PROGRAMMING, ALGORITHMS AND FLOWCHARTS

What is Programming?

Series of instructions to a computer to accomplish a task

Instructions must be written in a way the computer can understand

Programming languages are used to write programs

ALGORITHMS AND FLOWCHARTS

A typical programming task can be divided into two phases:

Problem solving phase produce an ordered sequence of steps that describe

solution of problem this sequence of steps is called an algorithm

Implementation phase implement the program in some programming

language

Devise an algorithm for cooking a sausage on a barbecue

Steps in Problem Solving

First produce a general algorithm. Refine the algorithm successively to get step by

step detailed algorithm that is very close to a computer language.

Pseudocode is an artificial and informal language that helps programmers develop algorithms. Pseudocode is very similar to everyday English.

Pseudocode & Algorithm

Example 1: Write an algorithm to determine a student’s final grade and indicate whether it is passing or failing. The final grade is calculated as the average of four marks.

Start with a general algorithm.

General Algorithm

Pseudocode: Input a set of 4 marks Calculate their average by summing and dividing

by 4 if average is below 50

Print “FAIL”else

Print “PASS”

Detailed Algorithm

Step 1: Input M1,M2,M3,M4Step 2: GRADE

(M1+M2+M3+M4)/4 Step 3: if (GRADE < 50) then

Print “FAIL” else

Print “PASS”endif

Task: add two numbers

Pseudocode:Start

Get two numbers

Add them

Print the answer

End

Next Step - The Flowchart

A graphical representation of the sequence of operations in an information system or program. Information system flowcharts show how data flows from source documents through the computer to final distribution to users. Program flowcharts show the sequence of instructions in a single program or subroutine. Different symbols are used to draw each type of flowchart.

The sequence of the flowchart and the algorithm MUST match.

The Flowchart

A Flowchartshows logic of an algorithmemphasizes individual steps and their

interconnectionse.g. control flow from one action to the next

Flowchart Symbols Basic

Oval

Parallelogram

Rectangle

Diamond

Hybrid

Name Symbol Use in Flowchart

Denotes the beginning or end of the program

Denotes an input operation

Denotes an output operation

Denotes a decision (or branch) to be made. The program should continue along one of two routes. (e.g. IF/THEN/ELSE)

Denotes a process to be carried oute.g. addition, subtraction, division etc.

Flow line Denotes the direction of logic flow in the program

Module

Denotes a self-contained section of the program

Example

PRINT“PASS”

Step 1: Input M1,M2,M3,M4Step 2: GRADE (M1+M2+M3+M4)/4 Step 3: if (GRADE <50) then

Print “FAIL” else

Print “PASS” endif

START

InputM1,M2,M3,M4

GRADE(M1+M2+M3+M4)/4

ISGRADE<50

PRINT“FAIL”

STOP

YN

Control Structures

Represent the flow of logic through the programme.

There are four main control structures:•Sequence•Decision – incorporating if-then-else•Repetition•Case

Sequence Structure• a series of actions are performed in sequence

START

Display message “How many

hours did you work?”

Read Hours

Display message “How much do you get paid per

hour?”

Read Pay Rate

Multiply Hours by Pay Rate.

Store result in Gross Pay.

Display Gross Pay

END

This is the flowchart for a program which calculates an employee’s gross pay.

Example 1

Write an algorithm and draw a flowchart to convert the length in feet to centimetres.

Pseudocode: Input the length in feet (Lft) Calculate the length in cm (Lcm) by

multiplying LFT with 30 Print length in cm (LCM)

Example

Algorithm Step 1: Input Lft Step 2: Lcm Lft x 30 Step 3: Print Lcm

START

InputLft

Lcm Lft x 30

STOP

Flowchart

PrintLcm

Example 2

Write an algorithm and draw a flowchart that will read the two sides of a rectangle and calculate its area.

Pseudocode Input the width (W) and Length (L) of a rectangle Calculate the area (A) by multiplying L with W Print A

Example 2

Algorithm Step 1: Input W,L Step 2: A L x W Step 3: Print A

START

InputW, L

A L x W

STOP

PrintA

DECISION STRUCTURES

One of two possible actions is taken, depending on a condition.

Decision Structure

A new symbol, the diamond, indicates a yes/no question.

Decision Structures

Sometimes only one choice is necessary

YES

NO

Example 1

Pseudocode:Start Get year bornCalculate agePrint age If age > 50 print OLDEnd

Flowchart Start Get year born Calculate age Print age If age > 50 print OLD End

Get yr

Calc age

Print age

Age>50

OLD

Y

N

Start

End

Decision Structure

Sometimes two choices are offered. If the answer to the question is yes, the flow follows one path. If the answer is no, the flow follows another path

YESNO

Decision Structure

In the flowchart segment below, the question “is x < y?” is asked. If the answer is no, then process A is performed. If the answer is yes, then process B is performed.

IF–THEN–ELSE STRUCTURE

The structure is as follows

If condition then

true alternative

else

false alternative

IF–THEN–ELSE STRUCTURE

The algorithm for the flowchart is as follows:

If A>B then print A

else print B

isA>B

Print APrint B

N Y

Example 1

A ticket seller is issuing show tickets at the gate. When the patrons arrive, he asks how old each one is. It they are under 12, they pay half price. If they are 12 or over, they pay full price.

Algorithm

Get age

Age < 12?

If Age < 12Then

Pay half priceElse

Pay full price

Flowchart

YESNO

Begin

Input age

Age < 12

Half priceFull price

End

Example 2

Write an algorithm that reads two values, determines the largest value and prints the largest value with an identifying message.

ALGORITHMStep 1: Input VALUE1, VALUE2Step 2: if (VALUE1 > VALUE2) then

MAX VALUE1else

MAX VALUE2endif

Step 3: Print “The largest value is”, MAX

Example 2

MAX VALUE2

STOP

N Y

START

InputVALUE1,VALUE2

MAX VALUE1

isVALUE1>VALUE2

PrintMAX

Example 3

Write the algorithm and draw the flowchart for the following:

Enter two numbers, x and y. If x is less than y, square x to give the result a, otherwise, add x + y to give the result a.

YESNO

x < y

Calculate a as x times 2.

Calculate a as x plus y.

Algorithm

Input xInput y

x < y?

If x <yThen

a = x^Else

a = x+y

NESTED IFS

One of the alternatives within an IF–THEN–ELSE statementmay involve further IF–THEN–ELSE

statement

Repetition StructureRepetition (Iteration) Structure

A repetition structure represents part of the program that repeats. This type of structure is commonly known as a loop.

Repetition Structure

Notice the use of the diamond symbol. A loop tests a condition, and if the condition exists, it performs an action. Then it tests the condition again. If the condition still exists, the action is repeated. This continues until the condition no longer exists.

Repetition Structure

In the flowchart segment, the question “is x < y?” is asked. If the answer is yes, then Process A is performed. The question “is x < y?” is asked again. Process A is repeated as long as x is less than y. When x is no longer less than y, the repetition stops and the structure is exited.

x < y Process A

YES

x < y Add 1 to x

YES

So this might look like…

When would this loop exit?

The action performed by a repetition structure must eventually cause the loop to terminate. Otherwise, an infinite loop is created.In this flowchart segment, x is never changed. Once the loop starts, it will never end.QUESTION: How can this flowchart be modified so it is no longer an infiniteloop?

Out of Control!

x < y Display x

YES

This type of structure is known as a pre-test repetition structure. The condition is tested BEFORE any actions are performed.

Pre-test ( Before) Loop

x < y? Display x

Add 1 to x

YES

Post-test (After) Loop

This flowchart segment shows a post-test repetition structure.The condition is tested AFTER the actions are performed.A post-test repetition structure always performs its actions at least once.

Display x

Add 1 to x

YESx < y

Case Structure

One of several possible actions is taken, depending on the contents of a variable.

Used for MORE THAN TWO decisions

The structure below indicates actions to perform depending on the value in years_employed.

CASEyears_employed

1 2 3 Other

bonus = 100 bonus = 200 bonus = 400 bonus = 800

CASEyears_employed

1 2 3 Other

bonus = 100 bonus = 200 bonus = 400 bonus = 800

If years_employed = 1, bonus is set to 100

If years_employed = 2, bonus is set to 200

If years_employed = 3, bonus is set to 400

If years_employed is any other value, bonus is set to 800

Case AlgorithmInput a Grade

Case based on Grade    Case =100        Report “Perfect Score” (or suitable command eg Grade = Perfect)     Case > 89        Report “Grade = A”    Case > 79        Report “Grade = B”    Case > 69        Report “Grade = C”    Case > 59        Report “Grade = D”    Default (or Else)        Report “Grade = F”End Case

Draw the Flowchart for the above example

CASEgrade

>89 >79 >69 > 59

Grade =A Grade = B Grade = C Grade = D

Flowchart

Grade = Perfect Grade = F

100 other

Construct the algorithm and the flowchart for the following procedure:

An egg packing business needs to devise a process to sort eggs for delivery to supermarkets. If the eggs weigh 50 grams, they are Class A eggs. If they weigh 60 grams, they are Class B eggs. If they weigh 70 grams, they are Class C eggs. If they weigh more than 70 grams, they are Class D eggs.

Input a Weight

Case based on Weight    Case =50        Class = A    Case =60        Class = B    Case = 70        Class = C    Case > 70        Class = D    OR        Default/ Else Class = D End Case

Algorithm

CASEweight

50 60 70 > 70

Class =A Class = B Class = C Class = D

Flowchart

Modules (stepwise refinement)A program module is a part of a program that makes sense on its own, just like a paragraph in an essay should make sense on its own. In large programs, such as those used in industry, programs are developed as separate modules and then put together.The process of representing modules in flowcharts is known as stepwise refinement. You will look at this more closely when you start programming.

•The position of the module symbol indicates the point the module is executed.

•A separate flowchart can be constructed for the module.

START

END

Read Input.

Call calc_pay function.

Display results.

START

Display message “How many

hours did you work?”

Read Hours

Display message “How much do you get paid per

hour?”

Read Pay Rate

Multiply Hours by Pay Rate.

Store result in Gross Pay.

Display Gross Pay

END

A simpler example… could look like

CASEweight

50 60 70 > 70

Class =A Class = B Class = C Class = D

Input weight

Calculate class

Display class

A complete program example When you use stepwise refinement, the complete, refined flowchart is drawn first. Then the modules are named and drawn in order.This program lists the AFL team name and points total for all teams that have a full-forward from the AFL competition who has kicked at least 10 goals.

Startup

Headings

MainLoop

AFL

Goals >= 10Check goals

Check pos

Check goalsCheck conf

Finish

Although this procedure is listed as a module in the opening flow chart, it’s unsuitable. Why?