1. CPS 120: Introduction to Computer Science
Algorithm Design
and
Problem Solving

2. Session Goals
Determine whether a problem is suitable for a computer solution
Describe the computer problem-solving process
Distinguish between following an algorithm and developing one
Apply top-down design methodology to develop an algorithm to solve a problem

3. The Program Development Cycle
Analyze the problem
Design the solution algorithm
Design the user interface
Write the code
Test and debug the program
Complete the documentation

4. Problem Solving
Problem solving is the act of finding a solution to a perplexing, distressing, vexing, or unsettled question

5. Problem Solving
In 1945 George Polya wrote How to Solve It: A New Aspect of Mathematical Method
His How to Solve It list is quite general
Written in the context of solving mathematical problems
The list becomes applicable to all types of problems

6. Ask Questions... …to understand the problem
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?
Flowcharting and Pseudocode

7. Look for Familiar Things
You should never reinvent the wheel
In computing, you see certain problems again and again in different guises
A good programmer sees a task, or perhaps part of a task (a subtask), that has been solved before and plugs in the solution
Functions and subroutines

8. Divide and Conquer
Break up a large problem into smaller units that we can handle
Applies the concept of abstraction
The divide-and-conquer approach can be applied over and over again until each subtask is manageable
Modularization

9. Computer Problem-Solving
The computer problem-solving process

10. The Interactions Between Problem-Solving Phases

11. The Program Development Cycle
Analyze the problem
Design the solution algorithm
Design the user interface
Write the code
Test and debug the program
Complete the documentation

12. Algorithms
Muhammad ibn Musa Al'Khowarizmi, a Tashkent cleric who in the twelfth century developed the concept of a written process to be followed to achieve some goal, and published a book on the subject that gave it is modern name -- algorithm.

13. What is an Algorithm?
An algorithm is merely the sequence of steps taken to solve a problem
Two parts
Actions to be executed
Order in which those actions are to be done
Computational steps that transform the input data into useful output data.
Algorithms are not programs
They need to be coded in a programming language like C++, Visual Basic, COBOL, Java, etc.
The logical sequence of steps

14. Algorithms
An algorithm is set of instructions for solving a problem or sub-problem in a finite amount of time using a finite amount of data
The instructions are unambiguous

15. Following an Algorithm
Preparing a Hollandaise sauce

16. Following an Algorithm (cont.)
Preparing a Hollandaise sauce

17. Programs are Solutions to Problems
Programmers arrive at these solutions by using one or more of these devices:
Structured Programming
Structure charts
Logic flowcharts
Pseudocode (sue-dough'-code)
Solutions to problems need to be developed before code is written

18. Developing an Algorithm
The plan must be suitable in a suitable form
Two methodologies that currently used
Top-down design
Object-oriented design

19. Structured Programming
Structured program languages lend themselves to flowcharts, structure charts, and pseudocode.
Structured programming languages work best where the instructions have been broken up into small, manageable parts.

20. Top-Down Design
Breaking the problem into a set of sub-problems called modules
Creating a hierarchical structure of problems and sub-problems

21. Structure Charts
Structure charts illustrate the structure of a program by showing independent hierarchical steps.
Major divisions are subdivided into smaller pieces of information.

22. Top-Down Design
An example of top-down design
This process continues for as many levels as it takes to expand every task to the smallest details
A step that needs to be expanded is an abstract step

23. A General Example Planning a large party
Subdividing the party planning

24. Flowcharts and Pseudocode
Forms of documentation used to build and communicate the detailed parts your structured designs

25. Flowchart
A graphical representation of an algorithm.

26. Pseudocode
Uses a mixture of English and formatting to make the steps in the solution explicit

27. Flowcharts & Pseudocode are Important
A graphical layout of the algorithm is often very useful in spotting “illogical” logic!
Pseudocode –
Make a detailed description of your algorithm’s logic before worrying about syntax and data layout.
An algorithm you develop using pseudocode should be capable of implementation in any procedural programming language
Pseudocode is generally independent of the implementation language

28. Reasons Programmers Draw Flowcharts
Drawing a flowchart gives the programmer a good visual reference of what the program will do
Flowcharts serve as program documentation
Flowcharts allow a programmer to test alternative solution to a problem before coding
Flowcharts provide a method for easy desk checking

29. Logic Flowcharts
These represent the flow of logic in a program and help programmers “see” program design.

30. Common Flowchart Symbols
Terminator. Shows the starting and ending points of the program. A terminator has flow lines in only one direction, either in (a stop node) or out (a start node).
Data Input or Output. Allows the user to input data and results to be displayed.
Processing. Indicates an operation performed by the computer, such as a variable
assignment or mathematical operation. With a heading – an internal subroutine
Decision. The diamond indicates a decision structure. A diamond always has two
flow lines out. One flow lineout is labeled the “yes” branch and the other is labeled the
“no” branch.
Predefined Process. One statement denotes a group of previously defined statements.
Such as a function or a subroutine created externally
Connector. Connectors avoid crossing flow lines, making the flowchart easier to read.
Connectors indicate where flow lines are connected. Connectors come in pairs, one with
a flow line in and the other with a flow line out.
Off-page connector. Even fairly small programs can have flowcharts that extend several
pages. The off-page connector indicates the continuation of the flowchart on another
page. Just like connectors, off-page connectors come in pairs.
Flow line. Flow lines connect the flowchart symbols and show the sequence of operations during the program execution.
Common Flowchart Symbols

31. How to Draw a Flowchart
Five steps which can be used as a guide for completing flowcharts.
Start with a 'trigger' event (it may be the beginning of the program)
Initialize any values that need to be defined at the start of the program
Note each successive action concisely and clearly
Go with the main flow (put extra detail in other charts -- this is the basis of structured programming)
Follow the process through to a useful conclusion (end at a 'target' point -- like having no more records to process)

32. Rules for Drawing Flowcharts
Top to bottom and left to right
Draw the flowchart the way you like to read
Use arrowheads on flow lines whenever the flow is not top to bottom, left to right
Be neat ! Use graphics software
Avoid intersecting lines

33. Sample Program Flowchart
Terminal Symbol
Start
Initialize Variables
Preparation Symbol or
Variables =
I/O Symbol
Open
Files
Read a
Record No
More
items?
Process
Record
(Total Time)
Close
Files
Decision Symbol
Stop
Yes
Process
Record
(Detail Time)
Process Symbol
Write
Record

34. Disadvantages to Flowcharts
Time consuming
A program flowchart shows how the input becomes output, but it does not show why a particular step is done
Flowcharts are subjective

35. Pseudocode
Pseudocode is an artificial and informal language that helps programmers develop algorithms.
Pseudocode is a "text-based" detail (algorithmic) design tool.
An English description of an algorithm in sufficient detail to allow its implementation to be easily written.

36. Pseudocode
This device is not visual but is considered a “first draft” of the actual program.
Pseudocode is written in the programmer’s native language and concentrates on the logic in a program—not the syntax of a programming language.

37. General Rules for Pseudocode
There is no standard pseudocode
The rules of Pseudocode are generally straightforward
Should be easily read and understood by non-programmers
All statements showing "dependency" are to be indented.
These include while, do, for, if, switch

38. Writing Pseudocode
You need to reach a balance between excessive and insufficient detail.
Write only what is necessary to understand and communicate the essential parts of your algorithm

39. Pseudocode Statement Rules
Statements are written in a simple English-like language
Each instruction is started on a separate line
Logic-showing keywords are written in UPPER CASE or typed in BOLD UPPERCASE
(e.g. IF, THEN, FOR, DO etc.)
These are the only uppercase words in this form of pseudocode.
Indentation is used to show structure
Instructions are written from top to bottom, with only one entry point and one exit point
Logically related groups of instructions can be formed into modules and given a name

40. Pseudocode for a Generalized Program
START
Intialize variables
LOOP
While More records do
READ record
PROCESS record
PRINT detail record
ENDLOOP
CALCULATE TOTALS
PRINT total record
END

41. Rules for Pseudocode Make the pseudocode language-independent
Indent lines for readability
Make keywords stick out by showing them capitalized, in a different color or a different font
Punctuation is optional
End every IF with ENDIF
Begin loop with LOOP and end with ENDLOOP
Show MAINLINE first; all others follow
TERMINATE all routines with an END instruction

42. A Computer Example Problem
Create an address list that includes each person’s name, address, telephone number, and address
This list should then be printed in alphabetical order
The names to be included in the list are on scraps of paper and business cards

43. A Computer Example

44. A Computer Example

45. A Computer Example

46. A Computer Example

47. A Computer Example

48. Relation of Flowcharts & Pseudocode

49. The Program Development Cycle
Analyze the problem
Design the solution algorithm
Design the user interface
Write the code
Test and debug the program
Complete the documentation

50. Data Processing Facts enter the computer They are:
Checked for accuracy
Organized
Calculated with
Stored