Topic02 Programming, Algorithms and Flow Charts

Programming, Algorithms and Flowcharts

Topic Video 022

1Tuesday, 14 July 2009

Algorithms A program is an ordered set of functions designed to perform a specific task. It is written according to a set of defined rules so that it can be translated to the binary codes that the computer understands.

• The programming language is the set of instructions that can be used to construct a program.

• Before writing a program, the programmer must clearly understand the problem, and how the proposed program will solve it.


Algorithms An algorithm is the step-by-step sequence of instructions that describes how to perform the task. In other words it is the method used to solve a problem.

For example:

Problem: Find the sum of 2 numbers:

Algorithm: Get first number

Get second number

Add the two numbers

Display the result.


Algorithms

There may be more than one way to perform the task. Therefore the one problem may have more than one possible algorithm.

• The algorithm can be described in any language or symbolic code.


Flowchart Symbols

Input/Output BlockGeneralized Input/Output Block; reading data from an input medium or writing data to an output medium. This block should be used in situation were data is being sent in and out of the processor via some sort of I/O peripheral.


Flowchart Symbols

Process BlockAny process step; an operation or group of operations that cause a change in value, form or location of the data. This can consist of arithmetic or logical operators or even move commands.


Flowchart Symbols

FlowlineSequence of operations and direction of data flow; arrowheads are required if linkage is not left-to-right or top- to-bottom. Generally arrowheads are included to avoid confusion.


Flowchart Symbols

AnnotationAdditional explanation or comments. This block is used for providing additional information to any other block in the flowchart.


Flowchart Symbols

DecisionDecision-making or switching type of operation, usually based on a comparison, that determines which of a number of paths should be followed.


Flowchart Symbols

PreparationAn operation performed on the program itself for control, initialization, overhead or cleanup; examples are to set a switch, modify an index register, or set a limiting value for iteration control.


Flowchart Symbols

Predefined ProcessOne or more operation defined in more detail elsewhere, such as in a booklet or on a different flowchart, but not on another part of the flowchart in which this symbol appears.


Flowchart Symbols

TerminalTerminal point in a flowchart – stop, start or break in the line of flow.


Flowchart Symbols

ConnectorsEntry to or exit from another part of the flowchart; if to or from step is on another page then the page reference should also be stated.


Anatomy of a Program

Flow of Control refers to the order in which a program’s statements are executed.

• Every program has common flow structures.• The control structure is the lowest fundamental

unit in a program.• Each control structure has one entry point and

one exit point.



The basic control structures are: Sequence (Single Step) ‏ Selection (Conditional) ‏

if then if then else



Iteration (Loops)‏ while do repeat until for

Control structures may be used in combination with each other. This is called nesting.


Sequence Simplest control structure. Performs one step followed by another in the

order they are placed in the program. The normal flow of a program is sequential,

unless the program is directed otherwise.

Step 1 Step 2 Step 3


Examples!"#$"%

&'(%

)*+,-%

.$/'"%)%


Selectional• Selectional also called conditional, branching, testing.

– if - then structure

IF THENTrue

False

if - then structures test a condition and act upon it.

if the condition is true, a task is performed and the structure is exited.

if the condition is false, no task is performed and the structure is exited.

Note that the THEN part is always the true condition.


Examples!"#$"%

&'(%

)*+,-%

.$/'"%

0)123%)12%

4$56%

7#896%


Selectional• if – then - else structure

IF THENTrue

FalseELSE

if - then - else structures test a condition and act upon it

if the condition is true, task 1 is performed and the structure is

exited if the condition is false, task 2 is

performed and the structure is exited

The THEN always refers to the true condition. The ELSE always refers to the false

condition.

TASK1

TASK2


Examples!"#$"%

&'(%

)*+,-%

.$/'"%

0)123%)12%

.$/'"%

0)4*23%

5$67%

8#9:7%


Selectional• case structure

A case structure is used instead of many if-then statements.

A case structure is used when one selection is made from many alternatives.

A case structure takes a value and compares that value to the choices from the first one down to the last one.

Once a match is found the then statement is executed and the case statement is exited.

IF THENTrue

FalseELSE

IF THENTrue

FalseELSE


Example!"#$"%

&'(%

)*+,-%

.$/'"%

0)123%)12%

.$/'"%

0)423%

5$67%

8#9:7%

)42%

5$67%

8#9:7%

.$/'"%

0)*23%


Iteration

Iterative Structures also called loops Used where the same program steps must be

repeated many times Always involves testing a condition to leave the loop.


Iteration

while - do structure while - do tests a condition

at the top of the loop and then performs the task

if condition is true, perform the task and loop back to test again

if condition is false, exit the structure

WHILE DOTrue

False


Examples

!"#$"%

&'(%

)'*"!+),)-./

012.3%

45"+6#$,)-./

012.73%789:;<%

2$=5%>#?@5%


Iteration

repeat - until structure repeat - until performs a task and

then tests a condition at bottom of the loop

if condition is false, loop back to perform task again

if condition is true, exit the structureUNTIL

REPEAT

True

False


Examples!"#$"%

&'(%

)'*"!+),)-./

012.3%

45"+6#$,)-./

012.73%

7889:;%

<#=>5%

2$?5%


Iterationfor structure A for structure is used when the

starting and ending values of a loop are known.

A for structure is a loop that requires the bounds of the loop to be specified. A counter is used as an indicator of the progress through the loop.

The starting value of the loop must be specified, the ending value of the loop must be specified, and the step size (or how the counter is altered) must be specified.

END?

BODY

YES

NO

START


Iterationfor structure A for structure initialises the

counter to the starting value.

The counter is compared to the ending value.

If the counter is greater than the ending value then the for structure is exited.

If the counter is less than or equal to the ending value, then the loop body is executed. After the loop body has been executed the counter is incremented by the step size ( ie counter = counter + step_size) and the end condition tested again.

END?

BODY

YES

NO

START


Examples!"#$"%

&'(%

)*+%

,$-'"%)%

).*/%

0$12%

3#452%

)*)67%


IterationWhich type of Loop to use?

The while-do, repeat-until structures are used when then number of times the loop will be repeated is not known.

These are typically problems that rely on some condition to stop the loop executing.

–For Example

• while the x key has not been pressed do

• repeat ... until the x key is pressed


Variables Variables are where the program / algorithm stores its data.

The variables are located in the memory of the computer. The size of the variable depends upon the type of data stored in it.

Variables are given a unique name to distinguish them from each other.

Variables should be given a name to reflect their use. For example, if you need a variable to store the number of cars in the car park, then you should name the variable number_of_cars. Generally, the more descriptive the name of the variable, the easier it is to understand what the program does, and what the variable is used for.


Variables Variables can be used to store numbers, letters, strings, or arrays.

Strings are groups of letters.

Strings are normally shown between single quotation marks (‘)

For example‏‘Hello World’, ‘Coffee Time’, etc


Modular Programming

A well designed program requires careful forethought and planning.

A program should be designed so that it is easy to develop, easy to correct (debug), and easy to modify.

Experience has shown that the best method of programming is to break a program up into smaller tasks called modules. This is known as modular programming.


Modular Programming

Each module can be considered as a separate subprogram.

Main Module

Module 2Module 3

Module 4

An example of modular programming.


Modular Programming

Each module must do what is required of a small program, it should receive data, process the data, and produce a result.

Large modules should be broken up into smaller modules.

It is easier to develop the code for a many small tasks, than to develop the code for a large program made up of many tasks. Each module should be no bigger than about a screen or two of code.


Modular Programming

Each module should be able to be verified by itself. When testing the program it is much easier to setup the test data and find the error in a small block of code (say 10 lines), than it is to setup the test data and find the error in a whole program (say 100 lines of code).

Once each module has been tested individually and found to be working correctly then all the modules can be integrated together in the main module. Since each of the modules have been verified then the whole program should work when put together.


Modular Programming: Subroutines

Subroutines are modules. Subroutines are called to perform a specific task by the main module or other modules.

A subroutine can have values passed to it. These values are called the arguments.

Arguments are passed to a subroutine through the processors registers or the stack.

For example

LDAB #’B’JSR PutChar



How subroutines are executed– Each program line is executed sequentially

until the line that jumps to the subroutine is reached.

– When the processor jumps to the subroutine, execution starts at the first line of the subroutine.

– When the end of the subroutine is reached, the processor returns to the line immediately after the line that caused it to jump.



Top-Level (Level 0)

Sub 1 (Part of Level 1)

Do something Jump to Subroutine (JSR) Sub1Do something

Subroutine: Sub1Do somethingDo something


Top Down Design

The top level consists of the problem being broken up into smaller very general blocks.

The next level consists of the blocks in the top level being broken up into smaller blocks. This is called successive refinement.

The breaking up of the blocks in the level above is continued until the task is broken up into blocks small enough to be individual modules (subroutines).


Example

Party of the Century

Guests Venue Entertainment

How Many? Who Why? Music Alcohol Food


Modular Flowchart

!"#$"%

&'(%

)#*+,-./0,

1%23)045%%

)#*+,-./0.1%

23)0%65%

&'(%

67.89%,7:%


Important Techniques for developing Flowcharts• Use a standardized flowcharting template,

with clearly recognizable symbols. Follow ANSI recommendations for symbol use.

• Do not crowd or clutter the flowchart, ensure proper spacing between symbols.

• Number the pages of your flowchart sequentially. Specifically the title of program, the date and the author on each separate page.


Important Techniques for developing Flowcharts• Chart the main line of data flow in the system or program first, then incorporate detail in later flowcharts.

• Write within symbols avoid using too many words. If necessary use the annotation symbol.

• Choose wording to suit the anticipated readers of the flowchart.

• Be legible, neatness counts.

• If flowchart becomes complex use connector symbols to reduce the number of flow lines.


Important Techniques for developing Flowcharts

• Collect incoming and outgoing flow lines so that the number of lines entering or leaving a symbol are minimized.

• Use the flowchart as a guide when coding; change it when necessary to ensure the flowchart reflects the steps implemented in the code.

• Cross-reference portion of the flowchart to the source language code.


Important Techniques for developing Flowcharts

• Be consistent with the level of detail shown in the flowchart. Do not chart every detail, but do not leave out important details.

• Put yourself in the position of the reader; try to anticipate the reader’s problems in understanding the flowchart.


Need Further Assistance?

• Ask your Demonstrator,

• Post a question on the Forum,

• Email the Convener, or

• Make an appointment.


Topic02 Programming, Algorithms and Flow Charts

Documents

Transcript of Topic02 Programming, Algorithms and Flow Charts