CSC240 Static method, array, and loop.

Transcription

1 CSC240 Static method, array, and loop. 1. Tracing programs Code public class Quiz1 public static void main(string [] args) String x = mystery("00"); public static String mystery(string s) s = s + "7"; return s; at end of main() x = 007 public class Quiz2 public static void main(string [] args) System.out.println("main 1"); mystery1(true); System.out.println("main 2"); mystery2(7); System.out.println("main 3"); public static void mystery1(boolean a) System.out.println("a = " + a); if(a) mystery2(2); System.out.println("end mystery1"); public static void mystery2(int x) System.out.println("x = " + x); main 1 a = true x = 2 end mystery1 main 2 x = 7 main 3 1 P age

2 public class WhatsPrinted00 public static void func(int x,int y,int z) x *= 2; y += x; z = (x + y) % 2; System.out.println(x + " " + y + " " + z); public static void main(string args[]) int x = 10, y = 20, z = 30; func(z, x, y); public class WhatsPrinted01 public static void func(int x,int y,int z) x *= 2; y += x; z = (x + y) % 2; public static void main(string args[]) int x = 10, y = 20, z = 30; func(z, x, y); System.out.println(x + " " + y + " " + z); public class NamingAndScope public static void main(string [] args) int x = 3; x = x(); System.out.println(x); x = x(x+1); System.out.println(x); for(int i=0; i<2; i++) x = x(x+3); System.out.println(x); public static int x() int x = 5; return x + 7; public static int x(int x) // method overloading, compared with the above // return x + 1; P age

3 public class WhatsPrinted02 public static void func(int A[]) for (int i=1; i<a.length; i++) A[i]+=A[i-1]; public static void main(string args[]) int A[] = 10,20,30; func(a); System.out.println(A[2]); public class WhatsPrinted03 public static int func(int A[], int B[]) A = B; return A[1]; public static void main(string args[]) int A[] = 10,20,30; int B[] = 40,50,60; int x = func(a, B); System.out.println(x + " " + A[1]); import java.util.arrays; public class Mystery2 public static void main(string [] args) double [] arr1 = 3.5, 5.5, 7.5; double [] arr2 = 1.0, 3.0, 5.0; mystery(arr1, arr2); System.out.println(Arrays.toString(arr1)); System.out.println(Arrays.toString(arr2)); public static void mystery(double [] a1, double [] a2) for(int i=0; i<a1.length && i<a2.length; i++) a1[i] = a1[i] + a2[i]; if(i>0) a2[i] = a2[i] - a2[i-1]; [4.5, 8.5, 12.5] [1.0, 2.0, 3.0] 3 P age

4 public class Mystery public static void mystery1 (double [ ] d) for(int i=1; i<d.length; i++) d[i] = d[i] d[i-1]; public static double [ ] mystery2 (double [ ] d) double [ ] ret = new double[d.length]; for(int i=0;i<ret.length;i++) ret[i] = d[i]-1; d[i] += 1; // POINT 2 return ret; public static void main (String [] args) double [ ] d1 = 1.0,-1.0,3.0; double [ ] d2 = 5.5,6.5,7.5; mystery1(d1); // POINT 1 double [ ] d3 = mystery2(d2); // POINT 3 1) Draw what d1 looks like at POINT 1. d1 1.0,-2.0,5.0, 2) Draw what d, ret, and d2 look like at POINT 2. ret 4.5,5.5,6.5, d=d2 6.5,7.5,8.5, 3) Draw what d2 and d3 look like at POINT 3. d2 6.5,7.5,8.5, d3 4.5,5.5,6.5, 4 P age

5 2. Writing a short method: a. Write a method that takes integers a, b, and c as arguments. It should return their average (as a double). public static double average (int a, int b, int c) return (a+b+c)/3.0; b. Write a method that takes an integer X as an argument, and returns true if X is even, and false if X is odd.. public static boolean iseven(int N) return (N % 2 == 0); c. Write a method that takes an integer N as an argument, and displays a square of NxN stars. Assume N is positive. public static void displaylineofstars(int N) for(int j=0; j<n; j++) for(int i=0; i<n; i++) System.out.print("*"); System.out.println(); d. Write a method that takes a positive integer N as an argument, and returns true if N is prime, and false otherwise. Assume N > 1. public static boolean isprime (int x) int i =2; while (x%i!= 0) i++; return (i==x); 5 P age

6 e. Write a method that takes an array of chars A, an integer j, and an integer k as arguments. The method should return a String consisting of the characters in A between positions j and k. Assume j and k are not out of range. public static String subchararray(char [] A, int j, int k) String ret = ""; for(int i=j; i<k; i++) ret = ret + char[i]; return ret; f. Write a method called copystring that takes a String s and an int x as arguments. The method should return a String containing x copies of s. For example, if you define the method correctly, then the command String t = copystring("blah", 3); should store "blahblahblah" in public static String copystring (String s, int x) String result = ; for(int j = 1; j<=x; j++) result += s; return result; 3. Writing a short method (more challenging): Write a static method named enoughtimeforlunch that accepts four integers hour1, minute1, hour2, and minute2 as parameters. Each pair of parameters represents a time on the 24-hour clock (for example, 1:36 PM would be represented as 13 and 36). The method should return true if the gap between the two times is long enough to eat lunch: that is, if the second time is at least 45 minutes after the first time. Otherwise the method should return false. You may assume that all parameter values are valid: the hours are both between 0 and 23, and the minute parameters are between 0 and 59. You may also assume that both times represent times in the same day, e.g. the first time won't represent a time today while the second time represents a time tomorrow. Note that the second time might be earlier than the first time; in such a case, your method should return false. Here are some example calls to your method and their expected return results: Call enoughtimeforlunch(11, 00, 11, 59) enoughtimeforlunch(12, 30, 13, 00) enoughtimeforlunch(12, 30, 13, 15) enoughtimeforlunch(14, 20, 17, 02) Value Returned true false true true 6 P age

7 enoughtimeforlunch(12, 30, 9, 30) enoughtimeforlunch(12, 00, 11, 55) false false public static boolean enoughtimeforlunch(int h1, int m1, int h2, int m2) int t1 = h1*60+m1, t2 = h2*60+m2; return (t2-t1>=45); Write a static method named printgrid that accepts two integer parameters rows and cols. The output is a commaseparated grid of numbers where the first parameter (rows) represents the number of rows of the grid and the second parameter (cols) represents the number of columns. The numbers count up from 1 to (rows x cols). The output are displayed in column-major order, meaning that the numbers shown increase sequentially down each column and wrap to the top of the next column to the right once the bottom of the current column is reached. You may assume that both parameters passed to your method are greater than 0. Here are some example calls to your method and their expected results: Call printgrid(3,6); printgrid(5,3); printgrid(4,1); printgrid(1,3); Output 1, 4, 7, 10, 13, 16 2, 5, 8, 11, 14, 17 3, 6, 9, 12, 15, 18 1, 2, 3 1, 6, 11 2, 7, 12 3, 8, 13 4, 9, 14 5, 10, public static void printgrid(int r, int c) for(int i = 1; i<= r; i++) int num = i; for(int j = 1; j< c; j++) system.out.print(num+, ); num+=r; system.out.println(num); 7 P age

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