1 ipc144 session 4 flowcharts. 2 objectives: by the end of this session, the student will be able...

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IPC144Session 4

Flowcharts

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Objectives:By the end of this session, the student will be able to:List the three elements of the Structure TheoremIdentify the symbols used in flowchartsRecognize the flowchart symbols and their meaningConstruct the elements of the Structure Theorem using Flowcharts

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Flowcharts

BackgroundThe third phase of the PDC is Develop Solution into an Algorithm. The points that were made are:

Computers cannot devise a plan or decide to actComputers can only do what they are told, exactly the way they are toldComputers do what you tell them to do, not what you want them to doAn algorithm is a set of precise instructions which describe the task to be performed and the order in which they are to be carried out

Flowcharting is a tool that helps develop a solution into an algorithm: A picture is worth a thousand words Uses only a limited number of symbols to define an algorithm of any

complexity

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Flowcharts

FlowchartsThis is where the program starts to take shape.

The Structure Theorem states that it is possible to write any computer program by using only THREE basic control structures:simple sequenceselectioniteration

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Flowcharts

Simple sequenceThe Simple Sequence represents the computer's ability to execute instructions in a step-by-step, sequential manner.

SelectionThe Selection represents the computer's ability to make a decision. By evaluating a condition, alternate execution paths are followed.

IterationIteration represents the computer's ability to repeat a series of instructions.

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Simple Sequence

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Flowcharting - Simple Sequence

Flow linesShows the direction of logic flow within the flowchart

Unless otherwise stated the symbols of a flowchart have one flow line entering it, and one flow line exiting it.

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Flowcharting - Simple Sequence, continued

Terminator or Terminal InterruptUsed to show the beginning or end of an algorithmThere must be these two symbols (only one each)

Start

Stop

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Terminator or Terminal Interrupt, continuedA simple flowchart that does nothing:

Start

Stop

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Process BoxUsed to show a processing operationThe operation is described inside the box (i.e. a calculation)Does not show a decision, or the input or output of data

Perform aprocess

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An ExamplePrepare for an exam:

Start

Stop

Reviewtexts

ReviewLectures

ReviewTests

ReviewLabs

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Off-page / On-page ConnectorWhat happens when you run out of room on the page?A unique identifier is written inside the symbol so that they can be paired up when reading the flowchartLike the terminator, in that there is either one flow line entering it, OR one flow indicator exiting it.

A

Off-page

A

On-page

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A three page example

Start

U

V

Y

Z

StopA

B

W

X

A

B

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Input / Output OperationUsed to show the input of data, or the output of dataThe operation is described inside the boxDoes not show a decision, or any processing steps

Read valueA

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Module Call BoxUsed to call or invoke another flowchart. Any variables in the current program that are required by the module are listed in parenthesis, each separated by a comma.

The new flowchart is executed, and when it completes (hits the Stop Terminator) the current flowchart resumes execution.

This is where the modularity of programming is represented

getData(a, b)

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Flowcharting the ModuleAll of the rules that are being discussed for flowcharts applies to the modules.

Modules are flowcharts that are composed of Simple Sequence, Decisions and Iteration.

The START and STOP terminal interrupts have new meaning in this case.

The START terminal interrupt shows the name of the of the module and the parameters to be used (some or all may be modified by the module).

The STOP terminal interrupt shows the module is complete and execution control is being returned to the flowchart that invoked the module.

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ParametersMuch more time will be spent working with parameters when we actually start programming in C.

Parameters are a means to pass values from a flowchart to a module.

The names of the parameter variables in the module need not be the same name, however, they are positionally dependent.

myModule(a, varB, x, name)

myModule(a, b, c, d)

a avarB bx cname d

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Parameters, continue

myModule(a, b, c, d)

myModule(a, varB, x, name)

Stop

Proce

ssing i

s pas

sed to

this

flowch

art

Control of execution is returned to the originating flowchart

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ExampleDevelop a program to accept three numbers, add them together, then print their total and average. Write the flowchart

Input Process Output

val1 Read val1, val2, val3 total

val2 Calculate total average

val3 Calculate average

Display total

Display average

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ExampleDevelop a program to accept three numbers, add them together, then print their total and average. Write the flowchart

Start

DISPLAY“Enter 1st Value”

total=val1 + val2 + val3

A

C

average=total / 3

PRINT total,average

Stop

GET val1


A


B

GET val2

DISPLAY“Enter 2nd Value”

B


C

GET val3

DISPLAY“Enter 3rd Value”

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Selection

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Flowcharting - Selection

Selection

There will be references for the remainder of flowcharting to "conditions"

A condition is a logical expression that is either TRUE or it is FALSE

The condition is expressed as some form of a mathematical expression

Expressions:A > BSIN(theta) = COS(theta)recordValue = "end of records"

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Flowcharting - Selection, continued

Simple selection boxUsed to show a change in the path of logic in the programThe condition being tested is written inside the diamond in a mathematical form using information available in the flowchart.Has 2 flow indicators exiting itThese flow lines MUST be labelled with the result of the decision (e.g. YES, NO, TRUE, FALSE...)

A > B + 5True

False

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Multiway selection (Case Statement)Used to show a change in the path of logic in the programHas more than 2 flow lines exiting itUsed for EQUALITY decisions (each flow line is labelled with the value)

You may not place a range of values on the flow linesYou may not place conditions on the flow lines

One branch may be labelled 'Default' to represent the path to be taken when none of the conditions are met

varC

1

23

4

DefaultvarC

1 - 6

7 - 910 x > 5

Default

INVALID

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CollectorUsed to collect the flow lines that were created from a decision box, and restore the logic of the program back to a single streamTHE PATHS FROM A DECISION MUST REJOIN IN A COLLECTOR

To be used with simple To be used with a multiwaydecision ONLY decision ONLY

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Example - Simple Decision B

amount < 0DB =

DB + amount

CR =CR + amount

C

True

False

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Example - Multiway Decision B

userOption

res = x * yres = x - y

C

res = x / yres = x + y

A

S M

D

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ExampleWrite a program to help a local Elementary School's library with the overdue fines on books. The librarians don't wish to always charge students for overdue books, however sometimes the number of books that are overdue is too much. When this happens the librarians enter into the program the student's name, the name of the overdue book they borrowed, and the date it was signed out (the librarian reads these fields from the sign-out card). The colour of the card is also entered, as this indicates what the normal sign-out period is for that book. White cards are due back in 14 days, Blue cards in 7 days, Green cards the next day, and Orange cards in 28 days. The colour code of the card is entered as a single character into the program (W, B, G and O). When this information has been entered, the program determines by how much the book is overdue (based on the date the program is run) and the overdue charge, ($0.05 per day). A list is produced that will show the student's name, the name of the book, the number of days the book is overdue and the fine that is payable.

Create the IPO Chart and Flowchart (a module daysBetween(date, n) exists where the date parameter is the date to be compared to and the n parameter is the number of days between date and today that is calculated. For example if today is February 16th and the date parameter is February 5th, then the value of n would be set to 11 by the function.

If date is invalid or the date is in the future, n will be set to a negative number.

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ExampleWrite a program to help a local Elementary School's library with the overdue fines on books. The librarians don't wish to always charge students for overdue books, however sometimes the number of books that are overdue is too much. When this happens the librarians enter into the program the student's name, the name of the overdue book they borrowed, and the date it was signed out (the librarian reads these fields from the sign-out card). The colour of the card is also entered, as this indicates what the normal sign-out period is for that book. White cards are due back in14 days, Blue cards in 7 days, Green cards the next day, and Orange cards in 28 days. The colour code of the card is entered as a single character into the program (W, B, G and O). When this information has been entered, the program determines by how much the book is overdue (based on the date the program is run) and the overdue charge, ($0.05 per day). A list is produced that will show the student's name, the name of the book, the number of days the book is overdue and the fine that is payable.

Create the IPO Chart and Flowchart (a module daysBetween(date, n) exists)

Input Process Output

studentNameRead studentName, bookName, signoutDate, colourCode studentName

bookName Calculate daysOverdue bookNamesignoutDate Calculate fine daysOverdue

colourCodeDisplay studentName, bookName, daysOverdue, fine fine

Main

getData calcFine showFine

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Example

getData(studentName, bookName,signoutDate, colourCode)

calcFine(signoutDate, colourCode,daysOverdue, fine)

showFine(studentName, bookName,daysOverdue, fine)

Start

Stop

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Example

DISPLAY“Enter student’s

name”

getData(sName, bName, date, cCode)

GET sName

DISPLAY “Enter book name”

GET bName

A

DISPLAY“Enter Sign-out Date”

GET date

DISPLAY “Enter card colour

code”

GET cCode

A

Stop

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ExamplecalcFine(outDate, cCode,

overdue, fine)

daysBetween(outDate, n)

n < 0 DISPLAY “Invalid date entered”

fine = 0

B C

TRUE

FALSE

overdue = -1

Any errors in this module will result in overdue being less than 0 when the module returns

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Example B C

cCode

overdue = n - 1

overdue = n - 7

overdue = n - 14

overdue = n - 28

DISPLAY “Invalid

colour code”

fine = overdue x 0.05

Stop

G BW

O Default

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Example

Stop

showFine(sName, bName,overdue, fine)

overdue > 0

DISPLAY sName, bName, overdue,

fine

TRUE

FALSE

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Alternate Example

You can replace the multiway decisionWith a set of nestedsimple decisions

colourCode='W'

ovrDue = n - 14

ovrDue = n - 7

ovrDue = n -1

ovrDue = n - 28

colourCode='B'

colourCode='G'

T

T

TF

F

F

colourCode=‘O'

T

DISPLAY“Invalid code”

F

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Iteration

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Flowcharting - Iteration

IterationIteration is also referred to as "looping"The goal of iteration is to repeat a sequence of steps until a certain condition has been metThe iteration is a special case of the decision structure already discussedThe sequence of steps in the loop may consist of simple sequences, decisions, loopsThere are two types of loops:

Pre-test Loop Post-test Loop

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Pre-Test LoopThe condition that terminates the loop is tested before the loop is enteredIt is therefore possible to not execute any of the statements within the loop at all

Repeatedstatement(s)

False

True

ConditionalExpression

UpdateStatement

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Post-Test LoopThe condition that terminates the loop is tested after the loop is enteredIt is therefore necessary to execute the sequence of statements at least once in order to reach the decision that terminates the loop.

Repeatedstatement(s)

TrueConditionalExpression

UpdateStatement

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Flowcharting, continued

Integrity of a FlowchartIn this course will represent ALL logic necessary to represent a program – every last tedious step. There are no 'magic' shortcuts to writing the algorithm.

A flowchart is not just a collection of symbols and lines.

As a rule of thumb – lines in a flowchart do NOT cross each other

Think of a flowchart as a story in print:

Symbols (rectangles, parallelograms, circles, diamonds, lines...) -> letters of the alphabet. Individually they have no meaning, they are used to make words.

Simple Sequence / Decision / Iteration -> Words of the story. Just as you don’t mix letters arbitrarily into a word, you don’t throw symbols into a flowchart.

The flowchart is the story you are composing from START to STOP.

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Nesting StructuresWhen combining structures, they must be nested.

Each structure (Simple Sequence, Simple Decision and Iteration) each have ONE entry point and ONE exit point. This is a rule that cannot be broken:

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Nesting Structures, continuedEmbedding a loop within a decision

Loopneeded

Becomes...

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Nesting Structures, continuedEmbedding a decision within a loop

Decisionneeded Becomes...

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Nesting Structures, continuedThink of the nesting of your program, as a series of self-contained objects. One inside the other. There is no connection / interaction across layers. For example, this is illegal:

1 ipc144 session 4 flowcharts. 2 objectives: by the end of this session, the student will be able...

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