1. Copyright © 2004-2017 by Curt Hill
Tracing
What, How, and Why
Copyright © by Curt Hill

2. Copyright © 2004-2017 by Curt Hill
What?
Tracing is the simulation of a computer program or part of a computer program
We “play computer”
The results should be the same as if the program were executed on a real machine
The simulation needs to cover every relevant aspect of the computation
Copyright © by Curt Hill

3. Copyright © 2004-2017 by Curt Hill
Elbonian Computers
Copyright © by Curt Hill

4. Copyright © 2004-2017 by Curt Hill
Why?
Essential debugging tool
Important learning technique
A correct trace demonstrates that you understand the fundamentals
We will have tracing exercises in class and on tests
Copyright © by Curt Hill

5. Copyright © 2004-2017 by Curt Hill
How?
Find the first statement to be executed
Write down all the variables
Used to record their current value
As new variables are encountered they may be added to the list
For each statement that is encountered:
Determine its action
Record changes to any variables
Show the final values when execution is complete
Copyright © by Curt Hill

6. Consider the following code:
#include <iostream.h>
int main(){
int a,b=2,c=5;
cin >> a;
b = a*c
c = b/2 – c + 2
cout << a << “ “ << b << “ “ << c;
return 0; }
Copyright © by Curt Hill

7. Copyright © 2004-2017 by Curt Hill
Now what?
What is the first executable statement?
The declaration
Record the variables and their value
Then step through the statements
Copyright © by Curt Hill

8. Copyright © 2004-2017 by Curt Hill
First step:
#include <iostream.h>
int main(){
int a,b=2,c=5;
cin >> a;
b = a*c
c = b/2 – c + 2
cout << a << “ “ << b << “ “ << c;
return 0; }
a b c ?
Copyright © by Curt Hill

9. Copyright © 2004-2017 by Curt Hill
Second step:
#include <iostream.h>
int main(){
int a,b=2,c=5;
cin >> a;
b = a*c
c = b/2 – c + 2
cout << a << “ “ << b << “ “ << c;
return 0; }
a b c ?
Copyright © by Curt Hill

10. Copyright © 2004-2017 by Curt Hill
Third step:
#include <iostream.h>
int main(){
int a,b=2,c=5;
cin >> a;
b = a*c;
c = b/2 – c + 2;
cout << a << “ “ << b << “ “ << c;
return 0; }
a b c ?
Copyright © by Curt Hill

11. Copyright © 2004-2017 by Curt Hill
Fourth step:
#include <iostream.h>
int main(){
int a,b=2,c=5;
cin >> a;
b = a*c;
c = b/2 – c + 2;
cout << a << “ “ << b << “ “ << c;
return 0; }
a b c ?
Copyright © by Curt Hill

12. Copyright © 2004-2017 by Curt Hill
Fifth step:
#include <iostream.h>
int main(){
int a,b=2,c=5;
cin >> a;
b = a*c;
c = b/2 – c + 2;
cout << a << “ “ << b << “ “ << c;
return 0; }
a b c ?
Copyright © by Curt Hill

13. Copyright © 2004-2017 by Curt Hill
Sixth step:
#include <iostream.h>
int main(){
int a,b=2,c=5;
cin >> a;
b = a*c;
c = b/2 – c + 2;
cout << a << “ “ << b << “ “ << c;
return 0; }
a b c ?
Copyright © by Curt Hill

14. Copyright © 2004-2017 by Curt Hill
Right and Wrong
There is a right way and a wrong way to do tracing
The right way is this
The wrong way is to attempt to analyze this algebraically
That approach does not allow for a change in variables
Copyright © by Curt Hill

15. Copyright © 2004-2017 by Curt Hill
Conclusion
Tracing is something that you see in every test and many quizzes and group assignments
It is very useful to determine if you understand what is happening
A very good debugging tool as well
Copyright © by Curt Hill