a[i] = e e e.g. set(4, e) 4

15 set(i,e){ // relaces the object at index i } if (i >= ) & (i < size) a[i] = e e e.g. set(4, e) 5 e

16 add( i, e) Make roo b shifting, and then change reference. e.g. add(2, e) e 6

17 add( i, e) Make roo b shifting, and then change reference. e.g. add(2, e) e e

18 add( i, e) { // in the figure below, add( 2, e) if (i >=) & (i <= size){ for (j = size; j > i; j--) a[j] = a[j-] // shift (co) } 2 } size a[i] = e size = size + j u j u // relace value // increase nuber of eleents j j u u j u e 8

19 add( i, e) { // in the figure below, i = 2 if (i >=) & (i <= size){ for (j = size; j > i; j--) a[j] = a[j-] // shift (co) } } a[i] = e size = size + // relace value // increase nuber of eleents 2 size j u j u j u j u j u e 9

20 How to add an eleent to an arra list when arra is full? add( i, e) { // Create an et bigger arra. // Co all eleents to bigger arra. } // Add new eleent to the bigger arra. 2

21 How to add an eleent to an arra list when arra is full? add( i, e) { if (a.size == a.length){ ake new bigger arra b for ( int i=; i < size; i++) b[i] = a[i] // is arra full? // e.g. b.length = 2*a.length // co eleents to b } a = b } // insert the add( i, e ) code fro earlier. 2

22 SLIDE ADDED What if ou want to add an eleent to the list because ou don t care where it goes? Or what if ou want to add an eleent to the end of the list? The add( i, e) code does not allow this. Instead we need another ethod add(e ). See Exercises. 22

23 Overloading add( e ) add( i,e) // inserts eleent e at end of list // Inserts eleent e into the i-th osition reove(i) reove(e) // Reoves the i-th eleent fro list // Reoves first occurrence of eleent e // fro the list (if it is there) 23

24 Adding N eleents to an arra list Suose we initialize an arra list with an et arra of length. We then add an eleent. arralist of size (length ) arralist of size (length ) What do we do to add a second eleent? add first eleent

25 Adding N eleents to an arra list Suose each tie we add to a full arra list, we double the length of the arra. arralist of size (length ) arralist of size (length 2) arralist of size 2 (length 2) add second eleent

26 Adding N eleents to an arra list. arralist of size 2 (length 2) arralist of size 2 (length 4) arralist of size 3 (length 4) add third eleent

27 Adding N eleents to an arra list. arralist of size 3 (length 4) arralist of size 4 (length 4) add fourth eleent

28 Adding N eleents to an arra list. arralist of size 4 (length 4) arralist of size 4 (length 8) arralist of size 5 (length 8) add fifth eleent

29 Adding N eleents to an arra list. Double length and co one eleent Double length and co two eleents Double length and co four eleents add one eleent add one eleent add two eleents add four eleents 29

30 Q: How an ties do we need to double the length of the arra so that it is of length? A: Q: How an co oerations are required to add eleents to an et arra list? A: 3

31 Q: How an ties do we need to double the length of the arra so that it is of length? A: 2 =, = Q: How an co oerations are required to add eleents to an et arra list? A: = 2 - = - 3

32 List Oerations get(i) set(i,e) add(i,e) reove(i) reove(e) clear() iset() size() : // Reoves the i-th eleent fro list // Reoves eleent e fro the list (if it is there) // Eties the list. // Returns true if et, false if not et. // Returns nuber of eleents in the list 32

33 reove( i ) // in the figure below, i = j u j u j u e j u j u size = 6 size = 5

34 reove(i) if ( (i >= ) and (i < size) ){ t = a[i] // ut aside and later return it for ( k = i; k < size-; k++){ a[ k ] = a[ k + ] // shift (co) } } size = size a[ size ] = // clean return t 34

35 Quiz : Test our Java skill - Worth % of our grade - Starting toda at noon until Monda night - Practice courses/quiz echanis and tiing - Allow us to test if the sste works as we think - Allow ou/us to calibrate 35