Programming Logic and Design Fourth Edition Comprehensive Chapter

Programming Logic and Design Fourth Edition, Comprehensive Chapter 9 Advanced Array Manipulation Programming Logic and Design, Fourth Edition, Comprehensive

Objectives • • • Describe the need for sorting data Swap two values in computer memory Use a bubble sort Use an insertion sort Use a selection sort Programming Logic and Design, Fourth Edition, Comprehensive 2

Objectives (continued) • Use indexed files • Use a linked list • Use multidimensional arrays Programming Logic and Design, Fourth Edition, Comprehensive 3

Understanding the Need for Sorting Records • Sequential order: records are arranged based on the value in a field (e. g. , SSN, employee ID) • Random order: records are in the order in which they were added • Sorting: placing the records in order, based on the values in one or more fields • Ascending order: arranged from lowest to highest • Descending order: arranged from highest to lowest Programming Logic and Design, Fourth Edition, Comprehensive 4

Understanding the Need for Sorting Records (continued) • Median value: the middle item when values are listed in order • Mean value: arithmetic average • Computer always sorts based on numeric values – Character data is sorted by its numeric code value – “A” is less than “B” – Whether “A” is greater than “a” is system-dependent Programming Logic and Design, Fourth Edition, Comprehensive 5

Understanding How to Swap Two Values • Swapping two values: reversing their position • Use a temporary variable to hold one value Programming Logic and Design, Fourth Edition, Comprehensive 6

Using a Bubble Sort • Bubble sort: – Items in a list are compared in pairs – If an item is out of order, it swaps places with the item below it – In an ascending sort, after a complete pass through the list, largest item has “sunk” to the bottom and smallest item has “bubbled” to the top Programming Logic and Design, Fourth Edition, Comprehensive 7

Using a Bubble Sort (continued) • Developing the application: – Mainline logic Programming Logic and Design, Fourth Edition, Comprehensive 8

Using a Bubble Sort (continued) • Developing the application: – housekeeping() module Programming Logic and Design, Fourth Edition, Comprehensive 9

Using a Bubble Sort (continued) • Developing the application: – switch. Values() module Programming Logic and Design, Fourth Edition, Comprehensive 10

Using a Bubble Sort (continued) • Developing the application: – When to call switch. Values() module Programming Logic and Design, Fourth Edition, Comprehensive 11

Using a Bubble Sort (continued) Programming Logic and Design, Fourth Edition, Comprehensive 12

Using a Bubble Sort (continued) • Start with x = 0, compare first pair and swap • With x = 1, compare next pair and swap Programming Logic and Design, Fourth Edition, Comprehensive 13

Using a Bubble Sort (continued) • With x = 2, compare next pair, but no swap needed • With x = 3, compare last pair, and swap • Largest value has “sunk” to the bottom Programming Logic and Design, Fourth Edition, Comprehensive 14

Using a Bubble Sort (continued) Programming Logic and Design, Fourth Edition, Comprehensive 15

Using a Bubble Sort (continued) • Use nested loops for sorting an array • Inner loop makes the pair comparisons • Greatest number of comparisons is one less than the number of array elements • Outer loop controls the number of times to process the list – One less than the number of array elements Programming Logic and Design, Fourth Edition, Comprehensive 16

Refining the Bubble Sort by Using a Constant for the Array Size • Store the number of array elements in a constant • Makes code more readable • If number of elements changes later, only one change to the program is needed Programming Logic and Design, Fourth Edition, Comprehensive 17

Refining the Bubble Sort by Using a Constant for Array Size (continued) Programming Logic and Design, Fourth Edition, Comprehensive 18

Refining the Bubble Sort by Using a Constant for Array Size (continued) Programming Logic and Design, Fourth Edition, Comprehensive 19

Sorting a List of Variable Size • Use a variable to hold the number of array elements • Declare the array with a large fixed size • Count the number of elements when the file is read into the array • Use the count to control the loops when sorting, ignoring unused slots in the array Programming Logic and Design, Fourth Edition, Comprehensive 20

Sorting a List of Variable Size (continued) Programming Logic and Design, Fourth Edition, Comprehensive 21

Sorting a List of Variable Size (continued) Programming Logic and Design, Fourth Edition, Comprehensive 22

Refining the Bubble Sort by Reducing Unnecessary Comparisons • After the first pass through the array: – Largest item must be at the end of array – Second largest item must be at the second-to-last position in the array • Not necessary to compare those two values again • On each subsequent pass through the array, stop the pair comparisons one element sooner Programming Logic and Design, Fourth Edition, Comprehensive 23

Refining the Bubble Sort by Reducing Unnecessary Comparisons (continued) Programming Logic and Design, Fourth Edition, Comprehensive 24

Refining the Bubble Sort by Eliminating Unnecessary Passes • Need one fewer pass than the number of elements to completely sort the array • If array is somewhat ordered already, can reduce the number of passes • Use a flag variable to indicate if there were any swaps during a single pass • If no swaps, the array is completely sorted Programming Logic and Design, Fourth Edition, Comprehensive 25

Refining the Bubble Sort by Eliminating Unnecessary Passes (continued) Programming Logic and Design, Fourth Edition, Comprehensive 26

Using an Insertion Sort • Bubble sort is one of the least efficient sorting methods • Insertion sort usually requires fewer comparisons • Technique: – Compare a pair of elements – If an element is smaller than the previous one, search the array backward from that point and insert this element at the proper location Programming Logic and Design, Fourth Edition, Comprehensive 27

Using an Insertion Sort (continued) Programming Logic and Design, Fourth Edition, Comprehensive 28

Using an Insertion Sort (continued) Programming Logic and Design, Fourth Edition, Comprehensive 29

Using a Selection Sort • Selection sort: uses two variables to store the smallest value and its position in the array • Technique: – Store first element value and its position in variables – Compare value of first element to value of next element – If next element is smaller, put its value and position in the variables – Continue until end of array, at which time the smallest value and its position are in the variables – Swap the first element value and position with the element and position stored in the variables Programming Logic and Design, Fourth Edition, Comprehensive 30

Using a Selection Sort (continued) • Technique (continued): – Start at the second element in the array and repeat the process – Continue until all elements except the last have been designated as the starting point – After making one less pass than the number of elements, the array is sorted Programming Logic and Design, Fourth Edition, Comprehensive 31

Using a Selection Sort (continued) Programming Logic and Design, Fourth Edition, Comprehensive 32

Using Indexed Files • When a large data file with thousands of records needs to be accessed in sorted order, usually one field determines the sorting • Key field: the field whose contents make the record unique • Indexing records: create a list of key fields paired with their corresponding positions in the file • Sorting the indexes is faster than physically sorting the actual records • Random-access storage device: disk or other device from which records can be accessed in any order Programming Logic and Design, Fourth Edition, Comprehensive 33

Using Indexed Files (continued) • • Address: location within computer memory or storage Every data record on disk has an address Index: holds physical addresses and key field values Data file is in physical order, but index is sorted in logical order • When a record is removed from an indexed file, does not have to be physically removed, just deleted from the index file Programming Logic and Design, Fourth Edition, Comprehensive 34

Using Linked Lists • Linked list: requires one extra field in every record to hold the physical address of the next logical record Programming Logic and Design, Fourth Edition, Comprehensive 35

Using Linked Lists (continued) • To add a new record: – Search the linked list for the correct logical location – Break the current link, and insert the new record by linking the previous record to the new record and linking the new record to the next record Programming Logic and Design, Fourth Edition, Comprehensive 36

Using Linked Lists (continued) • Two-way linked list: – Two fields are added to the data record – One holds address of previous logical record – One holds address of next logical record • List can be accessed in either a forward or backward direction Programming Logic and Design, Fourth Edition, Comprehensive 37

Using Multidimensional Arrays • Single-dimensional (or one-dimensional) array: represents a single list of values • Multidimensional array: – A list with two or more related values in each position • Two-dimensional array: – Represents values in a table or grid containing rows and columns – Requires two subscripts Programming Logic and Design, Fourth Edition, Comprehensive 38

Using Multidimensional Arrays (continued) Programming Logic and Design, Fourth Edition, Comprehensive 39

Using Multidimensional Arrays (continued) Programming Logic and Design, Fourth Edition, Comprehensive 40

Summary • Sort data records in ascending or descending order based on the contents of one or more fields • Swap two values by creating a temporary variable to hold one of the values • Bubble sort compares pairs and swaps pairs to obtain the desired order • Eliminate unnecessary comparisons in each pass and eliminate unnecessary passes to improve bubble sort • Insertion sort compares pairs, and moves the out-ofplace element to its proper place within the part of the array already processed Programming Logic and Design, Fourth Edition, Comprehensive 41

Summary (continued) • Ascending selection sort uses two variables to hold the smallest value and its position in the array • Index can be used to access records in logical order without physically sorting the data file • Linked lists use an extra field to hold the address of the next record in the logical order Programming Logic and Design, Fourth Edition, Comprehensive 42