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Recursion

Recursion. Recursion. What is recursion? When one function calls itself directly or indirectly. Gcd. Find largest integer d that evenly divides into p and q. base case. reduction step, converges to base case. public static int gcd ( int p , int q ) { if ( q == 0 ) return p ;

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Recursion

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  1. Recursion

  2. Recursion What is recursion? When one function calls itself directly or indirectly. Gcd. Find largest integer d that evenly divides into p and q. base case reduction step,converges to base case public staticint gcd(int p,int q) { if(q ==0)return p; elsereturn gcd(q, p % q); } base case reduction step

  3. H-tree Used in integrated circuits to distribute the clock signal. H-tree of order n. • Draw an H. • Recursively draw 4 H-trees of order n-1, one connected to each tip. tip and half the size size order 3 order 1 order 2

  4. Htree in Java public class Htree { public static void draw(int n, double sz, double x, double y) { if (n == 0) return; double x0 = x - sz/2, x1 = x + sz/2; double y0 = y - sz/2, y1 = y + sz/2; StdDraw.line(x0, y, x1, y); StdDraw.line(x0, y0, x0, y1); StdDraw.line(x1, y0, x1, y1); draw(n-1, sz/2, x0, y0); draw(n-1, sz/2, x0, y1); draw(n-1, sz/2, x1, y0); draw(n-1, sz/2, x1, y1); } public static void main(String[] args) { int n = Integer.parseInt(args[0]); draw(n, .5, .5, .5); } } draw the H, centered on (x, y) recursively draw 4 half-size Hs

  5. Computational Cost of the H-Tree Program • H-tree of Order 1 • 5 calls to draw() = 41 + 40 • H-tree of Order 2 • 21 calls to draw() = 42 + 41 + 40 • H-tree of Order 3 • 85 calls to draw() = 43 + 42 + 41 + 40 • Execution time grows exponentially

  6. Towers of Hanoi http://en.wikipedia.org/wiki/Image:Hanoiklein.jpg

  7. Towers of Hanoi Move all the discs from the leftmost peg to the rightmost one. • Only one disc may be moved at a time. • A disc can be placed either on empty peg or on top of a larger disc. start finish Towers of Hanoi demo Edouard Lucas (1883)

  8. Move largest disc left. cyclic wrap-around Towers of Hanoi: Recursive Solution Move n-1 smallest discs right. Move n-1 smallest discs right.

  9. Implementing the Recursive Solution • Discs 1…n: • 1 is the smallest disc, n is the largest • moves(int n, boolean left): • moves(n, true): Moves discs 1…n one pole to the left • moves(n, false): Move discs 1…n one pole to the right

  10. Towers of Hanoi: Recursive SolutionDr. Java Demo publicclass TowersOfHanoi { public staticvoidmoves(int n,boolean left){if(n ==0)return; //base case moves(n-1,!left); //move n-1 discs to right if(left) System.out.println(n + " left"); else System.out.println(n + " right"); moves(n-1,!left); //move n-1 discs to right } publicstaticvoidmain(String[] args){ int N = Integer.parseInt(args[0]); moves(N,true); } } moves(n, true) : move discs 1 to n one pole to the left moves(n, false): move discs 1 to n one pole to the right smallest disc

  11. Recursive Function Call TreeOn the Board

  12. Computational Cost of Towers of Hanoi • T(n) = Number of moves required for moving n disks • T(1) = 1 • T(n) = 2*T(n-1) + 1 • Recurrence Relation

  13. Computational Cost of Towers of HanoiT(n) = 2*T(n-1)+1; T(1) = 1 • 2 Discs; T(2) = 3 moves • 3 Discs; T(3) = 7 moves • 4 Discs; T(4) = 15 moves • What is a closed-form expression for n discs? • n Discs: T(n) = 2n -1 • Execution time grows exponentially

  14. So What if Time Grows Exponentially? • Assume that it takes 1 second to make a single move: • 1 disc: 21-1 = 1 second • 10 discs: 210-1 seconds ~ 17 minutes • 20 discs: 220-1 seconds ~ 12 days • 30 discs: 230-1 seconds ~ 34 years! • Algorithms with exponential running times are no match even for the fastest computers for large values of n • Can we implement a non-recursive solution?

  15. every other move is smallest disc Outputs of the Recursive Solution % java TowersOfHanoi 4 1 right 2 left 1 right 3 right 1 right 2 left 1 right 4 left 1 right 2 left 1 right 3 right 1 right 2 left 1 right % java TowersOfHanoi 3 1 left 2 right 1 left 3 left 1 left 2 right 1 left Need to make a total of 2n -1 moves

  16. Divide-and-Conquer Divide-and-conquer paradigm. • Break up problem into smaller subproblems of same structure. • Solve subproblems recursively using same method. • Combine results to produce solution to original problem. Many important problems succumb to divide-and-conquer. • FFT for signal processing. • Parsers for programming languages. • Multigrid methods for solving PDEs. • Quicksort and mergesort for sorting. • Hilbert curve for domain decomposition. • Quad-tree for efficient N-body simulation. • Midpoint displacement method for fractional Brownian motion. Divide et impera. Veni, vidi, vici. - Julius Caesar

  17. Summary How to write simple recursive programs? • Base case, reduction step. • Trace the execution of a recursive program. • Use pictures. Why learn recursion? • New mode of thinking. • Powerful programming tool. Divide-and-conquer. Elegant solution to many important problems.

  18. Homework 4

  19. Eureka!

  20. Two approaches • Random (“generate-and-test”) • Recursive

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