CS101 Introduction to Computing Lecture 17 Algorithms II

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CS101 Introduction to ComputingLecture
17Algorithms II
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Focus of the last lecture was on Algorithms

• Became familiar with the concept of algorithms
• What they are? (SEQUENCE OF STEPS)
• What is their use?
• What are their types?
• What are the techniques used for representing
  them?
• Pseudo code
• Flowcharts
• Actual code

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Today

• We will continue our discussion on algorithms
  that we started during the 16th lecture
• In particular, we will look at the building
  blocks that are used in all algorithms
• We will also discuss the pseudo code and
  flowcharts for particular problems
• In addition, we will outline the pros and cons of
  those two techniques

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Algorithm Building Blocks

• All problems can be solved by employing any one
  of the following building blocks or their
  combinations
• Sequences
• Conditionals
• Loops

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Review of Flowchart Elements
Start or stop
Process
Input or output
Decision
Flow line
Connector
Off-page connector
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This review was essential because we we will be
using these building blocks quite often
today.OK. Now on with the three building
blocks of algorithms. First ..
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Sequences

• A sequence of instructions that are executed in
  the precise order they are written in
• statement block 1
• statement block 2
• statement block 3

statement block 1
statement block 2
statement block 3
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Conditionals

• Select between alternate courses of action
  depending upon the evaluation of a condition
• If ( condition true )
• statement block 1
• Else
• statement block 2
• End if

condition
True
False
statement block 1
statement block 2
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Loops

• Loop through a set of statements as long as a
  condition is true
• Loop while ( condition true )
• statement block
• End Loop

condition
True
statement block
False
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We will now present the algorithm for a problem
whose solution is familiar to usWe will first
go through the problem statement and then present
the algorithm in three different formats 1.
Pseudo code 2. Flowchart 3. Actual code
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Problem Statement

• Convert a decimal number into binary

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Convert 75 to Binary

75
2
remainder
37
1
2
18
1
2
9
0
2
4
1
2
2
0
2
1
0
2
0
1
1001011
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We did write down the pseudo code for this
problem last timeLets do it again, and in a
slightly more formal way
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Solution in Pseudo Code

• Let the decimal number be an integer x, x gt 0
• Let the binary equivalent be an empty string y
• Repeat while x gt 0
• Determine the quotient remainder of x 2
• y CONCATENATE( remainder, y )
• x quotient
• Print y
• Stop

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• Q Is this the only possible algorithm for
  converting a decimal number into a binary
  representation?
• If not, then is this the best?In terms of
  speed?In terms of memory requirement?In terms
  of ease of implementation?
• You must ask these questions after writing any
  algorithm!

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Tips on Writing Good Pseudo Code

• Use indention for improved clarity
• Do not put code in pseudo code make your
  pseudo code language independent
• Dont write pseudo code for yourself write it
  in an unambiguous fashion so that anyone with a
  reasonable knowledge can understand and implement
  it
• Be consistent
• Prefer formulas over English language descriptions

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Flowchart of Decimal to Binary Conversion
Start
Get x
Find quotient remainder of x 2
y CONC(remainder, x) x quotient
Yes
xgt0 ?
No
Print y
x is the decimal number y is the binary equivalent
Stop
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• Does the flowchart depict the correct
  algorithm?
• What do we mean by correct, or better yet, what
  do we check for correctness?
• One way is to check the algorithm for a variety
  of inputs
• Does it perform satisfactorily for
• x 0 ?
• negative numbers?
• numbers with fractional parts?

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Decimal to Binary Conversion in JavaScript

• ltSCRIPTgt
• x 75 // x is the decimal number
• y // y is the binary equivalent
• while ( x gt 0)
• remainder x 2
• quotient Math.floor( x / 2 )
• y remainder y
• x quotient
• document.write(y y)
• lt/SCRIPTgt

NOTE Dont worry if you dont understand this
code for now you will - later!
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Another Example Sorting

• Sort the following objects w.r.t. their heights

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Expected Result
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Strategy

• There are many strategies for solving this
  problem. We demonstrate a simple one
• Repeat the following steps while the list is
  un-sorted
• Start with the first object in the list
• Swap it with the one next to it if they are in
  the wrong order
• Repeat the same with the next to the first object
• Keep on repeating until you reach the last object
  in the list

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Back to the Objects to be Sorted
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Q Is the list sorted?A No
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Sorting Step A1
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Sorting Step A1
Swap? Yes
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Sorting Step A2
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Sorting Step A2
Swap? Yes
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Sorting Step A3
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Sorting Step A3
Swap? No
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Sorting After Step A7
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Q Is the list sorted?A No
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Sorting Step B1
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Sorting Step B1
Swap? Yes
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Sorting Step B2
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Sorting Step B2
Swap? No
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Sorting After Step B7
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Q Is the list sorted?A No
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Sorting Step C1
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Sorting Step C1
Swap? No
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Sorting After Step C7
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Q Is the list sorted?A Yes
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STOP
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Lets now look at this same process of sorting
being applied to a bigger list---FLASH MOVIE
FOR BUBBLE SORT GOES HERE---
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Flowchart for the Sorting Process
Start
list is an array containing the heights N is the
total number of objects in the list
Get list
Yes
No
ngtN ?
n n1
list sorted?
listn gt listn1?
No
No
n 0
Yes
Yes
SWAP listn, listn1
Stop
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Start
Get list
Yes
No
ngtN ?
n n1
list sorted?
listn gt listn1?
No
No
n 0
Yes
Yes
SWAP listn, listn1
Stop
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• Dim swapFlag As Boolean, list(8) As Integer
• readList( list() ) this needs to be defined
• swapFlag True
• Do While swapFlag True
• For n 1 To 8
• If list(n) gt list(n 1) Then
• temp list(n)
• list(n) list(n 1)
• list(n 1) temp
• swapFlag True
• End If
• Next
• Loop
• For n 1 To 8
• Debug.Print list(n)
• Next

VisualBasic Code for the Sorting Function
NOTE Dont worry if you dont understand this
code
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• Q Is this the only possible algorithm for
  sorting a list?
• A Certainly not! In fact this one (called the
  Bubble sort) is probably the worst (reasonable)
  algorithm for sorting a list it is just too
  slow
• You will learn a lot more about sorting in your
  future courses

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Pros and Cons of Flowcharts (1)

• I personally dont find flowcharts very useful
• The process of writing an algorithm in the form
  of a flowchart is just too cumbersome
• And then converting this graphical form into code
  is not straight forward
• However, there is another kind of flowcharts
  called Structured Flowcharts that may be better
  suited for software developers

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Pros and Cons of Flowcharts (2)

• The good thing about flowcharts is that their
  symbols are quite intuitive and almost
  universally understood
• Their graphical nature makes the process of
  explaining an algorithm to ones peers quite
  straightforward

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Pros and Cons of Pseudo Code (1)

• Quite suitable for SW development as it is closer
  in form to real code
• One can write the pseudo code, then use it as a
  starting point or outline for writing real code
• Many developers write the pseudo code first and
  then incrementally comment each line out while
  converting that line into real code

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Pros and Cons of Pseudo Code (2)

• Pseudo code can be constructed quite quickly as
  compared with a flowchart
• Unlike flowcharts, no standard rules exist for
  writing pseudo code

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With that we have reached the end of the
materials that we wanted to cover
today.However, I still need to tell you about
your assignment 6
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Assignment 6

• There are many algorithms for sorting a list
  Bubble sort the sorting algorithm discussed
  today in - is just one example.
• For assignment 6, submit the pseudo code and the
  flowchart for a sorting algorithm other than the
  Bubble sort.
• For this purpose, you can either search on the
  Web for an algorithm or come up with a scheme on
  your own.
• Consult the CS101 syllabus for the
• submission instructions deadline

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In Todays Lecture, We

• We continued our discussion on algorithms that we
  had started during the 16th lecture
• In particular, we looked at the building blocks
  that are used in all algorithms
• We also discussed the pseudo code and flowcharts
  for particular problems
• In addition, we outlined the pros and cons of
  those two techniques

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Focus of the Next Lecture Programming Languages

• To understand the role of programming languages
  in computing
• To understand the differences among low-
  high-level, interpreted compiled, and
  structured object-oriented programming
  languages