Desk Checking Algorithms

The basic idea of desk checking is that you step through the algorithm step by step and document each variable and how it changes as it goes through the algorithm. Lets take an example algorithm: 1 BEGIN 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 END

Step 1: List the variables Okay, first thing you do is list all the variables in the program. In this case, that is only mark and grade. There is also, obviously, an output in any program. You input variables and output results in any program. You also want to test a certain number of sets of data for the program. In an exam, they will normally tell you what data to test.

Step 2: Create the table Set Mark Grade Output

Step 3: The desk check Now, you have to manually step through each line of the algorithm and record what happens (unless nothing happens). Our first set of data is going to be Mark=34 Set Mark Grade Output 1 34 So, stepping through the algorithm… Line 1 does nothing. Line 2, inputs mark

Line 3, IF mark >= 50 THEN Ok, so in your test data, is mark >=50? No, then skip lines 4 and 5 to the else

statement. Line 6 and 7. 6 ELSE 7 grade = "Not pass" This is a change of variable, so you have to record it on your deskcheck Set Mark Grade Output 1 34 Not Pass Line 9 9 Display grade is your output, so place it in the output column Set Mark Grade Output 1 34 Not Pass Not Pass So, if you have multiple sets of data: Set Mark Grade Output 1 34 Not Pass Not Pass 2 50 Pass Well done

Pass 3 65 Pass Well Done

Pass So what about more complex algorithms?

Firstly, go through and list all the variables. Number of Trains Train Train ID Location X Location Y X Y Now, a table… Set Number

of Trains

Train TrainID Location X

Location Y

X Y Output

The question gave you the set of values for number of trains as 0, 1 and 2..so…for 0 Set Number

of Trains

Train TrainID Location X

Location Y

X Y Output

1 0 1 Train001 Location X

Location Y

“Train 001 at X, Y”

2 Train002 Location X

Location Y

“Train 002 at X, Y”

3 Train003 Location X

Location Y

“Train 003 at

X, Y” Never ending loop due to Number of Trains never equalling Train Number

Train doesn’t equal number of trains (0 doesn’t equal 1, so the loop is entered) ReadTrainID(TrainID)…there’s no sub procedure listed for

ReadTrainID(TrainID). You can assume that it’s just inputting the trainID. As we’re not given this value to test, we can either make it up, or just put in trainID.

ReadLocation (TrainID,LocationX,LocationY) is giving us the train and, it’s X and Y co-ordinates (location)

As you don’t know what the next two subprocedures do, you can ignore them and pass over them. A good desk check should include them, however, you’re not given enough info in this question to do it.

Ok, you’re going back to line 7 in the algorithm, because you’ve finished that sub procedure.

DisplayTrainID(TrainID, LocationX, LocationY) takes you to line 17. So, the output is “Train ID at X, Y”

You then go back to line 8, train = train + 1, so train now equals 2 Go through all that again because Train<>Number of trains yet. As you go through this, you notice that you keep going up in train Numbers,

but not Number of Trains, so you see that the loop will never end. Continuing the rest of the desk check:

Number of Trains

Train TrainID X Y Output

0 1 Train 001

LocationX LocationY Train 001 is at Location X, Location Y

2 Train 002

LocationX LocationY Train 002 is at Location X, Location Y

3 Train 003

LocationX LocationY Train 003 is at Location X, Location Y

Endless loop due to value of Train never = Niumber of Trains

1 1 Terminates without looping due to value of train = number of trains initially

2 1 Train 001

LocationX LocationY Train 001 is at Location X, Location Y

2 Terminates as Train=Number of trains