Assignment of pseudo code

PMAS – Arid Agriculture
University Rawalpindi
Programming Fundamentals using C++ (CS-323)
Lecture # 6
Monday, October 23, 2013

Quote of the day

“You don't have to be great
to start, but you have to
start to be great.” by Zig
Ziglar
Thursday, October 24, 2013

Programming Fundamentals using C++(CS-323)

3

Quick recap
• Algorithm Development
• Stepwise refinement of Algorithm



4

Today’s Topics
• Representing algorithms
• Types of Algorithmic operations
• Examples of algorithmic problem solving



5

Pseudocode
 English language constructs modeled to look like statements available
in most programming languages
 Example: ADD X + Y
 No fixed syntax for most operations is required
 Everyone knows what you are talking about



6

Pseudocode (continued)
• Less ambiguous and more readable than natural language
• Emphasis is on the process, not the specific notation
• Can be easily translated into a programming language



7

Operations
• Types of algorithmic operations
• Sequential – input / output, assignment, printing, etc.
• Conditional – doing one thing or another based on a certain condition

• Iterative – looping – doing something more than once



8

Sequential Operations (continued)
• Computation operations
• Example
• Set the value of “variable” to “arithmetic expression”
• X = 10 (set x equal to 10)

• Variable
• Named storage location that can hold a data value
• X in the above example


9

Sequential Operations (continued)
• Input operations
• To receive data values from the outside world
• Get a value for r, the radius of the circle

• Output operations
• To send results to the outside world for display
• Print the value of Area



10

Figure 2.3
Algorithm for Computing Average Miles per Gallon

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Conditional and Iterative Operations
• Sequential algorithm
• Executes its instructions in a straight line from top to bottom and then stops

• Control operations – allow us to get more complex
• Conditional operations
• IF X=10 THEN … do some stuff

• Iterative operations
• WHILE X < 100 … do some other stuff



12

(continued)
• Conditional operations
• Ask questions and choose alternative actions based on the answers
• Example

• if x is greater than 25 then
print x
else

print x times 100



13

(continued)
• Iterative operations
• Perform “looping” behavior; repeating actions until a
continuation condition becomes false

• Loop
• The repetition of a block of instructions



14

(continued)
• Examples
• while j > 0 do
set s to s + aj
set j to j - 1
• repeat
print ak
set k to k + 1
until k > n



15

Figure 2.4
Second Version of the Average Miles per Gallon Algorithm

16

(continued)
• Components of a loop
• Continuation condition – when do we stop?
• Loop body – what we want to repeat

• Infinite loop
• The continuation condition never becomes false and the
loop runs forever

• This is usually an error


17

Figure 2.5
Third Version of the Average Miles per Gallon Algorithm

18

• Pretest loop
• Continuation condition tested at the beginning of each pass through the loop
• It is possible for the loop body to never be executed
• Ex: While loop



19

(continued)
• Post-test loop
• Continuation condition tested at the end of loop body
• Loop body must be executed at least once
• Ex: Do - While loop



20

Figure 2.6
Summary of
Pseudocode
Language
Instructions


21

Example 1: Looking, Looking, Looking
• Examples of algorithmic problem solving
• Sequential search: find a particular value in an unordered collection
• Find maximum: find the largest value in a collection of data
• Pattern matching: determine if and where a particular pattern occurs in a
piece of text



22

(continued)
• Task
• Find a particular person’s name from an unordered list of
telephone subscribers

• Algorithm outline
• Start with the first entry and check its name, then repeat
the process for all entries



23

(continued)
• Correct sequential search algorithm
• Uses iteration (loops) to simplify the task
• Handles special cases (like a name not found in the
collection)



24

Uses the variable Found to exit the iteration as soon as a match is found

Figure 2.9
The Sequential Search Algorithm

25

(continued)
• The selection of an algorithm to solve a problem is
greatly influenced by the way the data for that
problem are organized



26

Example 2: Big, Bigger, Biggest
• Task
• Find the largest value from a list of values

• Keep track of the largest value seen so far (initialized to be the first in the list)

• Compare each value to the largest seen so far, and keep the larger as the new
largest



27

Figure 2.10
Algorithm to Find the Largest Value in a List

28

Example 3: Meeting Your Match
• Task
• Find if and where a pattern string occurs within a longer
piece of text

• Try each possible location of pattern string in turn
• At each location, compare pattern characters against string
characters



29

Example 3: Meeting Your Match
(continued)
• Pattern-matching algorithm
• Contains a loop within a loop
• External loop iterates through possible locations of
matches to pattern
• Internal loop iterates through corresponding characters
of pattern and string to evaluate match



30

Figure 2.12
Final Draft of the Pattern-Matching Algorithm

31

Summary
• Algorithm design is a first step in developing an algorithm
• Must also:
• Ensure the algorithm is correct
• Ensure the algorithm is sufficiently efficient

• Pseudocode is used to design and represent algorithms
• Pseudocode is readable, unambiguous, and analyzable
• Algorithm design is a creative process; uses multiple drafts and top-down
design to develop the best solution


32

Reading Material
• http://courses.cs.vt.edu/~cs1104/Algorithms/TOC.html
• http://faculty.otterbein.edu/PSanderson/csc150/notes/chapter2.html
• http://books.google.com.pk/books?id=BEhlUfdMNucC&printsec=fron
tcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=true
• eBook : Invitation to Computer Science 5ed, Chapter # 2



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