1. Chapter 5
Desk Checking/Dry Running

2. Desk Checking/Dry running
Basically it is the manual simulation/process of walking through a program’s logic on paper before actually writing the program (coding).
Used to test for the correctness of the solution algorithm
Vital to check for errors during development of the solution algorithm (before coding) so that these errors are not implemented
Done during planning/design stage
In a large system, if there is logical error, it is very difficult to trace back the program as it involves 1000s lines of codes.
Eliminate most errors but cannot prove algorithm is 100% correct
Analyze codes = time consuming = costly

3. Desk Checking/Dry running
It involves
Use pen and paper
Tabulate
All variables (one column for each)
Conditions – result is T for True or F for False
Output
walk through the logic of the algorithm with specific test data.
and keep track of the values of all variables through out the process.

4. Desk Checking/Dry running
May involves 6 steps for desk checking the solution:
Choose 2 or 3 sets of simple input data (may include one invalid data to test the condition, if any) .
Define/calculate the expected result for each test case.
Make a table of relevant variable names within the algorithm e.g. Read number, total, while counter < 10 etc.

5. Desk Checking/Dry running
Walk the first test data through the algorithm, keeping a step-by-step record of the content of each variable as the data passes through the logic.
Repeat the walk-through process using the next test data until the algorithm has reached its logical end.
Check that the expected result matches the actual result in the walk-through.
For some cases you may omit steps 1 & 2 (if you do desk check
after you do analysis using stepwise)

6. Example 1– Problem spec
Analyze the problem to accept customer name, purchase amount, amount paid and payment type (Cash or Card). Calculate and display the change.

7. Example 1– solution algorithm
Read customerName, purchaseAmt, amtPaid, paymentType
Calculate change
change = amtPaid - purchaseAmt
Display change

8. Example 1– Desk Checking
Step 1 - Choose 2 or 3 valid simple input data
Variables
Data1
Data2
CustomerName
purchaseAmt
amtPaid
paymentType
Aziz
100.00
Card
Ahmad
55.00
Cash

9. Example 1– Desk Checking
Step 2 - Define/calculate the expected result for each test case.
Variables
Data1
Data2
change
0.00
45.00

10. Example 1- Desk Checking
Step 3 - Make a table of relevant
variable names
statement
Customer
Name
purchaseAmt
amtPaid
Payment
Type
change
Output
Data 1
Read
Display
Data 2

11. Example 1- Desk Checking
Step 4 & 5 - walk-through process + record the content of each variable
statement
Customer
Name
purchaseAmt
amtPaid
Payment
Type
change
Output
Data 1
Read
Display
Aziz
100.00
Card
0.00
Data 2
Ahmad
55.00
Cash
45.00

12. Example2 – Problem Spec
A program is required to read from the screen the length and width of a rectangular house block and the length and width of the rectangular house that has been built on the block. The algorithm should then compute and display the mowing time required to cut the grass around the house, at a rate of 2 square metres per minute.
Chp 3 – Simple Program Design

13. Example3 – Problem Spec
Find the average of 4 integers which lie between 0 and 100 (exclusive) using iterative control structure

14. Example3 – solution algorithm
1. Set num_count = 0, total = 0 2. While num_count < Read number 2.2 If number > 0 AND number < total = total + number Add 1 to num_count 3. Calculate average avg_num = total / num_count 4. Display avg_num

15. Example 1– Desk Checking
Step 1 - Choose input data
Data 1
Data 2
number
20,30,40,60
1, 40, -5, 29, 100, 99

16. Example 1– Desk Checking
Step 2 - Define/calculate the expected result for each test case.
Variables
Data 1
Data 2
avg_num
37.50
42.25

17. Example 3- Desk Checking
Step 3 – List out variables, conditions, output
statement
num_count
total
While OK?
number
IF OK?
avg_num
output
Data1
1. Set
2. while T
2.1. Read 20
2.2. IF
total
2.2.2 Add 1 30

18. Example 3- Desk Checking
Step 3 – List out variables, conditions, output
statement
num_count
total
While OK?
number
IF OK?
avg_num
output
total 50
2.2.2 Add 2
2. while T
2.1. Read 40
2.2. IF 90 3

19. Example 3- Desk Checking
Step 3 – List out variables, conditions, output
statement
num_count
total
While OK?
number
IF OK?
avg_num
output
2.1. Read 60
2.2. IF T
total
150
2.2.2 Add 4
2. while F
3. avg_num
37.50
4. Display

20. Example 3- Desk Checking
Step 3 – List out variables, conditions, output
statement
num_count
total
While OK?
number
IF OK?
avg_num
output
Data2
1. Set
2. while T
2.1. Read 1
2.2. IF
total
2.2.2 Add 40

21. Example 3- Desk Checking
Step 3 – List out variables, conditions, output
statement
num_count
total
While OK?
number
IF OK?
avg_num
output
total 41
2.2.2 Add 2
2. while T
2.1. Read -5
2.2. IF F 29

22. Example 3- Desk Checking
Step 3 – List out variables, conditions, output
statement
num_count
total
While OK?
number
IF OK?
avg_num
output
total 70
2.2.2 Add 3
2. while T
2.1. Read
100
2.2. IF F 99
189

23. Example 3- Desk Checking
Step 3 – List out variables, conditions, output
statement
num_count
total
While OK?
number
IF OK?
avg_num
output
2.2.2 Add 4
2. while F
3. avg_num
42.25
4. Display

24. Class Exercise 1
Calculate the discounted price of an item purchased. Design the algorithm and do the desk check

25. Class Exercise 2
Calculate the discounted price of an item purchased. Customers receive a discount of 15% on an item if the purchase price of the item is over $100. Design the algorithm and do the desk check