03-選擇
-
3.1 引言
- 程序可以基于條件決定執(zhí)行哪些語句。
-
3.2 boolean數(shù)據(jù)類型
- boolean數(shù)據(jù)類型聲明一個具有true或者flase的變量。
- 相等的關(guān)系操作符是兩個等號（==），而不是一個等號（=），比較的結(jié)果是一個布爾值：true或flase。true和flase都是字面值，他們被當做保留字一樣，不能用作程序的標識符.
操作符 名稱 < 小于 <= 小于等于 > 大于 >= 大于等于 == 等于 != 不等于
- 3.3 if語句
- if語句是一個構(gòu)造，允許程序確定執(zhí)行的可選路徑。
- Java中有幾種類型的選擇語句：單分支if語句、雙分支if-else語句、嵌套if語句、多分支if-else語句、switch語句和條件操作符。
- 單分支if語句是指當且僅當條件為true是執(zhí)行的一個動作。單分支if語句的語法是：
```java
if(布爾表達式){
語句（組）;
}
```
- 示例：
- 提示用戶輸入一個整數(shù)。如果該數(shù)字是5的倍數(shù)，打印HiFive。如果該數(shù)字能被2整除，打印HiEven。
```java
package chapter03;
import java.util.Scanner;
public class SimpleIfDemo {
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.print("Enter an integer: ");
int number = input.nextInt();
if (number % 5 ==0)
System.out.println("HiFive");
if (number % 2 ==0)
System.out.println("HiEven");
}
}
```
- 3.4 雙分支 if-else 語句
- if-else語句根據(jù)條件是真或者是假，決定執(zhí)行的路徑。下面是雙分支if-else語句的語法：
```java
if(布爾表達式){
布爾表達式為真時執(zhí)行的語句（組）；
}
else{
布爾表達式為假時執(zhí)行的語句（組）；
}
```
- 3.5 嵌套的if語句和多分支if-else語句
- if語句可以在另外一個if語句中，形成嵌套的if語句。
```java
if (i > k){
if (j > k)
System.out.println("i and j are greater than k");
}
else
System.out.println("i is less than or equal to k");
```
- 3.6 常見錯誤和陷進
- 常見錯誤1：忘記必要的括號
- 常見錯誤2：錯誤地在if行出現(xiàn)分號
- 常見錯誤3：對布爾值的冗余測試
- 常見錯誤4：懸空else出現(xiàn)的歧義（每一組if-else都要進行相應(yīng)的縮進進行對齊，避免造成if-else不匹配的現(xiàn)象）
- 常見錯誤5：兩個浮點數(shù)值的相等測試
- 常見陷阱1：簡化布爾變量賦值
```java
if (number % 2 == 0)
even = true;
else
even = false;
//上面的代碼就等價于下面的代碼，但是 下面的代碼形式會更加的好
boolean even
= number % 2 ==0;
```
- 常見陷阱2：避免不同情形中的重復代碼
```java
package chapter01;
public class Try {
public static void main(String[] args){
int tuition;
boolean inState = true;
if (inState){
tuition = 5000;
System.out.println("The tuition is " + tuition);
}
else {
tuition = 15000;
System.out.println("The tuition is " + tuition);
}
//上面的代碼等價于下面的代碼，但是下面的代碼的形式會更加的好，顯得更加的簡捷
if (inState){
tuition = 5000;
}
else {
tuition = 15000;
}
System.out.println("The tuition is " + tuition);
}
}
```
- 3.7 產(chǎn)生隨機數(shù)
- 使用Math.random()來獲得一個0.0到1.0之間的隨機double值，不包括1.0。
- 示例：假設(shè)你想開發(fā)一個讓一年級學生練習減法的程序。程序隨機生成兩個一位整數(shù)：number1，number2，且滿足number1>=number2。程序向?qū)W生顯示問題，例如，“What is 9-2?”。當學生輸入答案之后，程序會顯示一個消息表明改答案是否正確。
```java
package chapter03;
import java.util.Scanner;
public class SubtractionQuiz {
public static void main(String[] args){
int number1 = (int)(Math.random() * 10);
int number2 = (int)(Math.random() * 10);
if (number1 < number2){
int temp = number1;
number1 = number2;
number2 = temp;
}
System.out.print("What is " + number1 + "-" + number2 + "?");
Scanner input = new Scanner(System.in);
int answer = input.nextInt();
if (number1 - number2 == answer)
System.out.println("You are correct!");
else {
System.out.println("Your answer is wrong.");
System.out.println(number1 + "-" + number2 + " should be " + (number1 - number2));
}
}
}
```
-
3.8 示例學習：計算身體質(zhì)量指數(shù)
package chapter03; import java.util.Scanner; public class ComputeAndInterpretBMI { public static void main(String[] args){ Scanner input = new Scanner(System.in); System.out.print("Enter weight in pounds: "); double weight = input.nextDouble(); System.out.print("Enter height in inches: "); double height = input.nextDouble(); final double KILOGRAMS_PER_POUND = 0.45359237; final double METERS_PER_INCH = 0.0254; double weightInKilograms = weight * KILOGRAMS_PER_POUND; double heightInMeters = height * METERS_PER_INCH; double bmi = weightInKilograms / (heightInMeters * heightInMeters); System.out.println("BMI is " + bmi); if (bmi < 18.5) System.out.print("Underweight"); else if (bmi < 25) System.out.println("Normal"); else if (bmi < 30) System.out.println("Overweight"); else System.out.println("Obese"); } } -
3.9 示例學習：計算稅率
package chapter03; import java.util.Scanner; public class ComputeTax { public static void main(String[] args){ Scanner input = new Scanner(System.in); System.out.print("(0-single filer,1-married jointly or " + "qualifying window(er),2-married separately,3-head of " + "household) Enter the filing status: "); int status = input.nextInt(); System.out.print("Enter the taxable income: "); double income = input.nextDouble(); double tax = 0; if (status == 0){ if (income <= 8350) tax = income * 0.10; else if (income <= 33950) tax = 8350 * 0.10 + (income - 8350) * 0.15; else if (income <= 82250) tax = 8350 * 0.10 + (33950 - 8350) * 0.15 + (income - 33950) * 0.25; else if (income <= 171550) tax = 8350 * 0.10 + (33950 - 8350) * 0.15 + (82250 - 33950) * 0.25 + (income - 82250) * 0.28; else if (income <= 372950) tax = 8350 * 0.10 + (33950 - 8350) * 0.15 + (82250 - 33950) * 0.25 + (171550 - 82250) * 0.28 + (income - 171550) * 0.22; else tax = 8350 * 0.10 + (33950 - 8350) * 0.15 + (82250 - 33950) * 0.25 + (171550 - 82250) * 0.28 + (372950 - 171550) * 0.22 + (income - 372950) * 0.35; } else if (status == 1){ // } else if (status == 2){ // } else if (status == 3){ // } else { System.out.println("Error:invalid status"); System.exit(1); } System.out.println("Tax is " + (int)(tax * 100) / 100.0); } } -
3.10 邏輯操作符
- 邏輯操作符！、&&、||和^可以用于產(chǎn)生復合布爾表達式。
操作符 名稱 說明 ！ 非 邏輯非 && 與 邏輯與 | 或 邏輯或 ^ 異或 邏輯異或
```java
package chapter03;
import java.util.Scanner;
public class TestBooleanOperators {
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.print("Enter an integer: ");
int number = input.nextInt();
if (number % 2 == 0 && number % 3 == 0)
System.out.println(number + "is divisible by 2 and 3.");
if (number % 2 == 0 || number % 3 == 0)
System.out.println(number + "is divisible by 2 or 3.");
if (number % 2 == 0 ^ number % 3 == 0)
System.out.println(number + "is divisible by 2 or 3,but not both.");
}
}
```
- 從數(shù)學的角度看，1<=2<=3是正確的，但是在Java中必須將兩個部分分開的，1<=2&&2<=3。
-
3.11 示例學習：判定閏年
package chapter03; import java.util.Scanner; public class LeapYear { public static void main(String[] args){ Scanner input = new Scanner(System.in); System.out.print("Enter a year: "); int year = input.nextInt(); boolean isLeapYear = ((year % 4 ==0 && year % 100 == 0) || (year % 400 == 0)); System.out.println(year + " is a leap year? " + isLeapYear); } } -
3.12 示例學習：彩票
package chapter03; import java.util.Scanner; public class Lottery { public static void main(String[] args){ int lottery = (int)(Math.random() * 100); Scanner input = new Scanner(System.in); System.out.print("Enter your lottery pick (two digits): "); int guess = input.nextInt(); int lotteryDigit1 = lottery / 10; int lotteryDigit2 = lottery % 10; int guessDigit1 = guess / 10; int guessDigit2 = guess % 10; System.out.println("The lottery number is " + lottery); if (guess == lottery) System.out.println("Exact match: you win $10,000"); else if (guessDigit2 == lotteryDigit1 && guessDigit1 == lotteryDigit2) System.out.println("Match all digits: you win $3,000"); else if (guessDigit1 == lotteryDigit1 || guessDigit1 == lotteryDigit2 || guessDigit2 == lotteryDigit1 || guessDigit2 == lotteryDigit2) System.out.println("Match one digit: you win $1,000"); else System.out.println("Sorry, no match"); } } -
3.13 switch語句
- switch語句基于變量或者表達式的值來執(zhí)行語句，下面是switch語句的完整語句。
```java
switch（switch表達式）{
case 值1：語句（組）1；
break；
case 值2：語句（組）2；
break；
...
case 值N：語句（組）N；
break；
default:默認情況下執(zhí)行的語句（組）
}
```
- switch語句遵從下述規(guī)則：
- 1、switch表達式必須是能計算出一個char、byte、short、int或者String型值，并且需用括號括主。
- 2、value1，value2，...，valueN必須與swtch表達式具有相同的數(shù)據(jù)類型。注意：value1，value2，...，valueN都是常量表達式，也就是說這里的表達式是不能包含變量的，例如，不允許出現(xiàn)1+x。
- 3、當switch表達式的值與case語句的值相匹配時，執(zhí)行從case開始的語句，直到遇到一個break語句或到達該switch語句的結(jié)束。
- 4、默認情況下（default）是可選的，當沒有一個給出的case與switch表達式匹配時，則執(zhí)行該操作。
- 5、關(guān)鍵字break是可選的。break語句會立即終止switch語句。
```java
package chapter03;
import java.util.Scanner;
public class ChineseZodiac {
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.print("Enter a year: ");
int year = input.nextInt();
switch (year % 12){
case 0:System.out.println("monkey");break;
case 1:System.out.println("rooster");break;
case 2:System.out.println("dog");break;
case 3:System.out.println("pig");break;
case 4:System.out.println("rat");break;
case 5:System.out.println("ox");break;
case 6:System.out.println("tiger");break;
case 7:System.out.println("rabbit");break;
case 8:System.out.println("dragon");break;
case 9:System.out.println("snake");break;
case 10:System.out.println("horse");break;
case 11:System.out.println("sheep");break;
}
}
}
```
- 不要忘記在需要的時候使用break語句。一旦匹配其中一個case，就從匹配的case處開始執(zhí)行， 遇到break語句或者到達switch語句的結(jié)束。這種現(xiàn)象稱為落空行為。
- 3.14 條件操作
- 符號？和：一起出現(xiàn)，稱為條件操作符（也稱為三元操作符，是Java中唯一的一個三元操作符）該語法如下：
```java
boolean-expression？expression1：expression2
```
```java
if(x>0)
y=1;
else
y=-1;
```
和下面的三元操作符是等價的：
```java
y=(x>0)?1:-1;
```
-
3.15 操作符的優(yōu)先級和結(jié)合規(guī)則
在這里插入圖片描述 -
3.16 調(diào)試
- 調(diào)試是在程序中找到和修改錯誤的過程。
- 1、一次執(zhí)行一條語句
- 2、跟蹤進入或者一步運行一個方法
- 3、設(shè)置斷點
- 4、顯示變量
- 5、顯示調(diào)用堆棧
- 6、修改變量
-
編程小習題
-
1、在這里插入圖片描述
package chapter03; import java.util.Scanner; public class Code_01 { public static void main(String[] args){ Scanner input = new Scanner(System.in); System.out.print("Enter a,b,c: "); double a = input.nextDouble(); double b = input.nextDouble(); double c = input.nextDouble(); double discriminant = b * b - 4 * a * c; if (discriminant > 0){ double answer1 = (-b + Math.pow(discriminant,0.5)) / (2 * a); double answer2 = (-b - Math.pow(discriminant,0.5)) / (2 * a); System.out.println("The equation has two roots " + answer1 + " and " + answer2); } else if (discriminant == 0){ double answer1 = (-b + Math.pow(discriminant,0.5)) / (2 * a); System.out.println("The eqution has one root " + answer1); } else System.out.println("The eqution has no real roots"); } } -
2、在這里插入圖片描述
-
1、
```java
package chapter03;
import java.util.Scanner;
public class Code_03 {
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.print("Enter a,b,c,d,e,f: ");
double a = input.nextDouble();
double b = input.nextDouble();
double c = input.nextDouble();
double d = input.nextDouble();
double e = input.nextDouble();
double f = input.nextDouble();
double medius = a * d - b * c;
if (medius == 0){
System.out.println("The eqution has no solution");
}
else{
double x = (e * d - b * f) / medius;
double y = (a * f - e * c) / medius;
System.out.println("x is " + x + " and y is " + y);
}
}
}
```
- 3、
```java
package chapter03;
import java.util.Random;
public class Code_04 {
public static void main(String[] args){
Random r = new Random();
int month = r.nextInt(12) + 1;
switch (month){
case 1:System.out.println("January");break;
case 2:System.out.println("Febrary");break;
case 3:System.out.println("March");break;
case 4:System.out.println("April");break;
case 5:System.out.println("May");break;
case 6:System.out.println("June");break;
case 7:System.out.println("July");break;
case 8:System.out.println("August");break;
case 9:System.out.println("September");break;
case 10:System.out.println("October");break;
case 11:System.out.println("November");break;
case 12:System.out.println("December");break;
}
}
}
```
- 4、
```java
package chapter03;
import java.util.Scanner;
public class Code_05 {
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.print("Enter today's day: ");
int number1 = input.nextInt();
System.out.print("Enter the number of days elapsed since today: ");
int number2 = input.nextInt();
System.out.print("Today is ");
method(number1);
System.out.print(" and the future day is ");
method(number2);
}
static void method(int n){
switch (n){
case 0:System.out.print("Sunday");break;
case 1:System.out.print("Monday");break;
case 2:System.out.print("Tuesday");break;
case 3:System.out.print("Wednesday");break;
case 4:System.out.print("Thursday");break;
case 5:System.out.print("Friday");break;
case 6:System.out.print("Saturday");break;
}
}
}
```
- 5、[圖片上傳失敗...(image-b8f19-1601914568733)]
```java
package chapter03;
import java.util.Arrays;
import java.util.Scanner;
public class Code_08 {
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.print("Enter three number(int): ");
int a = input.nextInt();
int b = input.nextInt();
int c = input.nextInt();
int[] arr = {a,b,c};
Arrays.sort(arr);
for(int i = 0; i < arr.length; i++){
System.out.print(arr[i]+" ");
}
}
}
```
- 6、
```java
package chapter03;
public class Code_09 {
public static void main(String[] args){
java.util.Scanner input = new java.util.Scanner(System.in);
System.out.print("Enter the first 9 digits of an ISBN as integer: ");
int first9Digits = input.nextInt();
int d9 = first9Digits %10;
int d8 = first9Digits /10 % 10;
int d7 = first9Digits /100 % 10;
int d6 = first9Digits /1000 % 10;
int d5 = first9Digits /10000 % 10;
int d4 = first9Digits /100000 % 10;
int d3 = first9Digits /1000000 % 10;
int d2 = first9Digits /10000000 % 10;
int d1 = first9Digits /100000000;
int d10 = (int)((d1 *1 + d2 * 2 + d3 * 3 + d4 * 4 + d5 * 5 + d6 * 6 + d7 * 7 + d8 * 8 + d9 * 9)%11);
if(d10 == 10)
System.out.println("The ISBN-10 number is " +first9Digits+"X");
else
System.out.println("The ISBN-10 number is " +first9Digits+d10);
}
}
```
- 7、[圖片上傳失敗...(image-7c0eb1-1601914568733)]
```java
package chapter03;
import java.util.Random;
import java.util.Scanner;
public class Code_10 {
public static void main(String[] args){
Random r1 = new Random();
int one = r1.nextInt(100);
int two = r1.nextInt(100);
System.out.print(one + " - " + two + " = ");
Scanner input = new Scanner(System.in);
int answer = input.nextInt();
if (answer == one - two)
System.out.println("You are right!");
else
System.out.println("You are wrong!");
}
}
```
- 8、
```java
package chapter03;
import java.util.Scanner;
public class Code_12 {
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.print("Enter a three-digits integer: ");
String str1 = input.nextLine();
String str2 = "";
for (int i = str1.length() - 1;i >= 0;i--){
str2 += str1.charAt(i);
}
if (str1.equals(str2))
System.out.println(str1 + " is a palindrome");
else
System.out.println(str1 + " is not a palindrome");
}
}
```
- 9、[圖片上傳失敗...(image-c9e8f5-1601914568734)]
```java
package chapter03;
import java.util.Arrays;
import java.util.Scanner;
public class Code_19 {
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.print("Enter three integers: ");
int a = input.nextInt();
int b = input.nextInt();
int c = input.nextInt();
int[] arr = {a,b,c};
Arrays.sort(arr);
if (arr[0] + arr[1] > arr[2])
System.out.println("legitimate");
else
System.out.println("not legitimate");
}
}
```
- 10、[圖片上傳失敗...(image-56f103-1601914568734)]
```java
package chapter03;
import java.util.Random;
public class Code_24 {
public static void main(String[] args){
Random r1 = new Random();
Random r2 = new Random();
int size = r1.nextInt(13);
int color = r2.nextInt(4);
switch (size){
case 0:System.out.print("The card you picked is 1 of ");break;
case 1:System.out.print("The card you picked is 2 of ");break;
case 2:System.out.print("The card you picked is 3 of ");break;
case 3:System.out.print("The card you picked is 4 of ");break;
case 4:System.out.print("The card you picked is 5 of ");break;
case 5:System.out.print("The card you picked is 6 of ");break;
case 6:System.out.print("The card you picked is 7 of ");break;
case 7:System.out.print("The card you picked is 8 of ");break;
case 8:System.out.print("The card you picked is 9 of ");break;
case 9:System.out.print("The card you picked is 10 of ");break;
case 10:System.out.print("The card you picked is Jack of ");break;
case 11:System.out.print("The card you picked is Queen of ");break;
case 12:System.out.print("The card you picked is King of ");break;
}
switch (color){
case 0:System.out.print("Clubs");break;
case 1:System.out.print("Diamonds");break;
case 2:System.out.print("Hearts");break;
case 3:System.out.print("Spades");break;
}
}
}
```
- 11、
```java
package chapter03;
public class Code_28 {
public static void main(String[] args){
java.util.Scanner input = new java.util.Scanner(System.in);
System.out.print("Enter r1's center x-,y-cooordinates, width,and height:");
double x1 = input.nextDouble();
double y1 = input.nextDouble();
double w1 = input.nextDouble();
double h1 = input.nextDouble();
System.out.print("Enter r2's center x-,y-coordinates, width,and height:");
double x2 = input.nextDouble();
double y2 = input.nextDouble();
double w2 = input.nextDouble();
double h2 =input.nextDouble();
double xDistance = x1 -x2 >=0 ? x1-x2 : x2-x1;
double yDistance = y1-y2 >=0? y1-y2 : y2-y1;
if (xDistance <= (w1 - w2) / 2 && yDistance <= (h1 - h2) / 2)
System.out.println("r2 is inside r1");
else if (xDistance <= (w1 + w2) / 2 && yDistance <= (h1 + h2) / 2)
System.out.println("r2 overlaps r1");
else System.out.println("r2 does not overlap r1");
}
}
```
