Flowcharts and Algorithms. Review of Terms A computer is a machine that can represent and manipulate data –Ultimately the data and the instructions.

Flowcharts and Algorithms

Review of Terms

A computer is a machine that can represent and manipulate data

– Ultimately the data and the instructions for manipulating it will be stored in binary

A program is an algorithm, expressed in a language the computer can understand

For example Pascal, Java, Visual Basic

The Two Phases of ProgrammingThe Two Phases of Programming

The task of programming can be divided into

two distinct phases:

– The Problem Solving Phase

• When we think about the problem and design an algorithm to help us solve it

–The Implementation (Coding) Phase

• When we translate the algorithm into a programming language

Algorithm Design Tool: PseudocodeAlgorithm Design Tool: Pseudocode

We have used pseudocode to design algorithms

Pseudocode:

– Is a generic way of describing an algorithm, without use of any specific programming language

– Helps programmers to plan an algorithm

– Not an actual programming language, but uses programming constructs available in all popular programming languages

Writing Pseudocode AlgorithmsWriting Pseudocode Algorithms

We need to be certain that the problem we are

solving is fully understood

Questions to ask:

– What data is known before the program runs?

– What data must be input by the user?

– What computations will be performed on the data?

– What data will be output to the user?

ExampleExample

Real World Problem:

– Calculate your two childrens’ pocket money, based upon one euro per year old.

Known Values

– Rate = one euro per year Inputs

– Ages of children Calculations

– Pocket_Money = Age x Rate Outputs

– Pocket_Money for each child

Pseudocode AlgorithmPseudocode Algorithm

Obtain Age of Child1

Obtain Age of Child2

Pocket_Money for Child1 = Age of Child1 x Rate

Pocket_Money for Child2 = Age of Child2 x Rate

Display Pocket_Money for Child1 and Child2

Algorithm including SelectionAlgorithm including Selection

Real World Problem:– Calculate one child’s Pocket Money, based upon one euro per

year old if s/he is under 10, and two euros per year if 10 or over.

Known Values– YoungRate = one euro per year– OlderRate = two euros per year– BreakAge = 10

Inputs– Age of child

Calculations– Pocket_Money = Age x Rate

Outputs– Pocket_Money

Pseudocode AlgorithmPseudocode Algorithm

Obtain Childs_Age

IF Childs_Age less than 10

THEN Pocket_Money = Age x YoungRate

ELSE Pocket_Money = Age x OlderRate

Display Pocket_Money

Algorithm including a LoopAlgorithm including a Loop Real World Problem:

– Calculate the total Pocket Money paid to each of three children at one euro per year old.

Known Values– Rate = one euro per year– NumChildren = 3

Inputs– Ages of children

Calculations– Pocket_Money = Age x Rate

Outputs– Total Pocket Money paid

Pseudocode AlgorithmPseudocode Algorithm

Set NumOfChildrenPaid to 0

Set TotalPocketMoneyOwed to 0

WHILE NumOfChildrenPaid < 3 DO

Obtain Childs_Age

Pocket_Money = Age x Rate

ADD Pocket_Money to TotalPocketMoneyOwed

ADD 1 to NumOfChildrenPaid

ENDWHILE

DISPLAY Total

AbstractionAbstraction

We use procedures (or mini-algorithms) to break the solution into meaningful “chunks”

If NumOfChildren > 1

ThenCalculatePocketMoneyforManyChildren

ElseCalculatePoketMoneyforOneChild

Endif

Each procedure is then described in pseudocode

Flowcharts as an Alternative to Flowcharts as an Alternative to PseudocodePseudocode

Perhaps a picture is worth a thousand words Sometimes it is easier to follow an algorithm if it is

represented in a diagram Program flowcharts are widely used for this

purpose

Example FlowchartExample Flowchart

Flowchart SymbolsFlowchart Symbols

Rectangle is used for actions

Diamond is used for decisions

Symbols are connected by

arrows to represent the

order of the actions

Flowcharting Control StructuresFlowcharting Control Structures

IF Statements

IF statements are used to cause certain actions to occur

within a program when specific conditions are met.

The simplest is the If/Then statement:

If Balance < 0 Then Display "You are overdrawn“

Here, if the Balance is less than zero, the message “You

are overdrawn” is shown.

IF exampleIF example

“You are overdrawn”Balance<0yesP

rog

ram flo

w

If/Then/EndIfIf/Then/EndIf

You can allow multiple statements by using If/Then/End If

Example

If Balance  < 0 Then

Display "You are overdrawn“

Display "Authorities have been notified“

End If

In this case, if Balance is less than zero, the two lines of

information are shown (one at a time!)

If/Then/EndIf exampleIf/Then/EndIf example

“You are overdrawn”Balance<0yes

“Authorities have been notified”

End If

Pro

gram

flow

If/Then/Else/EndIfIf/Then/Else/EndIf

When more than one result needs to be catered for we can use-

If/Then/Else/EndIf blocks: If Balance < 0 Then

Display "You are overdrawn"

Display "Authorities have been notified"

Else

Display " Your balance is " , Balance

EndIf

Here, the same two lines are printed if you are overdrawn (Balance < 0), but, if you are not overdrawn (Else), a message indicating your Balance is displayed.

If/Then/Else/EndIf exampleIf/Then/Else/EndIf example

“You are overdrawn”Balance<0

yes

“Authorities have been notified”

End If

“Your balance is £x”

no

End If

ElseIfElseIf

Or, we can add the ElseIf statement:If Balance < 0 Then

Display "You are overdrawn"

Display "Authorities have been notified"

ElseIf Balance = 0 Then

Display "You have no money left"

Else

Display (" Your balance is “, Balance)

End If

Now, one more condition is added.

If your Balance = 0, a different message appears.

If/Then/Else/EndIf exampleIf/Then/Else/EndIf example

“You are overdrawn”Balance<0

yes

“Authorities have been notified”

“Your balance is £x”

no

End If

Balance=0

no

“You have no money left”

yes

End If End If

Flowchart for Pocket Money ExampleFlowchart for Pocket Money Example

Previous Pseudocode Algorithm:

PROMPT for Age

READ Age

IF Age less than 10

THEN Pocket_Money =

Age x YoungRate

ELSE Pocket_Money =

Age x OlderRate

ENDIF

DISPLAY Pocket_Money

PM = Age x YoungRate

start

Print PM

stop

prompt Age

PM = Age x OlderRate

Age < 10 FALSETRUE

input Age

NoteNote

In using branching statements, make sure you consider all

viable possibilities in the If/Else/End If structure.

Also, be aware that each If and ElseIf in a block is tested

sequentially.

The first time an If test is met, the code associated with that

condition is executed and the If block is exited.

If a later condition is also True, it will never be considered.

And /OrAnd /Or

You may want to test conditions that are combined in different ways.

AND

For example

You want to print “Accepted” if the amount in an account is greater than £100 and the number of bad debts is zero.

This would be written like this:If Balance > 100 AND BadDebtsNo = 0 then Print “Accepted”

Only if both conditions are met will the message be printed.

And /OrAnd /Or

OR

The OR statement is more tolerant and will allow a condition to be met if either of two conditions are met.

For example

A person can have a discount of 20% in a store if they are over 64 years of age or have a club card.

This would be written:

If Age > 64 OR Card = True then Discount = 0.20

These conditions can be incorporated into IF/Then/Else/EndIf and If/Then/ElseIf/Else/EndIf statements

Nested StructuresNested Structures

Control structures can be nested; that is one can be placed within another.

Consider the flow diagram below.

Enter Name

Type of drink

Cost = £0.75

Tea

Type of coffee

Cost = £1.75

Latte

Cost = £2.15

Espresso

Coffee

Looping StructuresLooping Structures

Times = 0

Times = Times + 1

Do this

Do that

Does Times = 5

End

Yes

No

A looping structure to repeat operations five times

Different Types of LoopDifferent Types of Loop

While conditions do begin : statements / commands : end;

Repeat : statements / commands :

Until condition;

Previous Pseudocode Algorithm:

Set KidsPaid to 0

Set Total to 0

WHILE KidsPaid < 3 DO

PROMPT for Age

READ Age

PocketMoney = Age x Rate

ADD PocketMoney to Total

ADD 1 TO KidsPaid

ENDWHILE

DISPLAY Total

start

Display Total

stop

Read Age

Calc PM

KidsPaid < 3FALSETRUE

KidsPaid = 0

Total = 0

Add PM to Total

Increment KidsPaid

Prompt Age

Flowchart for Pocket Flowchart for Pocket Money ExampleMoney Example

ExerciseExercise

Take another look at the problems in the pseudocode tutorial – develop flowcharts to solve the various problems we considered there

Any advantage over pseudocode – do you think the graphical notation pushes you towards developing better structured algorithms?

Review QuestionsReview Questions

What are the three major components of a flowchart? How many arrows can emanate from a process box in

a flowchart? How many arrows can emanate from a decision box? Draw a flowchart to describe the process of finding a

name in a telephone directory by opening the directory in the middle, then moving to the middle of the half where the name will be found and so on until either the name is found or it is established that the name is not in the directory

How do flowcharts compare with pseudocode as a means of specifying program logic?

– Would you have a personal preference?