COIT29222 Structured Programming 1 COIT29222-Structured Programming Lecture Week 02 Reading:...

COIT29222 Structured Programming 1

COIT29222-Structured Programming Lecture Week 02

Reading: Textbook(4th Ed.), Chapter 2 Textbook (6th Ed.), Chapters 4, 5 Study Guide Book 1, Module 4

Variables Branching or selection statementsLooping statements

Expressions, Operators, Operands, etc


VariablesInstead of using memory locations directly, all

modern programming languages use names to refer to memory locations

We call these things variables – the values they refer to vary as the program runs

Using names, instead of memory locations, makes programs easier to read

Instead of…


Example Pseudocodewrite “Number of marks in exam ==> “ read M1 write “Student’s mark ==> “read M2set M3 to M2 / M1set M4 to 100 * M3write “Student’s percentage: “write M4


Variableswrite “Number of marks in exam ==> “ read NbrMarks write “Student’s mark ==> “read StudentMarkset ProportionOfMarks to StudentMark / NbrMarks set Percentage to 100 * ProportionOfMarkswrite “ Student’s percentage: “write Percentage

4 variables: NbrMarks, StudentMark, ProportionOfMarks,

Percentage


Alternative Designwrite “Number of marks in exam ==> “ read NbrMarks write “Student’s mark ==> “read StudentMarkset Percentage to 100 * StudentMark / NbrMarkswrite “ Student’s percentage: “write Percentage

3 variables: NbrMarks, StudentMark,Percentage

In C++, this looks like…


Sample Program In C++

#include <iostream.h>void main (void){ int NbrMarks = 0; float StudentMark = 0, Percentage = 0; cout << "Number of marks in exam ==> "; cin >> NbrMarks; cout << "Student’s mark ==> "; cin >> StudentMark; Percentage = 100 * StudentMark / NbrMarks; cout << " Student’s percentage: "; cout << Percentage;

}


Declaring VariablesIn C++, you must declare variables before you

use them

Also, you must declare the type of value each variable will hold

The first variable declared is NbrMarks – it can hold any integer value

int NbrMarks = 0;

An integer is any +ve or –ve whole number, or zero: …-3, -2, -1, 0, 1, 2, 3, …


Naming Variables Start with a letter - A to Z

Can contain upper or lower case letters, or numerals - 0 to 9

Good idea to limit names to 32 characters

Must avoid C++ reserved words like int, float, void, etc

More later…


Initialising VariablesOur declaration of NbrMarks also gives it an

initial value - 0

int NbrMarks = 0;

If you do not give a variable an initial value, it will hold whatever data was left in memory by the previous program

Incorrect initialisation of variables is a very common logic error made by programmers

Please read the previous point again!


More DeclarationsOur program also declares two floating point

variables – StudentMark and Percentage

float StudentMark = 0, Percentage = 0;

We use variables of type float to hold values that may not be an integer

Examples:

An student’s mark: 15.5 (marks)

A distance: 123.456 (meters)

A temperature: -2.3 (degrees C)


Why “Floating Point”Some programs need to store very big numbers

– eg. 1,234,500,000,000,000,000; others need to store very small numbers - 0.000000000012345

These two numbers can be represented as:1.2345 * 1018 and 1.2345 * 10-12

float variables are stored in two parts:number part : 1.2345 in above casespower part : 18 or –12 in above cases

Decimal point “floats around”, depending on power part – more details later


More On DeclarationsWe could declare these variables like this:

float StudentMark = 0; float Percentage = 0;

Instead, a comma was used to separate two variables of the same type

float StudentMark = 0, Percentage = 0;Breaking the declaration over two lines is

optional in C++ - it’s “common practice”

float StudentMark = 0, Percentage = 0;

Notice how a comma is used here to separate two use of punctuation - , and ;


BranchingOur example program always performs the exact

same actions: get number of marks in exam get student’s mark calculate the student’s percentage display the student’s percentage

In practice, most programs are capable of performing different actions, depending on the inputs provided by the user

e.g., we may want to also state if the student passed the exam – so, sometimes the program will need to output “Pass”, otherwise “Fail”


Sample Dialogs

Number of marks in exam ==> 80

Student’s mark ==> 60

Student’s percentage: 75 (Pass)

or…

Number of marks in exam ==> 80

Student’s mark ==> 20

Student’s percentage: 25 (Fail)


Branching StatementsTo enable programs to perform different actions

(depending on user input) all programming languages include one or more branching or selection statements

The branching statement selects between one of more possible paths through the program

All programming languages include some form of if-then-else statement


Pseudocode Examplewrite “Number of marks in exam ==> “ read NbrMarks write “Student’s mark ==> “read StudentMarkset Percentage to 100 * StudentMark / NbrMarkswrite “ Student’s percentage: “write Percentageif Percentage < 50 then write “ (Fail)”else write “ (Pass)”


Example of if-then-elsewrite “Number of marks in exam ==> “ read NbrMarks write “Student’s mark ==> “read StudentMarkset Percentage to 100 * StudentMark / NbrMarkswrite “ Student’s percentage: “write Percentageif Percentage < 50 then write “ (Fail)” NbrMarks: ?else StudentMark: ? write “ (Pass)” Percentage: ?in C++, this looks like…


if-else Statements

#include <iostream.h>void main (void){ float StudentMark = 0; cout << "Student’s mark ==> "; cin >> StudentMark; if (StudentMark >= 50) {

cout << "Pass!"; } else {

cout << "Fail!"; }

}


if-else StatementsNotice that comparison between StudentMark

and 50 is enclosed in parentheses

if (StudentMark >= 50)

Also, braces mark start and end of the if-branch (shown below), and the else-branch {

cout << "Pass!"; }

Indentation makes code easier to read – it’s not required by the compiler


if-else-if Statements if-else-if statements can be coded as: if (StudentMark >= 75) {

cout << “Distinction!"; } else if (StudentMark >= 50) {

cout << “Pass!"; } else {

cout << "Fail!"; }


Activity – Code in C++write “Select conversion - (1) C to F, (2) F to C

==> “read ConversionTypewrite “Input temperature ==> “read Temperaturewrite “Converts to ”if ConversionType = 1 then write 32 + (Temperature * 1.8) write “ degrees Fahrenheit”else write (Temperature – 32) / 1.8 write “ degrees Centigrade”


A Start#include <iostream.h>void main (void){ int ConversionType = 0; float Temperature = 0; cout << "Select conversion - (1) C to F, (2) F to C ==> "; cin >> ConversionType;

}


A Solution#include <iostream.h>void main (void){ int ConversionType = 0; float Temperature = 0; cout << "Select conversion - (1) C to F, (2) F to C ==> "; cin >> ConversionType; cout << "Input temperature ==> "; cin >> Temperature; if (ConversionType == 1) {

cout << 32 + (Temperature * 1.8); cout << " degrees Fahrenheit";

} else { cout << (Temperature - 32) / 1.8; cout << " degrees Centigrade"; }}

note: C++ uses == instead of = for testing equality


LoopingOur program to calculate the exam

percentage for a single student performs the following 5 tasks: get number of marks in exam get student’s mark calculate the student’s percentage display the student’s percentage display “Pass” or “Fail”

To calculate the exam percentage for more than one student, a program must perform some of these tasks more than once…


LoopingTo calculate the percentage for 2 students, a

program must perform 9 tasks: get number of marks in exam get student’s mark calculate the student’s percentage display the student’s percentage display “Pass” or “Fail” get student’s mark calculate the student’s percentage display the student’s percentage display “Pass” or “Fail”


LoopingTo calculate the percentage for 3 students, a

program must perform 13 tasks: get number of marks in exam get student’s mark calculate the student’s percentage display the student’s percentage display “Pass” or “Fail” get student’s mark calculate the student’s percentage display the student’s percentage display “Pass” or “Fail” get student’s mark calculate the student’s percentage display the student’s percentage display “Pass” or “Fail”


LoopingWe could have:

-One program to handle 2 students, -Another program to handle 3 students, etc

It would be better to have a single program to handle any number of students

We would like to design a program to repeat some actions more than once

We want our program to: get the number of marks in exam from user- repeat as required - get student’s mark calculate the student’s percentage display the student’s percentage display “Pass” or “Fail”


LoopingOne design is…

Ask user for number of students at start: get number of marks in exam get number of students- repeat once for each student - get student’s mark calculate the student’s percentage display the student’s percentage display “Pass” or “Fail”

Another design is…


LoopingKeep asking if the user has more students:

get number of marks in exam- repeat while user wants more students - get student’s mark calculate the student’s percentage display the student’s percentage display “Pass” or “Fail” ask user if they have more students


Looping StatementsTo enable programs to perform different numbers

of actions (depending on data inputs) all programming languages include one or more looping or repetition statements

All programming languages include some form of while statement...

Other common looping statements are do-while and for


Pseudocode Examplewrite “Number of marks in exam ==> “ read NbrMarks write “Number of students ==> “ read NbrStudents set NbrProcessed to 0while NbrProcessed < NbrStudents write “Student’s mark ==> “ read StudentMark set Percentage to 100 * StudentMark / NbrMarks write “ Student’s percentage: “ write Percentage set NbrProcessed to NbrProcessed + 1


Example of whilewrite “Number of marks in exam ==> “

NbrMarks:

80read NbrMarks

NbrStudents:

2write “Number of students ==> “

NbrProcessed:

2read NbrStudents

StudentMark:

40set NbrProcessed to 0

Percentage:

50while NbrProcessed < NbrStudents write “Student’s mark ==> “ read StudentMark set Percentage to 100 * StudentMark / NbrMarks write “ Student’s percentage: “ write Percentage set NbrProcessed to NbrProcessed + 1


ActivityShow the output produced by the following pseudocode.

set TotalCost to 20set ItemCost to 5set NumberOfItems to 0while TotalCost > 0 set TotalCost to TotalCost - ItemCost set NumberOfItems to NumberOfItems + 1write “Number of items purchased: “write NumberOfItems


while Statement

#include <iostream.h>void main (void){ int NbrTimesTold = 0,

NbrTimesToTell = 0; cout << "How many times must I tell you? ==> "; cin >> NbrTimesToTell;

while (NbrTimesTold < NbrTimesToTell) {

cout << “ No new taxes!"; cout << endl; NbrTimesTold = NbrTimesTold + 1;

}}


Activity FeedbackThe program displays “No new taxes!”

the number of times requested by the user…

cout << endl; moves to start of next line

How many times must I tell you? ==> 3 No new taxes! No new taxes! No new taxes!


while StatementThings to notice:

Comparison enclosed in parentheses

while (NbrTimesTold < NbrTimesToTell) {

cout << endl; cout << " No new taxes!"; NbrTimesTold = NbrTimesTold + 1;

}

Use of braces and indentation


One More Detail About C++All compilers are fussy about punctuation

C++ compilers are also fussy about case

The following code has 3 compilation errors…

#Include <iostream.h> void Main (void) {

Cout << "Hello World!"; }


C++ is Case-Sensitive

#Include <iostream.h> void Main (void) {

Cout << "Hello World!"; }

This program has three errors:it should be #include , not #Includeit should be main , not Mainit should be cout , not Cout

C++ compilers are case-sensitive


Data ProcessingWe said that computers are data processing

devices – they convert input data into output data

Where does all the data processing occur in a program?

Recall our sample program...


Sample Programwrite “Number of marks: “ read NbrMarks write “Student’s mark: “read StudentMarkset ProportionOfMarks to StudentMark / NbrMarks set PercentageOfMarks to ProportionOfMarks *

100write “ Student’s percentage: “write PercentageOfMarks

In this program, all the data processing occurs in assignment statements


Anatomy of an AssignmentLet’s analyse the assignment statement…

Here, the two assignment statements are: set ProportionOfMarks to StudentMark / NbrMarks set PercentageOfMarks to ProportionOfMarks * 100

That is: set < variable > to < value of interest >

Here, the values of interest are: StudentMark / NbrMarks ProportionOfMarks * 100


ExpressionsWe use the term expression to mean:

the description of a value of interest

we describe the value that we wish to assign to a data object in an expression

so: StudentMark / NbrMarks ProportionOfMarks * 100

are two expressions


Data Processing

So, where does the data processing happen?

answer: some of it happens in

assignment statements

It can also happen in output statements…


Alternative Design

write “Number of marks: “ read NbrMarks write “Student’s mark: “read StudentMarkset ProportionOfMarks to StudentMark / NbrMarks write “ Percentage: “write ProportionOfMarks * 100


Anatomy of an OutputThe anatomy of our assignment statement is: set < variable > to < expression >

the anatomy of our output statement is: write < expression >

so, where does all the data processing happen?

Expressions !


ExpressionsClearly, expressions are important - that’s

where the data processing happens

Let’s take a closer look at expressions

Previously, we said that data was numbers and text -for now, we just deal with expressions to process numbers

The anatomy of an expression is one we’ve seen before...


Expressions as a Black BoxWe can think of an expression as a black

boxExpressions have one or more input values

and produce one output value - the input-process-output model again

Example: StudentMark / NbrMarks

input process output

StudentMark ? NbrMarks (a single value - depends on inputs)


Operatorswe use the term operator to mean:

a symbol, or name, used to represent an operation that can be performed on data

in the two expressions: StudentMark / NbrMarks ProportionOfMarks * 100

the operators are:/ for division* for multiplication

+ and - are used for addition and subtraction+, -, *, / all work in C++ as you would expect


OperandsWe use the term operand to mean:

an input to an expressionIn the two expressions: StudentMark / NbrMarks ProportionOfMarks * 100

The operands are:StudentMark and NbrMarks

ProportionOfMarks and 100


Binary OperatorsIn the following examples: StudentMark / NbrMarks ProportionOfMarks * 100 NbrMarks - StudentMark StudentMark + 10

Each operator is used with two operands

So / , * , - and + are binary operators –they can all be used with two operands


Unary OperatorsThe + and - operators are also unary

operators (they can be used with just one operand)

Examples: -273.15 as in set AbsoluteZero to -273.15 +100 as in set BoilingPointOfWater to

+100 expression - 273.15

operandoperator


Numeric Expressions expressions that evaluate to a number are

called numeric expressions numeric expression come in all shapes and

sizes: a number by itself – a literal: set NbrTimesTold to 0 the name of a variable: write Percentage expressions that use operators: set NbrTimesTold to NbrTimesTold + 1


Power of Expressions The arithmetic operators +, -, * and / give us a

powerful language to process numbersThe power comes from the ability to nest little

expressions inside bigger expressionsInstead of:set ProportionOfMarks to StudentMark / NbrMarks write ProportionOfMarks * 100We can write:write StudentMark / NbrMarks * 100

question: which operator is applied first here? and, does it matter?


Nested Expressionswhich operator is applied first here?

Is the division first?

StudentMark / NbrMarks * 100

divide StudentMark by NbrMarks, then multiply by 100

Or is the multiplication first?


multiply NbrMarks by 100, then divide StudentMark by result of multiplication

Activity: does it matter?


Activity FeedbackUsing StudentMark = 50, NbrMarks = 100…

Division first: (StudentMark / NbrMarks) * 100= (50 / 100) * 100= 50

Multiplication first:StudentMark / (NbrMarks * 100)= 50 / (100 * 100)= 0.005

Will a C++ program do it in the correct order?


Order of Use There are rules to decide the order in which

operators in an expression are appliedUnary operators before binary operatorsMultiplication (*) and division (/) before

addition (+) and subtraction (-)Otherwise, left to right

Evaluate the following: 4 * -2 + 3 2 + 12 / 4 * 3

Will the following be evaluated correctly?StudentMark / NbrMarks * 100


Activity Feedback Evaluate:

4 * -2 + 3unary operator first (- applies to 2)multiplication before addition (4 * -2) + 3= -8 + 3= -5


Activity FeedbackEvaluate the following:

2 + 12 / 4 * 3Multiplication and division before additionLeft to right otherwise – so division before

multiplication here2 + (12 / 4) * 3

= 2 + 3 * 3Multiplication before addition= 2 + (3 * 3)= 2 + 9= 11


Activity FeedbackWill the following be evaluated correctly?


Yes it will – since the division occurs before the multiplication, this is the same as:

(StudentMark / NbrMarks) * 100


Order of Use Avoid errors by using parentheses:

(4 * -2) + 3 2 + ( ( 12 / 4 ) * 3 )

Sometimes you can rewrite an expression to make it easier to read – instead of:StudentMark / NbrMarks * 100

You can write:100 * StudentMark / NbrMarks

Is this easier to understand? if so, why?


Activity FeedbackThe expression:

100 * StudentMark / NbrMarks

may seem easier to read than:StudentMark / NbrMarks * 100

Possibly because, in the first expression above, the order in which operators are applied doesn’t matter

– left for student to checkAlways keep you code as simple as possible

The following program is designed to convert temperatures between Fahrenheit and Centigrade

It has a logic error – fix it…


#include <iostream.h>void main (void){ int ConversionType = 0; float Temperature = 0; cout << "Select conversion - (1) C to F, (2) F to C ==> "; cin >> ConversionType; cout << "Input temperature ==> "; cin >> Temperature; if (ConversionType == 1) {

cout << 32 + Temperature * 1.8; cout << " degrees Fahrenheit";

} else { cout << Temperature - 32 / 1.8; cout << " degrees Centigrade"; }}



cout << 32 + Temperature * 1.8; cout << " degrees Fahrenheit";

} else { cout << Temperature - 32 / 1.8; cout << " degrees Centigrade"; }}

problem here: division occurs before subtraction

Feedback



cout << 32 + (Temperature * 1.8); cout << " degrees Fahrenheit";

} else { cout << (Temperature – 32) / 1.8; cout << " degrees Centigrade"; }}

a solution: enclose subtractionin parentheses

clarification: parentheses make intention clear

Feedback


C++ Syntax Summaryinput : cin >> <variable>; output : cout << <expression>;assignment : <variable> = <expression>;

A selection statement: if ( <test> ) { <if-block statements> } else { <else-block statements> }

A repetition statement: while ( <test> ) { <while-block statements> }


SummaryFive base programming statements are: input,

output, assignment, selection, repetition

These statements provide a platform for programming in all popular languages

Data processing happens in expressions

Different types of expressions – literals, variables names, ones that use operators…

Arithmetic operators are: +, -, *, /

Rules control order of application

Parentheses are used to impose ordering

COIT29222 Structured Programming 1 COIT29222-Structured Programming Lecture Week 02 Reading:...

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