CS61B 0A Exercises

虽然以前有Java基础，但是工作四年主要使用的还是python和shell，另外对于数据结构和算法的基础也不牢固。所以开一个坑从头开始学习CS61B，对于Dicussion/Lab/Homework做一个记录，学完之后回头看能有一个收获。整体流程参考CS自学社区，学习课程为B站CS61B，本篇为学习完Lectrue01后的 Homework0A。题目可以在https://practiceit.cs.washington.edu/上找到，用自己的邮箱注册即可

/*
　　Self-Check 1.26: Confusing
　　Question:
　　　　What is the output from the following Java program?
*/

public class Confusing {
    public static void method1() {
        System.out.println("I am method 1.");
    }

    public static void method2() {
        method1();
        System.out.println("I am method 2.");
    }

    public static void method3() {
        method2();
        System.out.println("I am method 3.");
        method1();
    }

    public static void main(String[] args) {
        method1();
        method3();
        method2();
        method3();
    }
}

//Answer:

I am method 1.
I am method 1.
I am method 2.
I am method 3.
I am method 1.
I am method 1.
I am method 2.
I am method 1.
I am method 2.
I am method 3.
I am method 1.

/*
这题没什么难度，涉及到函数的互相调用。
*/

 

/*
　　Exercise 2.5: starTriangle
　　Question:
　　　　Write for loops to produce the following output:
*
**
***
****
*****
*/

//Answer:

for (int i = 1; i <= 5; i++){
    for (int j = 1; j <= i; j++){
        System.out.print('*');
    }
    System.out.println();
}

/*
　　这题感觉我的解法稍微有点复杂，但是Java中没有 '*' * i 这种语法，ChatGPT也没有给出更好的方案
*/

 

/*
    Self-Check 2.25: numberTotal
    Question:
        What is the output of the following loop?
*/

int total = 25;
for (int number = 1; number <= (total / 2); number++) {
    total = total - number;
    System.out.println(total + " " + number);
}

//Answer:
24 1
22 2
19 3
15 4
10 5

/*
　　把number值和total值代入循环，最后要判断循环结束的条件是number==6 && total==10
*/

 

/*
    Question:
            Write a method called printIndexed that accepts 
            a String as its parameter and prints the String's 
            characters in order followed by their indexes in reverse order. 
            For example, the call of printIndexed("ZELDA");
            should print Z4E3L2D1A0 to the console.
*/

//Answer:

public static void printIndexed (String str){
    int len = str.length();
    for (int i = 0 ; i < len; i++){
        System.out.print(str.charAt(i));
        System.out.print(len - i - 1);
    }
}

/*
没啥好说的，用到charAt函数
*/

/*
    Exercise 4.17: stutter
    Question:
        Write a method called stutter that accepts a parameter and returns the String with its characters returns repeated twice. For example, stutter("Hello!") returns "HHeelllloo!!"
*/

//Answer
public static String stutter (String str){
    String result = "";
    for (int i = 0; i < str.length(); i++){
        result += str.charAt(i);
        result += str.charAt(i);
    }
    return result;
}

/*
    跟上一题差不多
*/

/*
    Self-Check 4.5: ifElseMystery1
    Question:
public static void ifElseMystery1(int x, int y) {
    int z = 4;
    if (z <= x) { 
        z = x + 1; 
    } else { 
        z = z + 9; 
    } 
    if (z <= y) { 
        y++; 
    } 
    System.out.println(z + " " + y); 
}
*/

//Answer:
ifElseMystery1(3, 20);    
13 21
ifElseMystery1(4, 5);    
5 6
ifElseMystery1(5, 5);    
6 5
ifElseMystery1(6, 10);    
7 11

/*
　　按顺序来呗，计算xyz的值
*/

 

/*
    Exercise 4.19: quadrant
    Question:
Write a static method called quadrant that takes as parameters a pair of real numbers representing an (x, y) point and that returns the quadrant number for that point. Recall that quadrants are numbered as integers from 1 to 4 with the upper-right quadrant numbered 1 and the subsequent quadrants numbered in a counter-clockwise fashion:


                     ^ y-axis
                     |
                     |
                     |
         Quadrant 2  |  Quadrant 1
                     |
<--------------------+--------------------> x-axis
                     |
         Quadrant 3  |  Quadrant 4
                     |
                     |
                     |
                     V
Notice that the quadrant is determined by whether the x and y coordinates are positive or negative numbers. If a point falls on the x-axis or the y-axis, then the method should return 0. Below are sample calls on the method.

Call    Value Returned
quadrant(12.4, 17.8)    1
quadrant(-2.3, 3.5)    2
quadrant(-15.2, -3.1)    3
quadrant(4.5, -42.0)    4
quadrant(0.0, 3.14)    0

*/

//Answer:
public static int quadrant(double x, double y){
    if (x > 0){
        if (y > 0)
            return 1;
        else if (y < 0)
            return 4;
        else
            return 0;
    } else if (x < 0){
        if (y > 0)
            return 2;
        else if (y < 0)
            return 3;
        else
            return 0;
    } else
        return 0;
}

/*
    投机取巧没有用到等于0的时候，还要学习下浮点数和int比较的方法，另外觉得if else有点多了。
*/
//好蠢,为什么不放在一起呢？
public static int quadrant(double x, double y){
    if (x > 0 && y > 0)
       return 1;
    else if (x < 0 && y > 0)
        return 2;
    else if (x < 0 && y < 0)
        return 3;
    else if (x > 0 && y < 0)
        return 4;
    else
        return 0;
}