1 IPC144 Session 6 Problem Analysis and Program Design Continued.

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IPC144Session 6

Problem Analysis and Program DesignContinued

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Objectives

To write pseudo codeTo convert Flowcharts into Pseudo codeTo perform walkthroughs on pseudo codeTo represent data files in flowcharts and pseudo codeTo represent arrays in flowcharts and pseudo code

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Pseudo code

Pseudo codePseudo code (pseudo = fake, code = computer language), a fake computer language (3GL)

Throughout these discussions we have been using the PDC to move from a high level, general analysis of a problem to a more specific solution

The IPO Chart gave us a brief description of the inputs, the outputs and a list of processes for transforming the inputs into the outputs

The Flowchart gave us, diagrammatically, the sequence or order in which processes and decisions are made to solve the problem

Some people are more comfortable with pseudo code, and may choose to design their algorithm directly in pseudo code.

In this course you are expected to be able to read and write both flowcharts and pseudo code.

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Pseudo code, continued

Pseudo code, continuedPseudo code is a hybrid between the structured computer languages (C, Pascal, BASIC...) and English

The control structures we defined in Flowcharting are represented hereBecause pseudo code is not a real computer language and it has loose rules for implementation, it can be used to develop an algorithm for any structured computer language

From the pseudo code (after testing the algorithm) the program can be coded by translating the statements in pseudo code to statements in the programming language (C in our case).

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Pseudo code, continuedThere is no real standard for pseudo code, however it must have the following characteristics:

Statements are written in simple EnglishEach instruction is written on a separate lineKeywords are used to consistently name the parts of control structuresIndentation is used to make parts of a control structure conspicuousEach set of instructions is written from top to bottom, with only one entry point and one exit point

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Pseudo code, continuedAgain the three basic structures are represented:

simple sequencesimple decisioniteration

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Simple sequence - Assigning Variables

This is where variables are manipulated within your algorithmThe New Webster's Computer Terms dictionary definition:A variable is a symbolic name representing a value that changes during the program's execution

Back to Secondary School algebra, remember these?

5 = 5x + 4y0 = 5(x + y) - 3x + 5

Solve for x and y.

x and y are the variables.

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Simple sequence - Assigning Variables, continued

When creating a name for a variable, make it meaningfulDon't make it too long (you will end up with writer's ramp now and carpal tunnel syndrome later)

custName may be not as meaningful ascustomers_first_and_last_name but it is easier to write and probably conveys enough information about how the variable is being used. The other extreme cn is obviously meaningless.

Use your judgement. Ask yourself: If I had to re-read this in another year, would the variable name be meaningful?

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There are two types of variable assignments:When you are initializing a variableAs a result of some processing

It will be considered BAD FORM not to initialize a variable before using it

Verbs used: INITIALIZESET

INITIALIZE counter to zeroSET recordSize to 500

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As a result of processing:

Expect to use the symbol "="

totalPrice = basePrice + salesTax

You can use this form for initializing variables as well

counterA = 0counterB = 0recordSize = 500

The value on the right side of the “=“ is calculated (if necessary) and then placed into the variable that is on the left side of the “=“. You can have only one variable on the left of the “=“.

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Pseudo code Flowchart

counterA = 0

SET recordSize to 500

counterA = 0

SET recordSizeto 500

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Simple sequence - Input of Data

Verbs used: READGET

READ usually refers to data being read from a file

GET usually refers to data being entered by the user at a keyboard

READ address FROM customerFileGET newAddress

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Simple sequence - Input of Data, continued

Flowchart Pseudo code

READ address FROM customerFile

GET newAddress

Read addressFROM

customerFile

Get newAddress

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Simple sequence - Output of DataVerbs used: PRINT

WRITEOUTPUTDISPLAY

PRINT usually refers to data being sent to a printer

WRITE usually refers to data being sent to a file

OUTPUT and DISPLAY usually refer to data being sent to a monitor.

In pseudo code The output is not necessarily formatted (made to look pretty), so it is common to simply list each piece of information separated by commas.

E.g.DISPLAY count, average

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Simple sequence - Output of Data, continuedExample:

PRINT "Program Completed"WRITE newAddress TO customerFileOUTPUT name, address, postalCodeDISPLAY "End of Data Reached"

Note that any phrase that is to be output EXACTLY as written is enclosed in quotes

This helps to differentiate between a literal string and a variable name.

E.g.

average = sum / countDISPLAY The average is average over a range of count values

Should be written as:DISPLAY “The average is”, average, “over a range of ” , count, “values”

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Simple sequence - Output of Data, continued


WRITE address TO customerFile

DISPLAY newAddress

Write addressTO

customerFile

Display newAddress

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Simple sequence - Computation

Some verbs used: ADDCOMPUTECALCULATEMULTIPLYDIVIDESUBTRACT

ADD 1 TO counterCOMPUTE tax = price x 0.15degreesC = (degreesF - 32) x (5/9)

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Simple sequence - Computation, continued


ADD 1 TO counter

degreesC = (degreesF - 32) × (5/9)

Add 1 to counter

degreesC =(degreesF - 32)

x (5/9)

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Simple sequence - Module Call

Verbs Used: CALL

CALL myModule

CALL yourModule(myvar)

CALL theirModule(yourVar, myVar, someVar)

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Simple sequence - Module Call


CALL dspMenu

CALL func(a, b, c)

dspMenu

func(a, b, c)

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Selection - Simple DecisionThe basic form looks like:

IF condition THENstatement(s)

ELSEstatement(s)

ENDIF

If the condition is TRUE, then the statements immediately following the THEN keyword are executed, until the ELSE clause. At which point the program resumes after the ENDIF keyword

If the condition is FALSE, then the statements immediately following the ELSE keyword are executed

Note the indentation that is being usedMakes it easier to see the components of the selectionProvides for readability

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Selection - Simple Decision, continued


IF a > b THENc = e + f

ELSEc = t x q

END IF

a > b c = e + f

c = t x q

TRUE

FALSE

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IF condition THENStatementAStatementBStatementC

ELSEStatementDStatementE

END IF

If condition StatementAStatementDTRUEFALSE

StatementB

StatementC

StatementE

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An example of no indentation: with indentation:

IF a = b THEN IF a = b THENDISPLAY "A equals B" DISPLAY "A equals B"c = (23 - d) / 2 c = (23 - d) / 2lifeUniverseEverything = 42 lifeUniverseEverything = 42ELSE ELSEDISPLAY "A does not equal B" DISPLAY "A does not equal B"vogons = 1 vogons = 1earth = 0 earth = 0ENDIF ENDIF

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What if you don't need an ELSE clause?

IF avg < 50 THENDISPLAY "Low average”

END IF

avg < 50DISPLAY

"Low Average"TRUE

FALSE

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Selection - Multiway DecisionThe basic form looks like:

SWITCH (variable)CASE (option)

statement(s)CASE (option)

statement(s). . . CASE (option)

statement(s)DEFAULT

statement(s)ENDSWITCH

variable is what is being testedoption is the possible value that variable takes on, and represents the new path of logic.

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Selection - Multiway Decision, continued


SWITCH (usrSel)CASE (A)

res = a + bCASE (S)

res = a - bCASE (M)

res = a * bCASE (D)

res = a / bDEFAULT

DISPLAY "Invalid option”ENDSWITCH

usrSel

res = a+ b res = a- b res = a* b res = a/ bDISPLAY"Invalidoption"

A S M D Default

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Iteration - Pre-Test Loop

The basic form looks like:

DOWHILE conditionstatement(s)

ENDWHILE

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Iteration - Pre-Test Loop, continued

Flowchart Pseudo codeDOWHILE i < 10

b = b x ci = i + 1

ENDWHILE

b = b x c

False

True

i < 10

i = i + 1

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Iteration - Post-Test Loop

The basic form looks like:

DOstatement(s)

WHILE condition

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Iteration - Post-Test Loop, continued

Flowchart Pseudo codeDO

b = b x ci = i + 1

WHILE i < 10

b = b x c

False

Truei < 10

i = i + 1

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Converting Flowcharts to Pseudo codeUse the indentation that has been depicted for each of the structures.

The statements that follow IF, CASE, DO and DOWHILE are indented an additional level.

ELSE is at the same level as it's corresponding IF. The statements after the ELSE are indented to the same level as the statements in the IF side

ENDIF, ENDCASE, ENDWHILE and WHILE end the indentation for their structure, and a re aligned with their corresponding IF, CASE, DOWHILE and DO

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Converting Flowcharts to Pseudo codeConvert the following flowchart into pseudo code:

hi = 101 resp ≠ “C”

Start

lo = 1

DISPLAY"Choose a number"

DISPLAY"I Guess", guess,

"is it Hi, Lo or Correct?"

resp = “N”

guess =(hi + lo) / 2

GET resp

resp = “L”

resp = “H”

hi = guess

lo = guess

A

A

B

B

C

C

Stop

DISPLAY"I found your number",

guess

TRUE

TRUE

TRUEFALSE

FALSE

FALSE

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Converting Flowcharts to Pseudo code, continued

hi = 101lo = 1resp = “N”DISPLAY "Choose a number"DOWHILE resp ≠ “C” guess = (hi + lo) / 2 DISPLAY "I guess", guess, "is it Hi, Lo or Correct?" GET resp IF resp = “L” THEN hi = guess ELSE IF resp = “H” THEN lo = guess ENDIF ENDIFENDWHILEDISPLAY "I found your number: ", guess

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Pseudo code Walkthroughs

What is the output of

num = 1var = 0DO

var = num + varDISPLAY varnum = num + 2

WHILE NUM <= 7

Note:

<=

means “less than or equal)

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num = 1var = 0DO

var = num + varDISPLAY varnum = num + 2

WHILE num <= 7

14916

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i = 5DOWHILE i > 0

j = if = 1DOWHILE j > 1

f = f x jj = j – 1

ENDWHILEDISPLAY fi = i – 1

ENDWHILEDISPLAY “Done”

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i = 5DOWHILE i > 0

j = if = 1DOWHILE j > 1

f = f x jj = j – 1

ENDWHILEDISPLAY fi = i – 1

ENDWHILEDISPLAY “Done”

12024621Done

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Data Files

Data FilesFrom Session1 we talked about Secondary Storage.

Secondary storage stores files in a hierarchy of directories.

There are two general types of data files:text filesbinary files

Binary files are not discussed in this course.

Text files contain data that is generally readable by people (the format might be difficult to understand).

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Data Files, continued

Data Files, continuedThe contents and format of a data file will be specified by the program requirements.

A file contains recordsRecords contain fieldsField contain characters

The end of the data file is represented by an 'End Of File' marker, typically referred to as EOF.

The end of a record is represented by a 'Carriage Return' character, typically referred to as CR or EOR.

Fields are either fixed size (the same size for the same field for all records) or they are delimited (a special character that will not be found in the input data is chosen for this purpose and separates each field within the record.

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Data Files, continuedRegardless of whether the data file has fixed size fields or delimited fields, all of the fields come in the same order.

For simplicity, this course assumes fixed size fields.

Field1 Field2 Field3 . . . Field n

Record1 2003-09-01 -1.00 refund 1345

Record2 2003-09-06 15.02 Belt 54588

Record3 2003-09-10 163.89 Shirt 45846

. . .

Record n 2003-12-24 -5.62 discount 3548

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Data Files, continuedTypical operations that are performed on a Data File:

Open - tell the O/S that you wish to perform some actions on the data in the fileRead - read some data from the data fileRewind - return to beginning of fileWrite - write some data to the data fileClose - tell the O/S that you are finished with the data file

Open, Close and Rewind are represented as processes

Read and Write use the traditional Input/Output box. The fields that are read or written are in the same order in the I/O Box as in the data file.

A file pointer keeps track of where you are in the file.

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Data Files, continuedFlowchart Pseudo code

OPEN file.datREAD a, b, c FROM file.datDISPLAY "Read", a, b, ca = b + cREWIND file.datWRITE a, b, c TO file.datCLOSE file.dat

open file.dat

close file.dat

DISPLAY"Read", a, b, c

READ a, b, cFROM file.dat

WRITE a,b ,cTO file.dat

a = b + c

REWIND file.dat

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Data Files, continuedWhile reading from a data file, how do you know you have reached the End-of-File?

In your condition, refer to a test for EOF. For example,

not EOF file.dat EOF file.datTRUEFALSE

TRUE FALSE

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Arrays

ArraysIn programming there are times when we need to work with lists of data such as a list of names or a list of data points from an experiment.

These lists can be stored in an Array.

The simplest explanation is to consider a single column from a spreadsheet:

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Arrays

Arrays

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Arrays

ArraysNote that the rows are numbered.

The same technique is used in computers.

The array can have any name, using the same techniques as selecting variable names.

Each element of the array is numbered.

The element number of the array follows the array name, enclosed in square brackets:

myList[0]myList[1]…myList[n]

** In C the first element of the array is number 0: myList[0]

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Arrays

ArraysAn example that places a calculated value into the first 10 rows of an array:

i = 0

i < 10

Start

a[i] = 2 * i + 1

i = i + 1

Stop

TRUE

FALSE

1 IPC144 Session 6 Problem Analysis and Program Design Continued.

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