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Data Structures 2

Data Structures 2. Stewart Blakeway blakews@hope.ac.uk. Aims of the Presentation . To demonstrate Arrays in Java Defining the Array object Determining the Range Assigning a value to an element Sorting the Array Searching the Array.

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Data Structures 2

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  1. Data Structures 2 Stewart Blakeway blakews@hope.ac.uk

  2. Aims of the Presentation • To demonstrate Arrays in Java • Defining the Array object • Determining the Range • Assigning a value to an element • Sorting the Array • Searching the Array

  3. Quick question – What is Shopping[1] and Shopping[4] ? • Shopping[1] is Crisps • Shopping[4] does not exist Shopping

  4. Another question • int[] cars = new int[7];

  5. Final question int[][] iceCreamSales = new int[7][15];

  6. What we have done before in data structures • A record is a collection of different data types • An array is a fixed size collection of same data types • Single-dimension • Multi-dimension • A sequence is a variable size collection of same data types • Stack • Queue

  7. Operations on arrays: assignment • Scores[4] = 29;

  8. Operations on arrays: retrieval • What would be the output of: System.out.println(Scores[3]+Scores[10]); 62

  9. Example of retrieval • Write a fragment of Java that would display the third name in the array. System.out.println(PlayOffTeams[3]);

  10. Example of retrieval • Write a fragment of Java that would display all the names in the array.

  11. Example of retrieval inti; for (i= 1; i<=4; i++) { System.out.println(PlayOffTeams[i]); }

  12. Initialisation • When arrays are created they should be initialised, otherwise they will possibly contain random numbers or NULL values • The array Scores would want to start out life like this – each element contains 0

  13. Initialisation • We can initialise Scores in Java like this: int j; for (j=1; j<=10; j++) { Scores[j] = 0; }

  14. Searching and sorting arrays

  15. Searching an array • Example: • How does a “hole in the wall” find your account from your credit card details ? • Need to search through the array of bank records

  16. Different kinds of search available • Linear Search • Binary search

  17. Linear Search • Start at the beginning and examine each in turn until a match found or end of records reached • Fastest find in - • 1 comparison • Slowest find in - • 1,000 comparisons • Average find in - • 500 comparisons for simplicity, we will assume that each search takes 1ms

  18. Search for a national insurance number Is a linear search appropriate ? No ! A better solution in this case is a binary search Linear Search Approximately 59 million people in the UK = Average Search Time of: 48.77 weeks Maximum Search Time of: 1y 43 weeks

  19. Binary Search • Think of a number between 1 and 1000 Figure 12

  20. Binary Search • Think of a number between 1 and 1000 • I can get that number in 10 yes/no questions • is it less than 500? No • is it less than 750? No • is it less than 825? Yes • ... keep going Figure 12

  21. Binary Search • Search for Evans in the array of names which have been sorted into alphabetical order Figure 12

  22. 1 Ball • 2 Davies • 3 Evans • 4 Galt • 5 Hurst • 6 Martin • 7 Mason • 8 Moore • 9 Perkins • 10 Stephens See pages 37, 38 in booklet Searching for Evans

  23. 1 Ball • 2 Davies • 3 Evans • 4 Galt • 5 Hurst • 6 Martin • 7 Mason • 8 Moore • 9 Perkins • 10 Stephens See pages 37, 38 in booklet Searching for Evans

  24. 1 Ball • 2 Davies • 3 Evans • 4 Galt • 5 Hurst • 6 Martin • 7 Mason • 8 Moore • 9 Perkins • 10 Stephens See pages 37, 38 in booklet Searching for Evans

  25. 1 Ball • 2 Davies • 3 Evans • 4 Galt • 5 Hurst • 6 Martin • 7 Mason • 8 Moore • 9 Perkins • 10 Stephens See pages 37, 38 in booklet Searching for Evans

  26. 1 Ball • 2 Davies • 3 Evans • 4 Galt • 5 Hurst • 6 Martin • 7 Mason • 8 Moore • 9 Perkins • 10 Stephens See pages 37, 38 in booklet Searching for Evans

  27. 1 Ball • 2 Davies • 3 Evans • 4 Galt • 5 Hurst • 6 Martin • 7 Mason • 8 Moore • 9 Perkins • 10 Stephens See pages 37, 38 in booklet Searching for Evans

  28. Found Evans ! • 1 Ball • 2 Davies • 3 Evans • 4 Galt • 5 Hurst • 6 Martin • 7 Mason • 8 Moore • 9 Perkins • 10 Stephens Searching for Evans

  29. Sorting • We need to sort to • be able to use the fast binary search which depends on the array being in ascending or descending order • produce reports in required order

  30. Sorting • Many different ways of carrying out a sort on an array. • We shall examine EXCHANGE SORT (Also known as the BUBBLE SORT) • We shall look at others

  31. Exchange Sort • Simple • Slow

  32. No swap • 1 Davies • 2 Martin • 3 Perkins • 4 Evans • 5 Mason • 6 Ball • 7 Stephens • 8 Moore • 9 Hurst • 10 Galt See pages 42, 43 in booklet

  33. No swap • 1 Davies • 2 Martin • 3 Perkins • 4 Evans • 5 Mason • 6 Ball • 7 Stephens • 8 Moore • 9 Hurst • 10 Galt See pages 42, 43 in booklet

  34. See pages 42, 43 in booklet • 1 Davies • 2 Martin • 3 Perkins • 4 Evans • 5 Mason • 6 Ball • 7 Stephens • 8 Moore • 9 Hurst • 10 Galt Swap

  35. See pages 42, 43 in booklet • 1 Davies • 2 Martin • 3 Evans • 4 Perkins • 5 Mason • 6 Ball • 7 Stephens • 8 Moore • 9 Hurst • 10 Galt Swap

  36. See pages 42, 43 in booklet • 1 Davies • 2 Martin • 3 Evans • 4 Mason • 5 Perkins • 6 Ball • 7 Stephens • 8 Moore • 9 Hurst • 10 Galt Swap

  37. See pages 42, 43 in booklet • 1 Davies • 2 Martin • 3 Evans • 4 Mason • 5 Ball • 6 Perkins • 7 Stephens • 8 Moore • 9 Hurst • 10 Galt No Swap

  38. See pages 42, 43 in booklet • 1 Davies • 2 Martin • 3 Evans • 4 Mason • 5 Ball • 6 Perkins • 7 Stephens • 8 Moore • 9 Hurst • 10 Galt Swap

  39. 1 Davies • 2 Martin • 3 Evans • 4 Mason • 5 Ball • 6 Perkins • 7 Moore • 8 Stephens • 9 Hurst • 10 Galt See pages 42, 43 in booklet Swap

  40. 1 Davies • 2 Martin • 3 Evans • 4 Mason • 5 Ball • 6 Perkins • 7 Moore • 8 Hurst • 9 Stephens • 10 Galt See pages 42, 43 in booklet Swap

  41. See pages 42, 43 in booklet • 1 Davies • 2 Martin • 3 Evans • 4 Mason • 5 Ball • 6 Perkins • 7 Moore • 8 Hurst • 9 Galt • 10 Stephens At the end of this first pass, Stephens is in the correct position alphabetically. This is repeated for a second pass starting at position 1 and again and again until the array is completely sorted.

  42. Java Exchange Sort Consider an array of 10,000 surnames, called names, indexed from 0 to 9,999. for (i=1; i<=9999; i++) { for (j=0; j< 9999; j++) { if (names[j] > names[j+1]) { temp = names[j]; names[j] = names[j+1]; names[j+1] = temp; } } }

  43. Summary • Operations on arrays • Initialisation • Assignment • Retrieval • Searching – linear and binary • Sorting – exchange sort

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