L05extraSteps in Program Development - Revision

7/31/2019 L05extraSteps in Program Development - Revision

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Steps in Program Development

Define the problem

Outline the solution

Develop the outline into an algorithm

Test the algorithm for correctness

Code the algorithm into a programming language

Run the program on computer Document and maintain the program


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Structured programming

Top-down development

Modular design

Structure theorem

Algorithms

Pseudocode


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Top-down development

Consider the problem as a whole and break

it down into component parts.

Each part becomes a problem to be solved

in its own right.

Using stepwise refinement, the problem is

repeatedly broken down into smaller partsuntil it becomes manageable.


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Modular design

A module is

a step or sub-task resulting from stepwise

refinement.

a problem solution which has a well-defined set

of inputs, processing and outputs.

Algorithms are written at module level.


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The Structure Theorem

Only three constructs should ever be used in

programming:-

1 Sequence

2 Selection

3 Iteration

i.e. no unstructured GOTOs or STOPs.


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Algorithm Definition

Lists the steps involved in accomplishing a taskwhich must

be lucid, precise and unambiguous

give the correct solution in all cases

eventually end

Algorithms can be written in pseudocode, or can

be expressed by means of a flowchart Other specification techniques include JSD or

Nassi-Schneiderman diagram.


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Pseudocode

Statements are written in simple English.

Each instruction is written on a separate line.

Keywords and indentation are used to signifyparticular control structures.

Each set of instructions is written from top to

bottom with only 1 entry and 1 exit. Groups of statements may be formed into modules

and that group can be given a name.


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Stored program concept

Data area

Procedural area

Input

Output input Output

Data

Instructions

Program


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Stored program concept

Data area is specific and rigorously defined

Procedural area can: -

Take data into the data area from outside

Put information out from the data area

Manipulate the data in the data area


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Writing pseudocode

6 basic operations:-

1. Receive a piece of data

2. Put out data

3. Perform arithmetic

4. Assign a value to a piece of data

5. Select a group of actions

6. Repeat a group of actions


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Receive data

Takes data in from outside the algorithm

Sample informal keywords used

READ student name

GET system date ...

INPUT number1, number2


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Put out data

Sends data out of the algorithm

PRINT student number

WRITE 'message

OUTPUT name, address, staff number


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Perform arithmetic

with specific pieces of data, where the

operands and destination are explicitly

specified.

An operator

An operand

Source and destination


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Operations in arithmetic

+ Add

- Subtract

* Multiply

/ Divide

() Precedence ** To the power of


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Assign a value to a piece of data

Initialise TOTAL to 0

Set COUNT to 0

COUNT = COUNT + 1 ...


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Consider a problem

Take three numbers into the program, add

them together and output the result


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Defining the problem

Determine INPUT

Determine OUTPUT

Determine processing

This can be done using an IPO chart(Input, Processing, Output):

INPUTNumber1Number2Number3

PROCESSINGRead 3 numbers

Add numbers togetherPrint total

OUTPUTTotal


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Designing a solution algorithm

Draw a rough sketch of the steps required tosolve the problem.

Add-Three-NumbersRead Number1, Number2, Number3Total = Number1 + Number2 + Number3Print Total

ENDThe algorithm is suitably named.

The end of the algorithm is marked as END.

Statements between the start and end are indented.


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Checking the algorithm Choose 2 or 3 simple input test cases which are valid.

Establish what the expected results should be.

Make a table of the relevant variable names within thealgorithm

Walk the first test case through the algorithm, keeping arecord of the contents of each variable in the table as thedata passes through the logic.

Repeat the walkthrough process, using the other test data

cases. Check the expected results against the actual results.

This is NOT a program proof.


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Selection - making choices

How do you decide what to do?

Look at the condition(s)

Specify the alternatives very clearly

Specify the conditions under which an

alternative option will be taken very clearly

Any instructions that are not optional shouldnot be subject to the condition.


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Selection control structure

Simple selection

Null ELSE statement

Combined IF statement

Nested IF statement

CASE statement


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Simple selectionif (accountbalance < 300) - condition

servicecharge = 5.00- executed if

condition true

else

servicecharge = 2.00- executed if

condition false

Endif

Null ELSE statementIf StudentAttendance = ParttimeAdd 1 to Parttime-Count

Endif


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Nested IF statement:

If oper = *

c = a * b

elseif oper = +

c = a + b

else

if oper = -

c= a - b

else

display Illegal operator

endif

endif

endif

Display c, =, a, oper, b


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Case statement:

Case oper of:

*: c = a * b

+ : c= a + b

-: c = a - bOther: display illegal operator

End Case

display c, =,a,operator,b


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switch (oper)

{

case '*':

c = a * b; break;

case '+':

c = a + b; break;

case '-':

c = a - b; break;

default:

printf("Illegal operator\n");

c = 0;}

printf("%f = %d%c%d\n",c,a,oper,b);

return 0;

}

Part B - case statement The block is enclosed

in braces

Once an appropriate

option has been

found, the code is

executed

If the break is

omitted, following

statements may also

be carried out.


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Structure theorem revisited

6 basic operations:-

1. Receive a piece of data

2. Put out data

3. Perform arithmetic

4. Assign a value to a piece of data

5. Select a group of actions6. Repeat a group of actions


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Repetition Control Structure

Coding which needs to be repeated for a

defined set of circumstances.

Define the coding which needs to berepeated.

Define the conditions under which the

coding should be repeated.


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1. Leading edge tested loop

DOWHILE conditioncoding

ENDDO

Condition is tested before the coding is executed - if thecondition is false, no coding is executed.

The coding is done WHILE the condition is TRUE. Thecondition is tested every time execution loops around.

When the condition is found to be false, execution continues atthe next statement after the ENDDO


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Flowchart notation of WHILE

Condition

true?

code

True False


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Sample leading edge loop

e.g. A temperature data entry consists of 15 entries,

each containing a temperature in degrees

Fahrenheit. A program is to be written which willread and print the temperatures in two columns on

a report, one column in degrees Fahrenheit and the

other in degrees Celsius. Column headings which

read Degrees F and Degrees C are to be printed atthe top of the page.


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IPO chart

INPUT

15 entries

F-temp

PROCESSING

Print column headings

For each entry

Read F-tempConvert F-temp to C-temp

Print F-temp, C-temp

OUTPUT

Headings

F-temp

C-temp


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Fahrenheit-Celsius conversion

Print 'Degrees F' and 'Degrees C'

Entry-counter = 0

DOWHILE Entry-counter < 15

Read F-tempC-temp = (F-temp - 32) * 5/9

Print F-temp, C-temp

Entry-counter = Entry-counter + 1

ENDDOEND

Don't forget to desk-check.


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2. Trailing edge tested loop

REPEATcoding

UNTIL condition

The coding is executed The condition is tested

If the condition is false, the process is repeated

If the condition is true, execution continues at the next line.

e.g.Display "Do you wish to continue [Y/N]?"Repeat

Input ANSWERuntil ANSWER = 'Y' or 'N


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Flowchart notation of Repeat

Condition

true?

code

True

False


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Example

A program is required to read a series of inventory entries

which contain item number and stock figure. The last

entry in the file has an item number of zero. The program

is to produce a 'Low Stock Items' report, by printing onlythose entries which have a stock figure of less than 20

items. A heading is to print at the top of the report and a

total low stock item count to print at the end.


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IPO chart

INPUT

Inventory entry with:

Item number

Stock figure

PROCESSING

Print heading

For each entry:-

Read EntryPrint Entry

Increment Total-

low-stock-items

Print Total-low-stock-

items

OUTPUT

Heading

'Low stock items'

Inventory list with:-Item number

stock-figure

Total-low-stock- items


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Process-inventory-entries (pseudocode)

Set Total-Low-Stock-Items to zeroPrint 'Low-Stock Items' HeadingREPEAT

Read inventory Entry

IF item number > 0 THENIF stock figure < 20 THEN

print item number,stock figure

increment Total-Low-Stock-ItemsENDIF

ENDIF

UNTIL item number = 0Print Total-Low-Stock-Items

END


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3. Auto-increment loop

If the exact number of loop iterations is known in advance,

then the execution of the loop can be controlled by a loop

index or counter, which is initialised for the first pass through

the loop, auto-increments on each pass through the loop andstops at a predestined point:-

DO loop-index = initial-value to final-value

statement blockENDDO


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How the DO loop works

The DO loop does the following:-

Initialise the loop-index to the required initial-value

increment the loop-index by 1 for each pass through the

loop

test the value of loop-index at the beginning of each

loop to ensure that it is within the stated range of valuesterminate the loop when the loop-index has

exceeded the specified final-value


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Example

A program is required to read a file of 350 student entries

containing student number, sex and age and to produce a

STUDENT LIST. If a student is under 18 years of age,

then an asterisk '*' is to be printed beside that student'sdetails. A total of the number of students under 18 years

of age is also to be printed at the end of the report.


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IPO chart

INPUT

350 student

entries

studnt-no

sex

age

PROCESSING

Print heading

For each student:

read Entry

print details

print '*

increment total-students-

under-18

Print total-students-under-18

OUTPUT

Heading'STUDENT LIST'

Student details containing:studnt-no

sex

age

Total-students-under-18


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Print-student-list1 Print 'STUDENT LIST' Heading

2 Set Total-Students-Under-18 to zero

3 DO loop-index = 1 to 350

4 read student Entry

5 print student details

IF age


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Desk checking

For two valid entries (change loop size to 2 from 350): -

Number Sex Age

Record 1 14872 M 25

Record 2 15543 F 17


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Desk checkingstatemnt Tot-u-18 Loop

idx

Studnt-

no

Sex Age PrintStud? Print * Final

1 - - - - - - -2 0 - - - - - -

3 0 1 - - - - -4 0 1 14872 M 25 - - -

5 0 1 14872 M 25 Yes - -6 / / / / / / /

7 / / / / / / / 3 0 2 14872 M 25 - - -

4 0 2 15543 F 17 - - -

5 0 2 15543 F 17 Yes - -6 0 2 15543 F 17 - Yes -

7 1 2 15543 F 17 - - -

8 Yes


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Comparison between WHILE and

REPEATWHILE

Add-numbers

Sum = 0Read number

While number not = 999

sum = sum + number

read number

end-while

Print sum

END

REPEAT

Add-Numbers

Sum = 0Repeat

read number

if number not = 999

sum = sum + number

end-if

until number = 999

Print sum

END

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L05extraSteps in Program Development - Revision