Algorithms the fundamentals, For computer Science.ppt

Chapter 7
Problem Solving
and Algorithms

2
Chapter Goals
• Describe the computer problem-solving process and
relate it to Polya’s How to Solve It list
• Distinguish between a simple type and a composite type
• Describe two composite data-structuring mechanisms
• Recognize a recursive problem and write a recursive
algorithm to solve it
• Distinguish between an unsorted array and a sorted
array
• Distinguish between a selection sort and an insertion
sort

3
Chapter Goals
• Describe the Quicksort algorithm
• Apply the selection sort, the bubble sort, insertion sort,
and Quicksort to an array of items by hand
• Apply the binary search algorithm
• Demonstrate an understanding of the algorithms in this
chapter by hand-simulating them with a sequence of
items

4
Problem Solving
Problem solving
The act of finding a solution to a perplexing,
distressing, vexing, or unsettled question
How do you define problem solving?

5
Problem Solving
How to Solve It: A New Aspect of
Mathematical Method by George Polya
"How to solve it list" written within the
context of mathematical problems
But list is quite general
We can use it to solve computer
related problems!

6
Problem Solving
How do you solve problems?
Understand the problem
Devise a plan
Carry out the plan
Look back

7
Strategies
Ask questions!
– What do I know about the problem?
– What is the information that I have to process
in order the find the solution?
– What does the solution look like?
– What sort of special cases exist?
– How will I recognize that I have found
the solution?

8
Strategies
Ask questions! Never reinvent the wheel!
Similar problems come up again and again
in different guises
A good programmer recognizes a task or
subtask that has been solved before and
plugs in the solution
Can you think of two similar problems?

9
Strategies
Divide and Conquer!
Break up a large problem into smaller units
and solve each smaller problem
– Applies the concept of abstraction
– The divide-and-conquer approach can be
applied over and over again until each
subtask is manageable

10
Computer Problem-Solving
Analysis and Specification Phase
Analyze
Specification
Algorithm Development Phase
Develop algorithm
Test algorithm
Implementation Phase
Code algorithm
Test algorithm
Maintenance Phase
Use
Maintain
Can you
name
a recurring
theme?

11
Phase Interactions
Should we
add another
arrow?
(What happens
if the problem
is revised?)

12
Algorithms
Algorithm
A set of unambiguous instructions for solving a
problem or subproblem in a finite amount of time
using a finite amount of data
Abstract Step
An algorithmic step containing unspecified details
Concrete Step
An algorithm step in which all details are specified

13
Developing an Algorithm
Two methodologies used to develop computer
solutions to a problem
– Top-down design focuses on the tasks to be
done
– Object-oriented design focuses on the data
involved in the solution (We will discuss this
design in Ch. 9)

14
Summary of Methodology
Analyze the Problem
Understand the problem!!
Develop a plan of attack
List the Main Tasks (becomes Main Module)
Restate problem as a list of tasks (modules)
Give each task a name
Write the Remaining Modules
Restate each abstract module as a list of tasks
Give each task a name
Re-sequence and Revise as Necessary
Process ends when all steps (modules) are concrete

15
Top-Down Design
Process continues for as many levels as it takes to make every step
concrete
Name of (sub)problem at one level becomes a module at next lower
level

16
Control Structures
Control structure
An instruction that determines the order in
which other instructions in a program are
executed
Can you name the ones we defined in the
functionality of pseudocode?

17
Selection Statements
Flow of control of
if statement

18
Algorithm with Selection
Problem: Write the appropriate dress for a given
temperature.
Write "Enter temperature"
Read temperature
Determine Dress
Which statements are concrete?
Which statements are abstract?

19
Algorithm with Selection
IF (temperature > 90)
Write “Texas weather: wear shorts”
ELSE IF (temperature > 70)
Write “Ideal weather: short sleeves are fine”
Write “A little chilly: wear a light jacket”
Write “Philadelphia weather: wear a heavy coat”
ELSE
Write “Stay inside”
Determine Dress

20
Looping Statements
Flow of control of while statement

21
Looping Statements
Set sum to 0
Set count to 1
While (count <= limit)
Read number
Set sum to sum + number
Increment count
Write "Sum is " + sum
Why is it
called a
count-controlled
loop?
A count-controlled loop

22
Looping Statements
Set sum to 0
Set allPositive to true
WHILE (allPositive)
Read number
IF (number > 0)
Set sum to sum + number
ELSE
Set allPositive to false
Write "Sum is " + sum
Why is it
called an
event-conrolled
loop?
What is the
event?
An event-controlled loop

23
Looping Statements
Read in square
Calculate the square root
Write out square and the square root
Calculate Square Root
Are there any abstract steps?

24
Looping Statements
Set epsilon to 1
WHILE (epsilon > 0.001)
Calculate new guess
Set epsilon to abs(square - guess * guess)
Calculate square root

25
Looping Statements
Set newGuess to
(guess + (square/guess)) / 2.0
Calculate new guess

26
Looping Statements
Read in square
Set guess to square/4
Set epsilon to 1
WHILE (epsilon > 0.001)
Calculate new guess
Set epsilon to abs(square - guess * guess)
Write out square and the guess

27
Composite Data Types
Records
A named heterogeneous collection of items in
which individual items are accessed by name. For
example, we could bundle name, age and hourly
wage items into a record named Employee
Arrays
A named homogeneous collection of items in
which an individual item is accessed by its position
(index) within the collection

Employee
name
age
hourly/Wage
Following algorithm, stores values into the fields of record:
Employee employee // Declare and Employee variable
Set employee.name to “Frank Jones”
Set employee.age to 32
Set employee.hourlyWage to 27.50
28

29
numbers[0]
numbers[4]
Insert 7.5

Arrays
• As data is being read into an array, a counter is
updated so that we always know how many data
items were stored
• If the array is called list, that with which we are
working is from list[0] to list [length-1]
30

31
An Unsorted Array
data[0]...data[length-1]
is of interest

32
integer data[20]
Write “How many values?”
Read length
Set index to 0
WHILE (index < length)
Read data[index]
Set index to index + 1
Fill an array numbers with limit values

Sequential Search of an
Unsorted Array
A sequential search examines each item in turn
and compares it to the one we are searching.
If it matches, we have found the item. If not, we
look at the next item in the array.
We stop either when we have found the item or
when we have looked at all the items and not
found a match
Thus, a loop with two ending conditions
33

Sequential Search Algorithm
Set Position to 0
Set found to FALSE
WHILE (position < length AND NOT found )
IF (numbers [position] equals searchitem)
Set Found to TRUE
ELSE
Set position to position + 1
34

35
Booleans
Boolean Operators
A Bolean variable is a location in memory that can contain
either true or false
Boolean operator AND returns TRUE if both operands are
true and FALSE otherwise
Boolean operator OR returns TRUE if either operand is true
and FALSE otherwise
Boolean operator NOT returns TRUE if its operand is false
and FALSE if its operand is true

36
Sorted Arrays
• The values stored in an array have unique keys of a
type for which the relational operators are defined.
• Sorting rearranges the elements into either
ascending or descending order within the array.
• A sorted array is one in which the elements are in
order

Sequential Search in a Sorted
Array
If items in an array are sorted, we can stop
looking when we pass the place where the
item would be it were present in the array
37
Is this better?

38
A Sorted Array
A sorted array of
integers

39
A Sorted Array
Read in array of values
Write “Enter value for which to search”
Read searchItem
Set found to TRUE if searchItem is there
IF (found)
Write “Item is found”
ELSE
Write “Item is not found”

40
A Sorted Array
Set found to TRUE if searchItem is there
Set index to 0
Set found to FALSE
WHILE (index < length AND NOT found)
IF (data[index] equals searchItem)
Set found to TRUE
ELSE IF (data[index] > searchItem)
Set index to length
ELSE

41
Binary Search
Sequential search
Search begins at the beginning of the list and
continues until the item is found or the entire list
has been searched
Binary search (list must be sorted)
Search begins at the middle and finds the item or
eliminates half of the unexamined items; process
is repeated on the half where the item might be
Say that again…

42
Binary Search
Set first to 0
Set last to length-1
Set found to FALSE
WHILE (first <= last AND NOT found)
Set middle to (first + last)/ 2
IF (item equals data[middle]))
Set found to TRUE
ELSE
IF (item < data[middle])
Set last to middle – 1
ELSE
Set first to middle + 1
RETURN found

43
Binary Search
Figure 7.10 Trace of the binary search

44
Binary Search
Table 7.1 Average Number of Comparisons
Is a binary search
always better?

45
Sorting
Sorting
Arranging items in a collection so that there is an
ordering on one (or more) of the fields in the items
Sort Key
The field (or fields) on which the ordering is based
Sorting algorithms
Algorithms that order the items in the collection
based on the sort key
Why is sorting important?

46
Selection Sort
Given a list of names, put them in alphabetical
order
– Find the name that comes first in the alphabet,
and write it on a second sheet of paper
– Cross out the name off the original list
– Continue this cycle until all the names on the original
list have been crossed out and written onto the
second list, at which point the second list contains the
same items but in sorted order

47
Selection Sort
A slight adjustment to this manual approach
does away with the need to duplicate space
– As you cross a name off the original list, a free
space opens up
– Instead of writing the value found on a second
list, exchange it with the value currently in the
position where the crossed-off item should go

48
Selection Sort
Figure 7.11 Example of a selection sort (sorted elements are shaded)

49
Selection Sort
Selection Sort
Set firstUnsorted to 0
WHILE (not sorted yet)
Find smallest unsorted item
Swap firstUnsorted item with the smallest
Set firstUnsorted to firstUnsorted + 1
Not sorted yet
current < length – 1

50
Selection Sort
Find smallest unsorted item
Set indexOfSmallest to firstUnsorted
Set index to firstUnsorted + 1
WHILE (index <= length – 1)
IF (data[index] < data[indexOfSmallest])
Set indexOfSmallest to index
Set index to indexOfSmallest

51
Selection Sort
Swap firstUnsorted with smallest
Set tempItem to data[firstUnsorted]
Set data[firstUnsorted] to data[indexOfSmallest]
Set data[indexOfSmallest] to tempItem

52
Bubble Sort
Bubble Sort uses the same strategy:
Find the next item
Put it into its proper place
But uses a different scheme for finding the next
item
Starting with the last list element, compare
successive pairs of elements, swapping whenever
the bottom element of the pair is smaller than the
one above it

54
Bubble Sort
Bubble sort is very slow!
Can you see a way to make it faster?
Under what circumstances is bubble
sort fast?

55
Bubble Sort
Bubble Sort
Set firstUnsorted to 0
Set index to firstUnsorted + 1
Set swap to TRUE
WHILE (index < length AND swap)
Set swap to FALSE
“Bubble up” the smallest item in unsorted part
Set firstUnsorted to firstUnsorted + 1

56
Bubble Sort
Bubble up
Set index to length – 1
WHILE (index > firstUnsorted + 1)
IF (data[index] < data[index – 1])
Swap data[index] and data[index – 1]
Set swap to TRUE
Set index to index - 1

Insertion Sort
If you have only one item in the array, it is already
sorted.
If you have two items, you can compare and
swamp them if necessary, sorting the first two with
respect to themselves.
Take the third item and put it into its place relative
to the first two
Now the first three items are sorted with respect to
one another
57

Insertion Sort
The item being added to the sorted portion can be
bubbled up as in the bubble sort
58

Insertion Sort
InsertionSort
Set current to 1
WHILE (current < length)
Set index to current
Set placeFound to FALSE
WHILE (index > 0 AND NOT placeFound)
IF (data[index] < data[index – 1])
Swap data[index] and data[index – 1]
Set index to index – 1
ELSE
Set placeFound to TRUE
Set current to current + 1
59

60
Subprogram Statements
We can give a section of code a name and
use that name as a statement in another
part of the program
When the name is encountered, the
processing in the other part of the program
halts while the named code is executed
Remember?

61
What if the subprogram needs data from the
calling unit?
Parameters
Identifiers listed in parentheses beside the
subprogram declaration; sometimes called formal
parameters
Arguments
Identifiers listed in parentheses on the
subprogram call; sometimes called actual
parameters

62
Figure 7.14 Subprogram flow of control

64
Recursion
Recursion
The ability of a subprogram to call itself
Base case
The case to which we have an answer
General case
The case that expresses the solution in terms of a
call to itself with a smaller version of the problem

65
Recursion
For example, the factorial of a number is defined
as the number times the product of all the numbers
between itself and 0:
N! = N * (N  1)!
Base case
Factorial(0) = 1 (0! is 1)
General Case
Factorial(N) = N * Factorial(N-1)

66
Recursion
Write “Enter n”
Read n
Set result to Factorial(n)
Write result + “ is the factorial of “ + n
Factorial(n)
IF (n equals 0)
RETURN 1
ELSE
RETURN n * Factorial(n-1)

67
Recursion
BinarySearch (first, last)
IF (first > last)
RETURN FALSE
ELSE
Set middle to (first + last)/ 2
IF (item equals data[middle])
RETURN TRUE
ELSE
IF (item < data[middle])
BinarySearch (first, middle – 1)
ELSE
BinarySearch (middle + 1, last

68
Quicksort
Ordering a list
using the Quicksort algorithm
It is easier to sort a smaller
number of items: Sort A…F,
G…L, M…R, and S…Z and
A…Z is sorted

Quicksort algorithm
With each attempt to sort the stack of data
elements, the stack is divided at a splitting value,
splitVal, and the same approach is used to sort
each of the smaller stacks (a smaller case)
Process continues until the small stacks do not
need to be divided further (the base case)
The variables first and last in Quicksort algorithm
reflect the part of the array data that is currently
being processed
69

70
Quicksort
Quicksort(first, last)
IF (first < last) // There is more than one item
Select splitVal
Split (splitVal) // Array between first and
// splitPoint–1 <= splitVal
// data[splitPoint] = splitVal
// Array between splitPoint + 1
// and last > splitVal
Quicksort (first, splitPoint - 1)
Quicksort (splitPoint + 1, last)

72
Quicksort
Split(splitVal)
Set left to first + 1
Set right to last
WHILE (left <= right)
Increment left until data[left] > splitVal OR left > right
Decrement right until data[right] < splitVal
OR left > right
IF(left < right)
Swap data[left] and data[right]
Set splitPoint to right
Swap data[first] and data[splitPoint]
Return splitPoint

74
Important Threads
Information Hiding
The practice of hiding the details of a module with
the goal of controlling access to it
Abstraction
A model of a complex system that includes only
the details essential to the viewer
Information Hiding and Abstraction are two
sides of the same coin

75
Important Threads
Data abstraction
Separation of the logical view of data from their
implementation
Procedural abstraction
Separation of the logical view of actions from their
implementation
Control abstraction
Separation of the logical view of a control structure from its
implementation

76
Important Threads
Identifiers
Names given to data and actions, by which
– we access the data and
Read firstName, Set count to count + 1
– execute the actions
Split(splitVal)
Giving names to data and actions is a form
of abstraction

77
Important Threads
Abstraction is the most powerful tool people have for
managing complexity!

78
Ethical Issues
Open-Source Software Development
What are the advantages and disadvantages
of open-source software?
What does the success of Linux suggest
about the future of open-source software?
Should open-source software be licensed
and subject to standard copyright laws?

79
Who am I?
My wife Jill and
I are holding
the medal I
received when
I was knighted.
What university
did I retire from
and where am
I working now?

80
Who am I?
I am a
mathematician.
Why is my
picture in a
book about
computer
science?

81
Do you know?
What writing system did the Rosetta stone
serve as a key to translating?
What are some of the adverse
consequences of piggybacking for free
off someone else’s paid Wi-Fi connection?
What, if any, legal privacy protections does a
blogger have to resist an employer seeking to
fire an anonymous blogging employee?

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