first part of the lecture explains Recursion topic in c language. Second half shows, what is a debugging, how to debug your program, debugging methods and debugging tool in Eclipse.

1. Recursion, Debugging
Saule Anay
Fall 2016, NU, CSCI 151

2. You should know already
• Functions
• Function parameters and arguments
• How to call function
• Global vs local variables

3. Today in the class
1. Recursion:
1. What is it?
2. How to use?
3. Examples
4. Recursion vs iteration
2. Inside my program
3. Debugging
1. what is it?
2. why we need?
3. How to use?
4. Examples.
4. Compiler errors

4. Recursion
• is a method where the solution to a problem
depends on solutions to smaller instances of
the same problem.
example: 2^11 = 2^10 * 2 = 1024 * 2
• one of the central ideas of computer science.

5. How do you empty a vase containing
five flowers?
Answer: if the vase is not empty, you take out one
flower and then you empty a vase containing four
flowers.
How do you empty a vase containing four flowers?
Answer: if the vase is not empty, you take out one
flower and then you empty a vase containing three
flowers.
…
How do you empty a vase containing one flower?
Answer: if the vase is not empty, you take out one
flower and then you empty a vase containing no
flowers.
How do you empty a vase containing no flowers?
Answer: if the vase is not empty, you take out one
flower but the vase is empty so you're done.

6. Base cases vs recursive cases
• one or more base cases:
– input(s) for which the function produces a result
trivially (without recurring),
– Base case for emptyVase: vase with zero flowers
• one or more recursive cases: input(s) for
which the program recurs (calls itself).
Example: vase with flowers bigger than zero.

7. How do you empty a vase containing
five flowers?
• Base: if Flowers=0  done. Vase is empty
• Recursive case: for all flowers>0, take out one
flower, empty Vase of (N-1) flowers

8. emptyVase program code
1. #include <stdio.h>
2. void emptyVase(int numFlowers){
3. if (numFlowers ==0){
4. printf("Vase is empty nown");
5. return;
6. }
7. if (numFlowers>0){
8. numFlowers--;
9. printf("one flower have been taken outn");
10. emptyVase(numFlowers);
11. } //end of IF statement
12. } //end of Function
13. int main () {
14. setvbuf(stdout, NULL, _IONBF, 0);
15. Int flowers =8;
16. emptyVase(flowers);
17. return 0;
18. }

9. Examples of recursive solution
1. Factorial:
Base case:0! = 1
Recursive case: for all n > 0, n! = n(n − 1)!
2. fibbonachi:
Base case:Fn(1) = 1, Fn(2) = 1
Recursive case: for all n > 2, Fn(n) = Fn(n-
1)+Fn(n-2)
3. X^n (x to the power n)
Base case: if n=0, x^n=1
Recursive case: for all n>0, x^n=x*x^(n-1)

10. Example with Factorial

11. Factorial recursive function
1. #include <stdio.h>
2. int factorial(int n){
3. //base case
4. if (n==0)
5. return 1;
6. //recursive case
7. if (n>0)
8. return n*factorial(n-1);
9. //default case if n<0
10. return -1;
11. } //end Factorial
12. int main () {
13. setvbuf(stdout, NULL, _IONBF, 0);
14. int number=8;
15. printf("fatorial of %i is %i n", number, factorial(number));
16. return 1;
17. }

12. Factorial iterative function
• #include <stdio.h>
• int factorial(int n){
• int i;
• int res=1;
• for(i=1; i<=n; i++)
• res=res*i;
• return res;
• } //end Factorial
• int main () {
• setvbuf(stdout, NULL, _IONBF, 0);
• int number=8;
• printf("fatorial of %i is %i n", number, factorial(number));
• return 1;
• }

13. Check list recursive function
• Have a base case
• Function call itself (recursive call)
• Recursive call MUST be with different input,
so that the base case must be reached
eventually

14. Recursion vs iteration
• Similarity
– Both repeat block of codes
• Difference
– iteration: loops to repeat the block of code
– recursion: call itself to repeat the block of code
Temination:
iteration: loop condition
recursion: base case
Infinite execution:
iteration: never reach loop condition
recursion: never reach base case

15. Which is better?
• It depends from situation:
– Some problem difficult to solve iteratively
– Some problem difficult to solve recursively
• Math people like recursion
– Shorter solution, abstraction
– To find good recursive solution is not always simple.
• CS people like iteration
– Easier to construct
– Easier to iterate and simulate (less magical)

16. Phillips head vs flat head screwdriver:
which is better?
It depends from situation
iterationrecursion

17. conclusion
• How to know if function recursive?
– It must call itself
• Recursive function must have 2 cases:
– Base case
– Recursive case
• Any recursion can be rewritten with iteration
• Which to choose depends on situation

18. What is inside my program?
Computer memory

19. What happens when?
1. you declare new variable?
2. You declare new array
3. You call a function
Computer allocates memory for variable, fro
array

20. function name var name value
continue
line console
main flowers 8 29
emptyVase(8) numFlowers 7 12
one flower have been taken
out
emptyVase(7) numFlowers 6 12
one flower have been taken
out
emptyVase(6) numFlowers 5 12
one flower have been taken
out
emptyVase(5) numFlowers 4 12
one flower have been taken
out
emptyVase(4) numFlowers 3 12
one flower have been taken
out
emptyVase(3) numFlowers 2 12
one flower have been taken
out
emptyVase(2) numFlowers 1 12
one flower have been taken
out
emptyVase(1) numFlowers 0 12
one flower have been taken
out
emptyVase(0) numFlowers 0 vase is empty

21. debugging

22. 1. Debugging
1. what is it?
2. why we need?
3. How to use?
4. Examples.

23. What is debugging?
Debugging is the process of finding and resolving of
errors in the code that prevent correct operation
of program or computer software.
• To debug a program means:
1. find problem
2. isolate problematic part of code
3. fix it.

24. Finding error with testing
• Testing:
– Give minimum input value
– Give maximum input value
– Give as much as possible diverse input value
Example:
find maximum in the array
time advancer problem

25. Finding errors by debugging
• When Output is incorrect, it is difficult to find an
error.
• What is wrong? How to find an error?
• Debugging:
1. Print debugging
1. Add Printf statements in code, to check internal states of
the program
2. Use debugging tool
1. Eclipse has one

26. Debugging tool
1. Running program Step-by-step
2. Stopping(Pausing) program at some instruction.
And see current program state(breakpoints)
3. Modify program state (modify the values of the
variables) while it is running

27. What is the bug?
• “I’ve debugged a program”
• “Worked the bugs out”
• I fixed the problem, that bugs no longer exist
in my program

28. Debugging perspective in Eclipse

29. Debugging perspective in Eclipse

30. Problem with perspective? Reset it

31. Control of debugging
• F5 – step into
• F6 – step over
• F7 – step return
• Ctrl+F2 (red square) - terminate

33. When you return from emptyVase(7)
continue from this place: Line 13

34. Debugging of Carrot problem
• int main () {
• setvbuf(stdout, NULL, _IONBF, 0);
• int rabbits[5];
• int carrots=1;
• int i;
• for (i=0; i<6; i++){
• rabbits[i]=carrots;
• carrots+=3;
• if (carrots>=16)
• carrots=1;
• }
• for (i=0; i<6; i++)
• Printf(“%i, ”, rabbits[i]);
• return 0;
• }

35. Compiler errors

36. Compiler errors
4 Errors in Line 29
1 Warning in line 27.
Error in Line 29 fixing: replace
comas with semicolons