Focus of the last Lesson 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
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
17.1 Algorithm Building Blocks
All problems can be solved by
employing any one of the following building blocks or their
combinations
Sequences
Conditionals
Loops
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 ..
Start or stop
Process
Input or output
Connector
Decision
Flow line
Offpage connector
Flowchart
Elements
We will now present the algorithm
for a problem whose solution is familiar to us
We will first go through the problem statement and then present the
algorithm in three different formats:
1. Pseudo code
2. Flowchart
3. Actual code
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
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
statement
block 1
True
conditioFnalse
statement
block 2
Loops
Loop through a set of statements as
long as a condition is true
Loop while ( condition = true )
statement block
End Loop
True
condition
False
statement
block
Problem Statement
Convert a decimal number into
binary
We did write down the pseudo code for this problem last time. Lets do it
again, and in a slightly more
formal way
17.2 Solution in Pseudo Code
Let the decimal number be an
integer x, x
> 0
Let the binary equivalent be an empty string
y
Repeat while
x
> 0 {
Determine the quotient & remainder of
x
÷ 2
y
= CONCATENATE( remainder,
y
)
x
= quotient
}
Print y
Stop
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
17.3 Tips on Writing Good Pseudo Code
Use indention for improved clarity
Do not put “code” in pseudo code – make your pseudo code language
independent
Convert 75 to Binary
2 75
37
1
2
18
1
2
9
0
2
4 1
2
2 0
2
1
0
2
0
1
1001011
remainder
Don’t 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
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?
Start
Find quotient
& remainder
Get x
x>0 ?
Print y
y = CONC(remainder, x)
x =
quotient
x
is the decimal number
y
is the binary equivalent
Flowchart of
Decimal
to Binary
Conversion
Yes No
<SCRIPT>
x = 75; // x is the decimal number
y = “”; // y is the binary equivalent
while ( x > 0) {
remainder = x % 2;
quotient = Math.floor( x / 2 );
y = remainder + y;
x = quotient;
}
document.write(“y = ” + y);
</SCRIPT>
Decimal to Binary Conversion in JavaScript
NOTE: Don’t worry if
you don’t understand
this code for now; you
will  later!
Strategy
There are many strategies for
solving this problem. We demonstrate a simple one:
Repeat the following steps while the list is unsorted:
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
Q:
Is the list sorted?
A:
No
Another Example: Sorting
Sort the following objects w.r.t. their
heights
Expected Result
Back to the Objects to be Sorted
Sorting: Step A1
Sorting: Step A1
Swap? Yes
Sorting: Step A2
Sorting: Step A2
Swap? Yes
Sorting: Step A3
Sorting: Step A3
Swap? No
Q:
Is the list sorted?
A:
No
Sorting: After Step A7
Sorting: Step B1
Sorting: Step B1
Swap? Yes
Sorting: Step B2
Q:
Is the list sorted?
A:
No
Sorting: Step B2
Swap? No
Sorting: After Step B7
Sorting: Step C1
Sorting: Step C1
Swap? No
Q:
Is the list sorted?
A:
Yes
STOP
Let’s now look at this same
process of sorting being applied to a bigger list
FLASH MOVIE FOR BUBBLE SORT GOES HERE
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) > list(n + 1) Then
temp = list(n)
list(n) = list(n + 1)
list(n + 1) = temp
swapFlag = True
Sorting: After Step C7
Start
n = n+1
Get list
list
sorted?
Stop SWAP
list[n], list[n+1]
list is an array containing
the heights
N is the total number of objects in the list
Flowchart for the Sorting Process
No
Yes
n = 0
list[n] >
list[n+1]?
Yes No
n>N ?
Yes
No
End If
Next
Loop
For n = 1 To 8
Debug.Print list(n)
Next
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
17.4 Pros and Cons of Flowcharts (1)
I personally don’t 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
17.5 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
one’s peers quite straightforward
17.6 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
17.7 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
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
In Today’s 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
Focus of the Next Lecture: Programming Languages
To understand the role of
programming languages in computing
To understand the differences among low & highlevel, interpreted &
compiled, and structured &
objectoriented programming languages
