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Lecture 4

# Lecture 4 - CHAPTER 2 A recursive function is a function...

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1 CHAPTER 2 Recursion 2 • A recursive function is a function that calls itself. 3 • Two functions A and B are mutually recursive, if A calls B and B calls A. • Recursion can always replace iteration (looping). 4 n! • A simple example: Factorial. Factorial(0) = 1 (By definition) Factorial(1) = 1 Factorial(2) = 2*1 Factorial(3) = 3*2*1 Factorial(4) = 4*3*2*1 Factorial(5) = 5*4*3*2*1 Factorial(n) = n * n-1 * n-2 * ... 1 5 DEFINITION OF n! 1 if n = 0 Factorial(n) = n * Factorial(n-1) if n > 0 This recursive definition screams for a recursive implementation . 6 int factorial (int n) { if (n = = 0) return 1; else return n * factorial(n - 1); }

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7 How to understand recursion: Every time a recursive function is called, that’s like it is a completely separate function. Just ignore the fact that this function was called by itself. Specifically, if “factorial” calls itself, then there are two parameters “n” that have NOTHING to do with each other. 8 factorial(3): n = 3 calls factorial(2) factorial(2): n = 2 calls factorial(1) factorial(1): n = 1 calls factorial(0) factorial(0): returns 1 to factorial(1). Inside factorial(1), 1 * factorial(0) becomes: 1 * 1 factorial(1):
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