Transcript Desk Checking - Education Mailing Lists

Desk Checking
Desk Checking
•
•
Desk checking is a manual (non computerised) technique for checking the logic of an algorithm.
The person performing the desk check effectively acts as the computer, using pen and paper to
record results. The desk checker carefully follows the algorithm being careful to rigidly adhere to
the logic specified. The desk check can expose problems with the algorithm.
Desk checks are useful to check an algorithm (before coding) thereby confirming that the algorithm
works as expected and saves time possibly writing a program that doesn't do what was intended.
Another benefit of a desk check is that it confirms to the programmer/designer that the algorithm
performs as intended.
A desk check is normally done as a table with columns for:
1.
2.
3.
4.
Pseudo code line number column (Pseudo code doesn't normally have lines numbers, but these
are necessary in a desk check to specify the line(s) being executed)
One column per variable used. The columns should be in alphabetical order on variable name
with the variable name at the top of the column. As the algorithm is executed, the new values of
the variables are put in the appropriate column. Show working for calculations.
A condition column. The result of the condition will be true (T) or false (F). As the algorithm is
executed, conditions are evaluated and the details are recorded in the column. Show working when
evaluating the conditions. This is used whenever a condition is evaluated - IF WHILE or FOR
statements all have explicit or implicit conditions.
An Input/Output column is used to show what is input by the user and displayed by the program.
Show inputs with: the variable name, a "?" and the value input e.g. price ? 200. Show outputs with
the variable name, an =, and the value displayed (or just the value displayed) e.g. discountPrice=
180 .
Desk Checks vs Test Plans
• A Desk Check concentrates on the
value of variables and the logic i.e.
what is the value of variable x after
statement n; what is the next
statement to be executed?
• A Test Plan focuses on the values of
inputs and outputs required to test a
program without concern for the
internal workings i.e. are the results
(outputs) correct for the inputs?.
Sequence Desk Check
Example 1
• Problem Description: Calculate the discounted price of an
item purchased. (Note: a problem description is not
normally specified in a desk check, it is only included to
assist in understanding the problem.)
• The following example shows desk checking involving
sequence, executing instructions one after the other.
• Sequence involves executing all instructions once, from top
to bottom, one after the other.
• Algorithm (with line numbers added for the Desk
Check)
1
2
3
4
5
calcDiscountPrice()
Input price, discountPercent
discount = price * discountPercent / 100
discountPrice = price - discount
Display discountPrice 6 STOP
Sequence Desk Check
Test Data
Inputs: price = $200, discountPercent = 10%.
Correct results: discount = $20, discountPrice = $180.
Line
discount
Number
Discount
Percent
Discount
Price
price
Input/Output
200
price ? 200;
discountPercent ? 10
1
2
3
4
5
6
10
200 * 10 / 100 =
20
200 - 20 =
180
discountPrice = 180
Selection Desk Check
Example 2
• Problem Description: 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.
• The following example shows desk checking involving selection
using an IF.
• For an IF without an ELSE, if the condition evaluates to True then
execution continues to the next line and the lines between the IF
and ENDIF are executed (inclusive), otherwise (the condition is
False) execution jumps from the IF to the ENDIF.
• Algorithm (with line numbers added for the Desk Check)
1 calcPrice()
2 Input price
3 IF price > 100 THEN
4 discount = price * 15 / 100
5 price = price - discount
6 ENDIF
7 Display price 8
STOP
Selection Desk Check
• Inputs: price = $200
Correct results: price = $170.
Line Number
discount
price
Conditions
Input/Output
1
2
200
3
4
5
price ? 200
200 > 100 ? is T
200 * 15 / 100 = 30
200 - 30 = 170
6
7
8
price = 170
Selection Desk Check cont.
Inputs: price = $50
Correct results: price = $50.
Line Number
discount
price
Conditions
Input/Output
1
2
3
50
price ? 50
50 > 100 ? is F
6
7
8
price = 50
Iteration Desk Check
•
Problem Description: Display a table of values of x and x squared. X is to
start at 1 and increase by 1 up to 3.
• The following example shows desk checking involving iteration (repetition)
using a FOR loop.
• The counter variable is initialized once at the top of the loop, the first time
through the loop. The implied condition is evaluated at the top of the loop,
with execution going to the next line if the condition is True and going to
the line after the ENDFOR if the condition is False. In this case the implied
condition is x <= 3 ?. On the ENDFOR line the counter variable is
incremented by 1, then program execution loops from the ENDFOR line,
back to the FOR line.
Algorithm (with line numbers added for the Desk Check)
1 calcSquares()
2 Display "X", "X Squared"
3 FOR x = 1 TO 3 DO
4 xSquared = x * x
5 Display x, xSquared
6 ENDFOR
7 Display "-----------"
8 STOP
Input: None
Correct results: x = 1, xSquared = 1; x = 2, xSquared = 4; x
= 3, xSquared = 9.
Line Number
x
xSquared
Conditions
Input/ Output
1
2
3
X, X Squared
1
4
1 <= 3 ? is T
1*1=1
5
6
x = 1, xSquared = 1
1+1=2
3
2 <= 3 ? is T
4
2*2=4
5
6
x = 2, xSquared = 4
2+1=3
3
3 <= 3 ? is T
4
3*3=9
5
6
3
7
8
x = 3, xSquared = 9
3+1=4
4 <= 3? is F
-----------
Example from Case Study
1. Dim CashPaid, Change, GrandTotal As Currency
2. GrandTotal = txtnettotal.Text
3. CashPaid = txtcashtendered.Text
4. If GrandTotal > CashPaid Then
5. txtcashtendered.Text = ""
6. Else
7. Change = CashPaid - GrandTotal
8. End If
GrandTotal = 120
CashPaid = 200
Change should = 80
Line Number
GrandTotal
2
120
3
CashPai
d
Change
8
Input/Output
200
120 > 200 ?
is F
4
7
Conditions
Change =
200 - 120
Change = 80
Try this one…
Dim Count As Integer
Adodc1.Recordset.MoveFirst
Do While Not Adodc1.Recordset.EOF
Count = Count + 1
Adodc1.Recordset.MoveNext
Loop
txtNo_Records.Text = Count
Adodc1.Recordset.MoveFirst
End Sub
*5 records in database