Www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Variables and Trace Tables Stewart...

www.hope.ac.uk Faculty of Sciences and Social Sciences

HO

PE

Variables and Trace Tables

Stewart Blakeway

FML 213

[email protected]

http://hopelive.hope.ac.uk/computing/


HO

PE

What we have done so far

• Introduced Java as a programming language with three constructs:– Sequence– Selection– Repetition

• Introduced algorithms expressed in:– Structured English– Flow charts

2


HO

PE

What we shall do today

• Introduce the concept of a variable• Apply variables to algorithms• Introduce the trace table as a method of

checking the operation of a program by monitoring its variables

3


HO

PE

Counting & Trace Tables

• Pages 29 to 36

4


HO

PE

Any job we do requires short term memory

• Coming to class– Where is it?– What time does it start?

• Getting a round in– Who ordered what?

5

What time is it ?

http://www.youtube.com/watch?v=VSSGiA4f5cs


HO

PE

Some things we remember have to be continually updated

• Following a recipe– Where are we up to?

• Counting people coming into a room– What was the last count?

• Watching a game and you can’t see the scoreboard– What score is it?

6


HO

PE

Question:• How do we record what we need to

remember in algorithms ?

7


HO

PE

Question:• How do we record what we need to

remember in algorithms ?

8

Answer:Use

variables


HO

PE

Variables

• A variable is somewhere in memory you store a piece of information you want to remember

9


HO

PE

Variables

• A variable is somewhere in memory you store a piece of information you want to remember

• As you work through a task, that piece of information stored in memory may get

updated – its contents vary over time

10


HO

PE

Representing a variable

11

14Number of people in the room

2-1Current score

• A box represents an area of memory

• The box is labelled


HO

PE

Rules for naming variables

12

14NumberOfPeopleInRoom

2-1CurrentScore


HO

PE

Rules for naming variables

• No spaces in the name• Use capitals to make name more readable• Preparing for converting to a computer

programming language where normally no spaces are allowed in variable names

13

14NumberOfPeopleInRoom

2-1CurrentScore


HO

PE

Making variables in Java

int NumberOfPeopleInRoom;

String CurrentScore;

14


HO

PE




15


HO

PE




NumberOfPeopleInRoom = 25;

CurrentScore = "2-1";

16


HO

PE

Making variables in algorithms

NumberOfPeopleInRoom := 25

CurrentScore := “2-1”

17

Making the variable and putting in the first value are done in one step


HO

PE

Count := 0

This does 2 things

1. Establishes an area of memory called Count

2. Puts zero into this location

18

?????Count

0Count


HO

PE

• This can be read as

– becomes

or –becomes equal to

19

:=


HO

PE

Fred := 12

20

???????1 Fred

2 12Fred

There is no significance in the name of the variable

It is not a keyword like while, if, …

It is up to the programmer


HO

PE

Total := 0Total := Total + 1

21

These 2 lines do 3 things.

1

2

3

?????Total

0

1

Total

Total

Set up the variable Total

Initialise it to zero

The new contents of Total become equal to the old

contents plus 1


HO

PE

display Sum

22

156Sum

156

Copies the contents of the variable Sum to the default output device


HO

PE

read(Fred)

23

This does three things

1

2

3

Establishes a variable called Fred

???Fred

Waits for input from the input device:

For example key press 8

Places 8 into the variable Fred

8Fred


HO

PE

Reading input in Java

int Fred;

System.in.read(Fred);

24


HO

PE

Putting it altogether so far

25

Brian := 0Amanda := 0read(Alma)while Alma greater than 0

begin Brian := Brian + 1Amanda := Amanda + Almaread(Alma)

end display Brian , Amanda


HO

PE


26




?

?

Amanda

Brian?

Alma

Variables


HO

PE





0

?

Amanda

Brian?

Alma

Variables


HO

PE


28




0

?

Amanda

Brian0

Alma

Variables


HO

PE


29




Assume that 6 is input

0

6

Amanda

Brian0

Alma

Variables


HO

PE


30




Alma is greater than 0

so we go into loop body

0

6

Amanda

Brian0

Alma

Variables


HO

PE


31




1

6

Amanda

Brian0

Alma

Variables


HO

PE


32




1

6

Amanda

Brian6

Alma

Variables


HO

PE


33




1

4

Amanda

Brian6

Alma

Variables



HO

PE


34




1

4

Amanda

Brian6

Alma

Variables

Alma is greater than 0

so we go into loop body


HO

PE


35




2

4

Amanda

Brian6

Alma

Variables


HO

PE


36




2

4

Amanda

Brian10

Alma

Variables


HO

PE


37




2

0

Amanda

Brian10

Alma

Variables



HO

PE


38




2

0

Amanda

Brian10

Alma

Variables

Alma is no longer greater than 0

so the loop is finished


HO

PE


39




2

0

Amanda

Brian10

Alma

Variables

2

10


HO

PE

Java versionint Brian;int Stewart;int Alma;

Brian = 0;Stewart = 0;System.in.read(Alma);while (Alma > 0) { Brian = Brian + 1; Stewart = Stewart + Alma; System.in.read(Alma); }System.out.println(Brian , Stewart);

40


HO

PE

Good Practice: Variable Names

41

Variable names should reflect their intended purpose.

This purpose, like a well designed user interface, should be guessable.

This helps with the readability of a program, together with comments.

What is the intended purpose of each of the variables: Brian, Stewart and Alma ?


HO

PE

Better Variable Names

42

1 is added to the contents of Brian (initially zero) each time through the while loop.

Each number input is stored in (not added to) the variable Alma.

The value stored in Alma is added to the current value of Stewart (initially zero).


HO

PE

The Names at Last

43

Brian seems to be counting each loop.

Alma seems to be storing each number input

Stewart seems to be keeping a running total.

Brian Count

Alma Number

Stewart Total

Refer to your course booklet on page 31


HO

PE

Trace Tables

44

The above method of keeping track of the contents of variables is obviously tedious. A better way would be to employ a tabular method. This is called a trace table.

Consider the following algorithm :

A := 6 (Step 1)

B := 8 (Step 2)

C := A (Step 3)

A := B (Step 4)

B := C (Step 5)


HO

PE

The Empty Trace Table

45

Step Number

A B C

123

4

5

Variables

A := 6B := 8C := AA := BB := C


HO

PE

Here we go

46

Step Number

A B C

1 623

4

5

A := 6B := 8C := AA := BB := C


HO

PE

Next Step

47

Step Number

A B C

1 62 83

4

5

A := 6B := 8C := AA := BB := C


HO

PE

Next Step

48

Step Number

A B C

1 62 83 6

4

5

A := 6B := 8C := AA := BB := C


HO

PE

Next Step

49

A := 6B := 8C := AA := BB := C

Step Number

A B C

1 6

2 8

3 6

4 8

5


HO

PE

Next Step

50

A := 6B := 8C := AA := BB := C

Step Number

A B C

1 6

2 8

3 6

4 8

5 6


HO

PE

Tracing a Loop

51

Answer := 1(Step 1)I := 0 (Step 2)Number := 0(Step 3)while (I is less than 3 ) (Step 4) begin

read(Number) (Step 5)Answer := Answer * Number (Step 6)I := I + 1 (Step 7) end display Answer

Multiply

Steps are numbered wherever variable contents are changed or conditions are tested.


HO

PE

Here is the Blank Trace Table Step

NumberAnswe

rI Numbe

rIs I < 3 ?

52

Answer := 1I := 0Number := 0while (I is less than 3) begin

read(Number)Answer := Answer * NumberI := I + 1 end display Answer

1

2

3

4

5

6

7


HO

PE

Initialisation stepsStep Number

Answer

I Number

Is I < 3 ?

1 1

53



1

2

3

4

5

6

7


HO

PE


Answer

I Number

Is I < 3 ?

1 1

2 0

54



1

2

3

4

5

6

7


HO

PE


Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

55



1

2

3

4

5

6

7


HO

PE

Now the Test: Is 0 < 3 ?Step Number

Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

56



1

2

3

4

5

6

7


HO

PE

Into the LoopStep Number

Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

5 5

57



1

2

3

4

5

6

7

Assume 5 is input


HO

PE


Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

5 5

6 5

58



1

2

3

4

5

6

7

5 times 1 = 5


HO

PE


Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

5 5

6 5

7 1

59



1

2

3

4

5

6

7

1 is added to I


HO

PE

Back to the test: Is 1 < 3 ?Step Number

Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

5 5

6 5

7 1

4 True

60



1

2

3

4

5

6

7


HO

PE

Inside the loop againStep Number

Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

5 5

6 5

7 1

4 True

5 2

61



1

2

3

4

5

6

7

Assume 2 is input


HO

PE


Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

5 5

6 5

7 1

4 True

5 2

6 10

62



1

2

3

4

5

6

7

5 times 2 = 10


HO

PE


Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

5 5

6 5

7 1

4 True

5 2

6 10

7 2

63



1

2

3

4

5

6

7

1 is added to I


HO

PE


Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

5 5

6 5

7 1

4 True

5 2

6 10

7 2

4 True

64



1

2

3

4

5

6

7


HO

PE


Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

5 5

6 5

7 1

4 True

5 2

6 10

7 2

4 True

5 4

65



1

2

3

4

5

6

7

Assume 4 is input


HO

PE


Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

5 5

6 5

7 1

4 True

5 2

6 10

7 2

4 True

5 4

6 40

66



1

2

3

4

5

6

7

10 times 4 = 40


HO

PE


Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

5 5

6 5

7 1

4 True

5 2

6 10

7 2

4 True

5 4

6 40

7 3

67



1

2

3

4

5

6

7

Add 1 to I


HO

PE


Answer

I Number

Is I < 3 ?

1 1

2 0

3 0

4 True

5 5

6 5

7 1

4 True

5 2

6 10

7 2

4 True

5 4

6 40

7 3

4 False68



1

2

3

4

5

6

7


HO

PE

Back to the test: Is 3 < 3 ? Step

NumberAnswe

rI Numbe

rIs I < 3 ?

1 1

2 0

3 0

4 True

5 5

6 5

7 1

4 True

5 2

6 10

7 2

4 True

5 4

6 40

7 3

4 False69



1

2

3

4

5

6

7


HO

PE

That’s the End of the Trace Table.

70

Last line of Program: display Answer

40Output the contents of variable Answer


HO

PE

Java Versionint Answer;int Number;int I;

Answer = 1;I = 0;Number = 0;while (I < 3) { System.in.read(Number); Answer = Answer * Number; I = I + 1; }System.out.println(Answer);

71


HO

PE

Next?• Assessment due – this has been emailed to

you.

if (!receivedAssessment)

begin

email [email protected]

end

else

begin

do assessment

end

mailto:[email protected]

Www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Variables and Trace Tables Stewart...

Documents

Transcript of Www.hope.ac.uk Faculty of Sciences and Social Sciences HOPE Variables and Trace Tables Stewart...