Desk Check Guide

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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:

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. If variable names consist of a number of words it is permissible to put a space between each word in the name so that the name fits better in the column by wrapping to the next line. e.g. the variable column heading discount Price could be used rather than the actual variable name discountPrice.
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 .

Normally portrait page layout would be used for the desk check, however if the table is too wide, landscape layout may be used.

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?.

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 calcDiscountPrice()
2     Input price, discountPercent 
3     discount = price * discountPercent / 100 
4     discountPrice = price - discount 
5     Display discountPrice 
6 STOP

Desk Check

Test Data

Inputs: price = $200, discountPercent = 10%.
Correct results: discount = $20, discountPrice = $180.

Line Number	discount	discount Percent	discount Price	price	Input/Output
1
2		10		200	price ? 200; discountPercent ? 10
3	200 * 10 / 100 = 20
4			200 - 20 = 180
5					discountPrice = 180
6

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

Desk Check

Inputs: price = $200
Correct results: price = $170.

Line Number	discount	price	Conditions	Input/Output
1
2		200		price ? 200
3			200 > 100 ? is T
4	200 * 15 / 100 = 30
5		200 - 30 = 170
6
7				price = 170
8

Inputs: price = $50
Correct results: price = $50.

Line Number	discount	price	Conditions	Input/Output
1
2		50		price ? 50
3			50 > 100 ? is F
6
7				price = 50
8

Example 3

Problem Description: Display whether a student passed or failed a subject depending on their mark.

The following example shows desk checking involving selection using an IF-ELSE.

For an IF-ELSE, if the condition evaluates to True, execution continues to the next line after the IF and the lines up to but excluding the ELSE are executed, then execution jumps to the ENDIF. If the condition is False, execution jump from the IF to the ELSE, and the lines from the ELSE to the ENDIF (inclusive) are executed.

Algorithm (with line numbers added for the Desk Check)

1  displayGrade() 
2      Input mark
3      IF mark >= 50 THEN
4          grade = "Pass"
5          Display "Well done"
6      ELSE
7          grade = "Not pass"
8      ENDIF
9      Display grade 
10 STOP

Desk Check

Inputs: mark = 75
Correct results: grade = "Pass", "Well done"

Line Number	grade	mark	Conditions	Input/Output
1
2		75		mark ? 75
3			75 >= 50 ? is T
4	Pass
5				"Well done"
8
9				Pass
10

Desk Check

Inputs: mark = 40
Correct results: grade = "Not pass"

Line Number	grade	mark	conditions	Input/Output
1
2		40		mark ? 40
3			40 >= 50 ? is F
6
7	Not pass			"Well done"
8
9				Not pass
10

Example 4

Problem Description: Calculate the cost of electricity based on the electricity used.

The following example shows desk checking involving selection using a linear nested IF-ELSEs.

Algorithm (with line numbers added for the Desk Check)

1  calcElectrictyCost()
2      Input electricityUsed
3      IF electricityUsed < 1000 THEN
4          cost = 100
5          usageLevel = "Low"
6      ELSE
7          IF electricityUsed < 5000 THEN
8              cost = 100 + (electricityUsed - 1000) * 0.1
9              usageLevel = "Moderate"
10         ELSE
11             cost = 500 + (electricityUsed - 5000) * 0.08
12             usageLevel = "High"
13         ENDIF
14     ENDIF
15     Display cost, usageLevel
16 STOP

Desk Check

Inputs: electricityUsed = 500,
Correct results: cost = 100, usageLevel = "Low".

Line Number	cost	electricity Used	usage Level	Conditions	Input/ Output
1
2		500			electricityUsed ? 500
3				500 < 1000 ? is T
4	100
5			Low
14
15					cost = 100, usageLevel = "Low"
16

Inputs: electricityUsed = 3000,
Correct results: cost = 300, usageLevel = "Moderate".

Line Number	cost	electricity Used	usage Level	Conditions	Input/ Output
1
2		3000			electricityUsed ? 3000
3				3000 < 1000 ? is F
6
7				3000 < 5000 ? is T
8	100 + (3000-1000) * 0.1 = 300
9			Moderate
13
14
15					cost = 300, usageLevel = "Moderate"
16

Inputs: electricityUsed = 15000,
Correct results: cost = 1300, usageLevel = "High".

Line Number	cost	electricity Used	usage Level	Conditions	Input/ Output
1
2		15000			electricityUsed ? 15000
3				15000 < 1000 ? is F
6
7				15000 < 5000 ? is F
10
11	500 + (15000-5000) * 0.08 = 1300
12			High
13
14
15					cost = 1300, usageLevel = "High"
16

Example 5

Problem Description: Calculate the cost of water based on the water used and whether the customer is a Domestic or Commercial user.

The following example shows desk checking involving selection using a non-linear nested IF-ELSEs.

Algorithm (with line numbers added for the Desk Check)

1  calcWaterCost()
2      Input waterUsed
3      IF waterUsed < 100 THEN
4          cost = 50
5      ELSE
6          Input customerType
7          IF customerType = "D" THEN
8              cost = waterUsed * 0.5
9          ELSE
10             cost = waterUsed * 0.6
11             Display "Commercial rate"
12         ENDIF
13         Display "High usage"
14     ENDIF
15     Display cost
16 STOP

Desk Check

Inputs: waterUsed = 70, customerType (not required)
Correct results: cost = 50

Line Number	cost	customer Type	water Used	Conditions	Input/ Output
1
2			70		waterUsed ? 70
3				70 < 100 ? is T
4	50
14
15					cost = 50
16

Inputs: waterUsed = 200, customerType = D
Correct results: cost = 100

Line Number	cost	customer Type	water Used	Conditions	Input/ Output
1
2			200		waterUsed ? 200
3				200 < 100 ? is F
5
6		D			customerType ? D
7				D = D ? is T
8	200 * 0.5 = 100
12
13					High usage
14
15					cost = 100
16

Inputs: waterUsed = 200, customerType = C
Correct results: cost = 120

Line Number	cost	customer Type	water Used	Conditions	Input/ Output
1
2			200		waterUsed ? 200
3				200 < 100 ? is F
5
6		C			customerType ? C
7				C = D ? is F
9
10	200 * 0.6 = 120
11					Commercial rate
12
13					High usage
14
15					cost = 120
16

Example 6

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

Desk Check

Test Data

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				X, X Squared
3	1		1 <= 3 ? is T
4		1 * 1 = 1
5				x = 1, xSquared = 1
6	1 + 1 = 2
3			2 <= 3 ? is T
4		2 * 2 = 4
5				x = 2, xSquared = 4
6	2 + 1 = 3
3			3 <= 3 ? is T
4		3 * 3 = 9
5				x = 3, xSquared = 9
6	3 + 1 = 4
3			4 <= 3? is F
7				-----------
8

Example 7

Problem Description: Total a series of number input by the user. Allow the user to continue inputting numbers until a negative number is input, then display the total of the numbers input.

The following example shows desk checking involving iteration (repetition) using a WHILE loop.

The 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 ENDWHILE if the condition is False. Program execution loops from the ENDWHILE line, back to the WHILE line.

Algorithm (with line numbers added for the Desk Check)

1 totalNumbers()
2     total = 0
3     Input number 
4     WHILE number >= 0 DO 
5         total = total + number 
6         Input number 
7     ENDWHILE 
8     Display total
9 STOP

Desk Check

Inputs: numbers = 10, 6, -1.
Correct results: total = 16

Line Number	number	total	Conditions	Input/Output
1
2		0
3	10			number ? 10
4			10 >= 0 ? is T
5		0 + 10 = 10
6	6			number ? 6
7
4			6 >= 0 ? is T
5		10 + 6 = 16
6	-1			number ? -1
7
4			-1 >= 0 ? is F
8				total = 16
9

Example 8

Problem Description: Calculate the membership fee for club members.

The following example shows desk checking involving sequence, selection and iteration (repetition).

Algorithm (with line numbers added for the Desk Check)

1  calcMembershipFees()
2      totalFees = 0
3      Input membershipType
4      WHILE membershipType <> "X" DO
5          IF membershipType = "F" THEN
6              membershipFee = 160
7              membershipName = "Full"
8          ELSE
9              IF membershipType = "J" THEN
10                 membershipFee = 80
11                 membershipName = "Junior"
12             ELSE
13                 IF membershipType = "P"
14                     membershipFee = 30
15                     membershipName = "Pensioner"
16                 ELSE
17                     membershipFee = 10
18                     membershipName = "Life"
19                 ENDIF
20             ENDIF
21         ENDIF
22         totalFees = totalFees + membershipFee
23         Display membershipFee, membershipName
24         Input membershipType
25     ENDWHILE
26     Display totalFees
27 STOP

Desk Check

Inputs: membershipType = "J", membershipType = "F", membershipType = "X".
Correct results: membershipFee = 80, membershipName = "Junior"; membershipFee = 160, membershipName = "Full"; totalFees = 240.

Line Number	membership Fee	membership Name	membership Type	total Fees	Conditions	Input/ Output
1
2				0
3			J			membership Type ? J
4					J <> X ? is T
5					J = F? is F
8
9					J = J ? is T
10	80
11		Junior
20
21
22				0 + 80 = 80
23						membership Fee = 80, membership Name = Junior
24			F			membership Type ? F
25
4					F <> X ? is T
5					F = F? is T
6	160
7		Full
21
22				80 + 160 = 240
23						membership Fee = 160, membership Name = Full
24			X			membership Type ? X
25
4					X <> X ? is F
26						totalFees = 240
27

Example 9

Problem Description: find the smallest of 3 numbers.

The following example shows desk checking involving subroutines.

When a subroutine is called, execution goes from the calling line (subroutine call e.g. line 3 in this example) to the first line in the subroutine (e.g. line 6). The subroutine is then executed, and when the end of the subroutine is reached (e.g. line 14), execution goes back to the line after where it was originally called (e.g. line 4).

Algorithm (with line numbers added for the Desk Check)

1  compareNumbers()
2      Input number1, number2, number3
3      getSmallestNumber()
4      Display smallest   
5  STOP

6  getSmallestNumber()
7      smallest = number1
8      IF number2 < smallest THEN
9          smallest = number2
10     ENDIF
11     IF number3 < smallest THEN
12         smallest = number3
13     ENDIF
14 EXIT

Desk Check

Inputs: number1 = 5, number2 = 3, number3 = 8.
Correct result: smallest = 3.

Line Number	number1	number2	number3	smallest	Conditions	Input/ Output
1
2	5	3	8			number1 ? 5; number2 ? 3; number3 ? 8
3
6
7				5
8					3 < 5 ? is T
9				3
10
11					8 < 3 ? is F
13
14
4						smallest = 3
5

Inputs: number1 = 7, number2 = 12, number3 = 9.
Correct result: smallest = 7.

Line Number	number1	number2	number3	smallest	Conditions	Input/ Output
1
2	7	12	9			number1 ? 7; number2 ? 12; number3 ? 9
3
6
7				7
8					12 < 7 ? is F
10
11					9 < 7 ? is F
13
14
4						smallest = 7
5