Steps in Program Development
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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
Structured programming
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Top-down development
Modular design
Structure theorem
Algorithms
Pseudocode
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 parts
until it becomes manageable.
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.
The Structure Theorem
• Only three constructs should ever be used in
programming:–1
–2
–3
Sequence
Selection
Iteration
• i.e. no unstructured GOTOs or STOPs.
Algorithm Definition
• Lists the steps involved in accomplishing a task
which 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.
Pseudocode
• Statements are written in simple English.
• Each instruction is written on a separate line.
• Keywords and indentation are used to signify
particular 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.
Stored program concept
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Data area
Procedural area
Input
input
Output
Data
Instructions
Output
Program
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
Writing pseudocode
• 6 basic operations:–
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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
Receive data
• Takes data in from outside the algorithm
• Sample informal keywords used
– READ student name …
– GET system date ...
– INPUT number1, number2
Put out data
• Sends data out of the algorithm
– PRINT student number
– WRITE 'message’
– OUTPUT name, address, staff number
Perform arithmetic
• with specific pieces of data, where the
operands and destination are explicitly
specified.
• An operator
• An operand
• Source and destination
Operations in arithmetic
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+ Add
- Subtract
* Multiply
/ Divide
() Precedence
** To the power of
Assign a value to a piece of data
• Initialise TOTAL to 0 …
• Set COUNT to 0
• COUNT = COUNT + 1 ...
Consider a problem
• Take three numbers into the program, add
them together and output the result
Defining the problem
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Determine INPUT
Determine OUTPUT
Determine processing
This can be done using an IPO chart
(Input, Processing, Output):
INPUT
Number1
Number2
Number3
PROCESSING
Read 3 numbers
Add numbers together
Print total
OUTPUT
Total
Designing a solution algorithm
Draw a rough sketch of the steps required to
solve the problem.
Add-Three-Numbers
Read Number1, Number2, Number3
Total = Number1 + Number2 + Number3
Print Total
END
The algorithm is suitably named.
The end of the algorithm is marked as END.
Statements between the start and end are indented.
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 the
algorithm
• Walk the first test case through the algorithm, keeping a
record of the contents of each variable in the table as the
data 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.
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 should
not be subject to the condition.
Selection control structure
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Simple selection
Null ELSE statement
Combined IF statement
Nested IF statement
CASE statement
Simple selection
if (accountbalance < 300) - condition
servicecharge = 5.00 - executed if
condition true
else
servicecharge = 2.00 - executed if
condition false
Endif
Null ELSE statement
If StudentAttendance = Parttime
Add 1 to Parttime-Count
Endif
Nested IF statement:
If oper = ‘*’
c=a*b
else
if oper = ‘+’
c=a+b
else
if oper = ‘-’
c= a - b
else
display “Illegal operator”
endif
endif
endif
Display c, ‘=‘, a, oper, b
Case statement:
Case oper of:
‘*’:
c=a*b
‘+’ :
c= a + b
‘-’:
c=a-b
Other:
display ‘illegal operator’
End Case
display c, ‘=‘,a,operator,b
Part B - case statement
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;
}
• 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.
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 actions
6. Repeat a group of actions
Repetition Control Structure
• Coding which needs to be repeated for a
defined set of circumstances.
• Define the coding which needs to be
repeated.
• Define the conditions under which the
coding should be repeated.
1. Leading edge tested loop
DOWHILE condition
coding
ENDDO
• Condition is tested before the coding is executed - if the
condition is false, no coding is executed.
• The coding is done WHILE the condition is TRUE. The
condition is tested every time execution loops around.
• When the condition is found to be false, execution continues at
the next statement after the ENDDO
Flowchart notation of WHILE
Condition
true?
True
code
False
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 will
read 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 at
the top of the page.
IPO chart
INPUT
15 entries
F-temp
PROCESSING
Print column headings
For each entry
Read F-temp
Convert F-temp to C-temp
Print F-temp, C-temp
OUTPUT
Headings
F-temp
C-temp
Fahrenheit-Celsius conversion
Print 'Degrees F' and 'Degrees C'
Entry-counter = 0
DOWHILE Entry-counter < 15
Read F-temp
C-temp = (F-temp - 32) * 5/9
Print F-temp, C-temp
Entry-counter = Entry-counter + 1
ENDDO
END
Don't forget to desk-check.
2.
Trailing edge tested loop
REPEAT
coding
UNTIL condition
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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 ANSWER
until ANSWER = 'Y' or 'N’
Flowchart notation of Repeat
code
False
Condition
true?
True
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 only
those 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.
IPO chart
INPUT
Inventory entry with:
Item number
Stock figure
PROCESSING
Print heading
For each entry:Read Entry
Print Entry
Increment Totallow-stock-items
Print Total-low-stockitems
OUTPUT
Heading
'Low stock items'
Inventory list with:Item number
stock-figure
Total-low-stock- items
Process-inventory-entries (pseudocode)
Set Total-Low-Stock-Items to zero
Print 'Low-Stock Items' Heading
REPEAT
Read inventory Entry
IF item number > 0 THEN
IF stock figure < 20 THEN
print item number,stock figure
increment Total-Low-Stock-Items
ENDIF
ENDIF
UNTIL item number = 0
Print Total-Low-Stock-Items
END
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 and
stops at a predestined point:DO loop-index = initial-value to final-value
statement block
ENDDO
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 values
– terminate the loop when the loop-index has
exceeded the specified final-value
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's
details. A total of the number of students under 18 years of
age is also to be printed at the end of the report.
IPO chart
INPUT
350 student
entries
studnt-no
sex
age
PROCESSING
Print heading
For each student:
read Entry
print details
print '*’
increment total-studentsunder-18
OUTPUT
Print total-students-under-18
Total-students-under-18
Heading'STUDENT LIST'
Student details containing:
studnt-no
sex
age
Print-student-list
1 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 <18 THEN
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print '*’
7
increment Total-Students-Under-18
ENDIF
ENDDO
8 Print Total-Student-Under-18
END
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
Desk checking
statemnt Tot-u-18 Loop
idx
1
2
0
3
0
1
4
0
1
5
0
1
6
7
3
4
5
6
7
8
/
/
0
0
0
0
1
/
/
2
2
2
2
2
Studntno
14872
14872
/
/
14872
15543
15543
15543
15543
Sex Age PrintStud?
Print *
Final
M
M
25
25
Yes
-
-
M
F
F
F
F
/
/
25
17
17
17
17
/
/
Yes
-
/
/
Yes
-
/
/
Yes
Comparison between WHILE and
REPEAT
WHILE
REPEAT
Add-numbers
Sum = 0
Read number
While number not = 999
sum = sum + number
read number
end-while
Print sum
END
Add-Numbers
Sum = 0
Repeat
read number
if number not = 999
sum = sum + number
end-if
until number = 999
Print sum
END