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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
Analyze the problem
Design the solution algorithm
Design the user interface
Write the code
Test and debug the program
Complete the documentation
The Program Development Cycle
Problem Solving
• Problem solving is the act of finding a solution to a perplexing, distressing, vexing, or unsettled question
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
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?
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
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
Analyze the problem
Design the solution algorithm
Design the user interface
Write the code
Test and debug the program
Complete the documentation
The Program Development Cycle
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.
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.
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
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
Developing an Algorithm
• The plan must be suitable in a suitable form
• Two methodologies that currently used– Top-down design – Object-oriented design
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.
Top-Down Design
• Breaking the problem into a set of sub-problems called modules
• Creating a hierarchical structure of problems and sub-problems
Structure Charts
• Structure charts illustrate the structure of a program by showing independent hierarchical steps.
• Major divisions are subdivided into smaller pieces of information.
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
An example of top-down design
Flowcharts and Pseudocode
• Forms of documentation used to build and communicate the detailed parts your structured designs
Flowcharts & Pseudocode are Important
• Flowcharts – – 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
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
Logic Flowcharts
• These represent the flow of logic in a program and help programmers “see” program design.
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 witha flow line in and the other with a flow line out.
Off-page connector. Even fairly small programs can have flowcharts that extend severalpages. 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
Common Flowchart Symbols
How to Draw a Flowchart• Five steps which can be used as a guide for
completing flowcharts. 1. Start with a 'trigger' event (it may be the beginning of
the program)
2. Initialize any values that need to be defined at the start of the program
3. Note each successive action concisely and clearly
4. Go with the main flow (put extra detail in other charts -- this is the basis of structured programming)
5. Follow the process through to a useful conclusion (end at a 'target' point -- like having no more records to process)
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
Start
Variables =
Open Files
Moreitems?
ProcessRecord
(Detail Time)
WriteRecord
Stop
No
Yes
Sample Program FlowchartTerminal Symbol
I/O Symbol
Process Symbol
Decision Symbol
Preparation Symbol or Initialize Variables
Read aRecord
CloseFiles
ProcessRecord
(Total Time)
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
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.
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.
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
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
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
STARTIntialize variablesLOOPWhile More records do READ record PROCESS record PRINT detail recordENDLOOPCALCULATE TOTALSPRINT total recordEND
Pseudocode for a Generalized Program
Rules for Pseudocode1. Make the pseudocode language-independent
2. Indent lines for readability
3. Make keywords stick out by showing them capitalized, in a different color or a different font
4. Punctuation is optional
5. End every IF with ENDIF
6. Begin loop with LOOP and end with ENDLOOP
7. Show MAINLINE first; all others follow
8. TERMINATE all routines with an END instruction
A Computer Example
• Problem
– Create an address list that includes each person’s name, address, telephone number, and e-mail 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
Analyze the problem
Design the solution algorithm
Design the user interface
Write the code
Test and debug the program
Complete the documentation
The Program Development Cycle