1. How Are Algorithms Developed?
CPS120:
Introduction to Computer Science
Lecture 6

2. 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.

3. 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++

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

5. Writing C++ (Source) Code
C++ syntax can obscure understanding
Write out solutions intuitively – on paper
Use flowcharts or pseudocode
Translate ideas to C++ syntax on paper
Try to NEVER compose code at the terminal
Don't try to crate the perfect solution on the first try – it's too difficult
Begin with a partial solution that runs
Make it better iteratively through refinement
Prototyping

6. Design Philosophy
Spend a little more on an organized software development approach to save a lot during testing, maintenance, and support phases

7. Pseudocode & Flowcharts are Important
Make a detailed description of your algorithm’s logic before worrying about C++ 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
Flowcharts –
A graphical layout of the algorithm is often very useful in spotting “illogical” logic!

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

9. Flowchart
A graphical representation of an algorithm.

10. 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

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

12. 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

13. 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

14. Flowchart for a Cash Register Program
Terminal Symbol
Start
sum=
Input price
sum=sum+price
More
items?
tax=sum x
total=sum+tax
Output sum, tax,
and total
Stop No
Yes or
Preparation Symbol
I/O Symbol
Process Symbol
Decision Symbol

15. 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

16. What is 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.

17. 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.

18. 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

19. Writing Pseudocode
You need to reach a balance between excessive and insufficient detail.
In some cases you only need a skeleton of the logic and in other cases you may want to show the intricacies of your algorithm
Write what is necessary to understand and communicate the essential parts of your algorithm without becoming bogged down in detail

20. 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

21. Pseudocode for a Cash Register Program
sum=0
While More items do
Input price
sum=sum+price
End While
tax=sum x
total=sum+tax
Output sum, tax, total

22. Rules for Pseudocode Make the pseudocode language-independent
Indent lines for readability
Make key words 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
TERMINAE all routines with an END instruction

23. 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.

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