CS1101X: Programming Methodology Recitation 9 Recursion II.

CS1101X: Programming Methodology

Recitation 9 Recursion II

CS1101X Recitation #9 2

Task 1: Mystery (1/6)

Study the code below and if you like, trace the recursive method mystery(). What is the smaller version of the task on which the recursive call works? How does the original problem relate to the smaller problem? What does the method compute?


Task 1: Mystery (2/6)public static void main(String[] args) { // code to read values into array - omitted System.out.print("Answer = " + mystery(list, list.length); }}

// pre-cond: n > 0public static int mystery(int[] a, int n) { if (n == 1) return a[0]; else { int m = mystery(a, n-1); return a[n-1] > m ? a[n-1]: m; }}


Task 1: Mystery (3/6)

Is the code a “going-up”, “going-down” or “split-half” recursion?

Write the other two versions. You may also try other versions.


Task 1: Mystery (4/6)“Going-up” version


Task 1: Mystery (5/6)“Split-half” version


Task 1: Mystery (6/6)Other version


Task 2: Transpose Matrix (1/6)

To transpose a square matrix, the columns become the rows and the rows become columns.

For example, the 55 matrix on the left below is transposed into the matrix on the right.

716345315

31530518

12224126

83111020

2398414

73112823

1652239

343041118

5352104

151862014


Task 2: Transpose Matrix (2/6)

Write a recursive method to transpose a square matrix.

Hint: If the original problem is an nn matrix that extends from m[0][0] to m[n-1][n-1], then the smaller problem is the (n-1)(n-1) matrix that extends from m[0][0] to m[n-2][n-2].


Task 2: Transpose Matrix (3/6)import java.util.*;

class Transpose { public static void main(String[] args) { int[][] matrix = createMatrix();

System.out.println("Before transpose:"); printMatrix(matrix);

transpose(matrix, matrix.length);

System.out.println(); System.out.println("After transpose:"); printMatrix(matrix); } . . .}

Write the methods createMatrix, printMatrix, and transpose.


Task 2: Transpose Matrix (4/6)createMatrix


Task 2: Transpose Matrix (5/6)printMatrix


Task 2: Transpose Matrix (6/6)transpose


End of Recitation #9

CS1101X: Programming Methodology Recitation 9 Recursion II.

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