IS12 - Introduction to Programming Lecture 23: Recursion

Peter Brusilovskyhttp://www2.sis.pitt.edu/~peterb/0012-051/

Recursive Definitions

Factorial function can be defined recursively (function definition use this function as a part of definition)

Recursive Calculations

Recursive definition can be used to calculate factorial

Recursive factorial

void main(){int i;printf("Enter an integer (non-negative): ");scanf("%d", &i);if(i >= 0) printf("%d! = %d\n", i, factorial(i));

}

int factorial(int n) {if(n == 0)

return 1;else

return (n * factorial(n-1));}

Rules for Using Recursion

Every recursive call must either solve the problem or reduce its size

Terminal case: solving (no calls) Base case: reducing size (recursive call) To make a recursive program:

– Identify base case(s) and terminal case(s)– Combine them (usually with if-else)

Recursive Power

void main(){int a, b;

printf("Enter a and b: ");scanf("%d %d", &a, &b);if(b >= 0)

printf("%d power %d = %d\n", a, b, power(a, b));}

int power(int base, int exp) {if(exp == 0)

return 1;else

return (base * power(base, exp-1));}

How it works? Power Case

Stackframe and system stack

When one function calls another function in C all parameters and return value are transferred via system stack

At each call a stackframe is placed into stack– Parameters of the call– Local variable of calling function– Next statement in the calling function– Place for a return value

The process of calling a function

Calling function: pushes stackframe Called function:

– Gets parameters from the stack– Performs calculations– Places return value to the stack– Transfers control to the next statement

Calling function– Restores local variables from the stack– Uses return value from the stack