Programming Logic and
Design
Fourth Edition, Comprehensive
Chapter 9
Advanced Array Manipulation
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
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Objectives (continued)
• Use indexed files
• Use a linked list
• Use multidimensional arrays
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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
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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
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Understanding How to Swap Two
Values
• Swapping two values: reversing their position
• Use a temporary variable to hold one value
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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
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Using a Bubble Sort (continued)
• Developing the application:
– Mainline logic
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Using a Bubble Sort (continued)
• Developing the application:
– housekeeping() module
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Using a Bubble Sort (continued)
• Developing the application:
– switchValues() module
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Using a Bubble Sort (continued)
• Developing the application:
– When to call switchValues() module
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Using a Bubble Sort (continued)
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Using a Bubble Sort (continued)
• Start with x = 0, compare first pair and swap
• With x = 1, compare next pair and swap
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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
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Using a Bubble Sort (continued)
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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
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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
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Refining the Bubble Sort by Using a
Constant for Array Size (continued)
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Refining the Bubble Sort by Using a
Constant for Array Size (continued)
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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
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Sorting a List of Variable Size
(continued)
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Sorting a List of Variable Size (continued)
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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
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Refining the Bubble Sort by Reducing
Unnecessary Comparisons (continued)
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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
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Refining the Bubble Sort by Eliminating
Unnecessary Passes (continued)
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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
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Using an Insertion Sort (continued)
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Using an Insertion Sort (continued)
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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
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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
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Using a Selection Sort (continued)
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Using Indexed Files
• When a large data file with thousands of records
needs to be accessed in sorted order, usually only
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
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Using Indexed Files (continued)
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•
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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
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Using Linked Lists
• Linked list: requires one extra field in every record to
hold the physical address of the next logical record
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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
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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
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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
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Using Multidimensional Arrays
(continued)
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Using Multidimensional Arrays
(continued)
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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
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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
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