1. Introduction to C Programming CE00312-1 Lecture 21 Recursion and Linear Linked Lists

2. Factorial of an Integer Factorial of 6 is expressed as 6! whose value is6  5  4  3  2  1 or it can be rewritten as6  5! We have defined 6! in terms of 5! - a recursive definition In general n! is n  (n-1)! when n > 0 and 0! is 1

3. Factorial as a C function int fact (int n) { if (n == 0) { return 1; } else { return n * fact(n - 1); }

4. An equivalent “functional” version is: int fact(int n) { return (n == 0 ? 1 : n * fact(n - 1)); } This can be read as “return the result of if n is 0 then 1 else factorial of n-1”.

5. Recursion with Linear Linked Lists Recursive functions are more convenient, shorter and easier to modify (once you get used to them!) compared to iterative functions (ie with while loops).

6. Printing a linear linked list Print 3 then 35812 - list Print the rest

7. Recursive function print_list #include "LLL.h“// node structure, type void print_list(Listpointer list) { if (list != NULL)// not empty list { printf("%d\n", list -> value);// print first print_list(list -> next);// print rest }// else empty list do nothing } For the current node given by, pointer list, print its value and then the recursive call, print_list, deals with the rest.

8. Printing a linear linked list in reverse Print the rest in reverse then print 3 35812 - list

9. Print the list in reverse #include "LLL.h“// node structure, type void print_reverse(Listpointer list) { if (list != NULL)// not empty list { print_reverse(list -> next);// print rest printf("%d\n", list -> value);// print first }// else empty list do nothing } This function recurses down the list, including the last, and prints values as it returns back up the list.

10. Adding the items in a linear linked list 3+sum of the rest 35812 - list

11. Recursive function to add all the items in a list int sum(Listpointer list) { if (list == NULL) { return 0;// empty list, sum is 0 } else { return list->value + sum(list->next); }

12. The previous function could also be written as: int sum(Listpointer list) { return (list == NULL) ? 0 : list -> value + sum(list -> next); } This can be read as “return the result of if the list is empty then zero else this node’s value plus the sum of the rest”