Java语言程序设计-4.pptx

1、语言程序设计,刘炯 西安电子科技大学 通信工程学院,第四章 循环,学习目标 使用while、do while和for语句来控制语句的重复 理解循环语句的控制流程 使用布尔表达式来控制循环语句 了解三种循环类型的异和同 编写嵌套循环 使用break和continue语句来实现程序控制,while循环,while (loop-continuation-contition) / loop body statement(s); ,Loop continuation condition?,statement(s) （loop body）,true,false,/ 1 加到 100 int sum = 0

2、; int number = 1; while (number = 100) sum = sum + number; number+; ,例子,流程图,sum=0; number=1;,(number = 100)?,sum = sum + number; number+;,true,false,跟踪 while loop：,int sum = 0; int number = 1; while (number = 100) sum = sum + number; number+; ,初始化 sum、number,跟踪 while loop：,int sum = 0; int number =

3、1; while (number = 100) sum = sum + number; number+; ,(number = 100) is true,跟踪 while loop：,int sum = 0; int number = 1; while (number = 100) sum = sum + number; number+; ,执行加法,跟踪 while loop：,int sum = 0; int number = 1; while (number = 100) sum = sum + number; number+; ,number+1，现在是 2,跟踪 while loop

4、：,int sum = 0; int number = 1; while (number = 100) sum = sum + number; number+; ,现在 number 是 2,跟踪 while loop：,int sum = 0; int number = 1; while (number = 100) sum = sum + number; number+; ,执行加法,跟踪 while loop：,int sum = 0; int number = 1; while (number = 100) sum = sum + number; number+; ,number+1，

5、现在是 3,依此类推 . .,跟踪 while loop：,int sum = 0; int number = 1; while (number = 100) sum = sum + number; number+; ,现在 number 是 100,跟踪 while loop：,int sum = 0; int number = 1; while (number = 100) sum = sum + number; number+; ,执行加法,跟踪 while loop：,int sum = 0; int number = 1; while (number = 100) sum = sum

6、 + number; number+; ,number+1，现在是101,跟踪 while loop：,int sum = 0; int number = 1; while (number = 100) sum = sum + number; number+; ,现在 number 是 101,跟踪 while loop：,int sum = 0; int number = 1; while (number = 100) sum = sum + number; number+; ,跳出循环，执行下一条语句,结束循环 执行的循环次数通常都是不能预测的。可能要使用一个输入值来标志循环的结束。这样的

7、值称为标志值(sentinel value). 下面的例子用来计算若干个整数之和，输入0则表示结束。,import javax.swing.JOptionPane; public class SentinelValue public static void main(String args) String dataString = JOptionPane.showInputDialog( Enter value ( 0 to exit); int data = Integer.parseInt(dataString); int sum = 0; while (data != 0) sum =

8、sum + data; dataString = JOptionPane.showInputDialog( Enter value ( 0 to exit); data = Integer.parseInt(dataString); / Display result JOptionPane.showMessageDialog(null, Sum = + sum); ,注意： 在循环控制中不要比较浮点数相等。因为浮点数是近似的，使用它们可能导致不精确的循环次数和不准确的结果。,double data = Math.pow(Math.sqrt(2), 2) - 2; if (data = 0) S

9、ystem.out.println(data is zero); else System.out.println(data is not zero); ,do-while 循环,do / loop body statement(s); while (loop-continuation-contition);,Loop continuation condition?,statement(s) （loop body）,true,false,流程图,同 while 循环相比，在do-while循环中， 循环体至少会被执行一次。,for 循环,for (initial-action; loop-con

10、tinuation-condition; action-after-each-iteration) / loop body statement(s); ,Loop continuation condition?,statement(s) （loop body）,true,false,initial-action,action-after-each-iteration,/ 1 加到 100 int sum = 0; int number; for (number=1; number=100; number+) sum = sum + number; ,(number=100)?,sum=sum+

11、number,true,false,number=1,number+,跟踪 for 循环：,int sum = 0; int number; for (number=1; number=100; number+) sum = sum + number; ,初始化 sum、定义number,跟踪 for 循环：,int sum = 0; int number; for (number=1; number=100; number+) sum = sum + number; ,初始化 number = 1,跟踪 for 循环：,int sum = 0; int number; for (number

12、=1; number=100; number+) sum = sum + number; ,此时 number = 1，满足条件,跟踪 for 循环：,int sum = 0; int number; for (number=1; number=100; number+) sum = sum + number; ,执行加法,跟踪 for 循环：,int sum = 0; int number; for (number=1; number=100; number+) sum = sum + number; ,一轮循环完成，执行number+ number现在是2,跟踪 for 循环：,int s

13、um = 0; int number; for (number=1; number=100; number+) sum = sum + number; ,此时 number = 2，满足条件,跟踪 for 循环：,int sum = 0; int number; for (number=1; number=100; number+) sum = sum + number; ,执行加法,依此类推 . .,跟踪 for 循环：,int sum = 0; int number; for (number=1; number=100; number+) sum = sum + number; ,一轮循环

14、完成，执行number+ number现在是101,直到 number=100,跟踪 for 循环：,int sum = 0; int number; for (number=1; number=100; number+) sum = sum + number; ,此时 number = 101，不满足条件,跟踪 for 循环：,int sum = 0; int number; for (number=1; number=100; number+) sum = sum + number; ,退出 for 循环,注意： for 循环的初始条件可以是 0 个或多个逗号分隔开的表达式。 for 循环

15、中迭代之后的动作也可以是 0 个或多个逗号隔开的语句。 因此，下面的两个for 循环是正确的，尽管实际中很少用到它们。,for (int i = 1; i 100; System.out.println(i+);,for (int i = 0, j = 0; (i + j 10); i+, j+) / Do something ,注意： 如果for 循环中的继续循环条件（ loop-continuation-condition）被忽略，那么它是隐式为true 的,for ( ; ; ) / loop statement(s) ,等效于,while (true) / loop statement

16、(s) ,这是一个无限循环，建议使用这种形式。,举例：,编写程序计算从0.01到1.0的数列之和，数列以0.01递增，即0.01+0.02+0.03 + 等等,import javax.swing.JOptionPane; public class SumTest public static void main(String args) double sum = 0; for (double data = 0.01; data = 1.0; data += 0.01) sum = sum + data; / Display result JOptionPane.showMessageDialo

17、g(null, Sum = + sum); ,举例：嵌套循环,编写九九乘法表,import javax.swing.JOptionPane; public class MultiplicationTable public static void main(String args) for (int i=1; i=9; i+) for (int j=1; j=i; j+) System.out.printf(%d*%d=%2d , j, i, i*j); System.out.println(); ,1*1= 1 1*2= 2 2*2= 4 1*3= 3 2*3= 6 3*3= 9 1*4= 4

18、 2*4= 8 3*4=12 4*4=16 1*5= 5 2*5=10 3*5=15 4*5=20 5*5=25 1*6= 6 2*6=12 3*6=18 4*6=24 5*6=30 6*6=36 1*7= 7 2*7=14 3*7=21 4*7=28 5*7=35 6*7=42 7*7=49 1*8= 8 2*8=16 3*8=24 4*8=32 5*8=40 6*8=48 7*8=56 8*8=64 1*9= 9 2*9=18 3*9=27 4*9=36 5*9=45 6*9=54 7*9=63 8*9=72 9*9=81,输出结果：,格式化输出,JDK 1.5引入一种新的printf方法

19、 System.out.printf(format, items); format 是一个子串和格式描述符构成的字符串。格式描述符（specifier）指定每个项目应该如何显示。项目可以是数值、字符、布尔值或字符串。每个描述符以百分号开头。 format 的格式同c/c+中的相同,常用的格式描述符,int studentNumber = 80; double averageScore = 76.3; System.out.printf(Java course: total %d students, average score is %f, studentNumber, averageScore

20、);,输出： Java course: total 80 students, average score is 76.300000,创建格式化的字符串 String.format(format, items); 使用方法同 System.out.printf(format, items);,采用哪种循环？,三种形式的循环语句 while，do-while 和 for 在表达上都是等价的 。这也就说，可以用三种循环的任一种。,等效于,while (loop-continuation-condition) / loop statement(s) ,for ( ; loop-continuation

21、-condition; ) / loop statement(s) ,for (initial-action; loop-continuation-condition; action-after-each-iteration) / loop statement(s) ,initial-action; while (loop-continuation-condition) / loop statement(s) action-after-each-iteration; ,等效于,while 循环转换为 for 循环,for 循环转换为 while 循环,建议使用自己觉得最自然、最舒服的一种。 通

22、常，如果重复次数已经知道，就采用for循环，例如将一条信息打印100次。 如果不知道重复次数，就采用while循环，例如读入一些数值直到读入0为止。 如果在检验循环条件前需要执行循环体，就用do-while循环替代while循环。,注意,在for/while循环子句之后循环体之前多写分号是常见的错误，如下所示： 但是在 do-while 循环中，while 子句必须要以分号结尾：,for (int i=0; i100; i+); System.out.println(i); ,int i=0; while (i100); System.out.println(i); i+; ,int i=0;

23、 do System.out.println(i); i+; while (i 1; System.out.println(); ,举例：输出二进制格式(32bit版本),import java.util.Scanner; public class BinaryFormat2 public static void main(String args) System.out.print(Please input a integer number:); Scanner scanner = new Scanner(System.in); int number = scanner.nextInt();

24、System.out.print(number + =); int cutter = 0 x01 1; System.out.println(); ,举例：输出二进制格式(使用JDK),import java.util.Scanner; public class BinaryFormat3 public static void main(String args) System.out.print(Please input a integer number:); Scanner scanner = new Scanner(System.in); int number = scanner.next

25、Int(); String binaryString = Integer.toBinaryString(number); System.out.print(number + = + binaryString); ,提示： 尽量使用 JDK 中已有的功能，可以起到事半功倍的效果。,使用break和continue,break：用于跳出循环 continue：执行下一次循环 break 和 continue 只用于 for/do.while/while 循环中 注：break 还可以用在 switch 语句中,break的例子：求最大公约数（改进版）,TODO：例子、,import javax.s

26、wing.JOptionPane; public class GreatestCommonDivisor2 public static void main(String args) String s1 = JOptionPane.showInputDialog(Enter first integer); int n1 = Integer.parseInt(s1); String s2 = JOptionPane.showInputDialog(Enter second integer); int n2 = Integer.parseInt(s2); int gcd = 1; int k = M

27、ath.min(n1, n2); while (k 1) if ( (n1 % k = 0) ,continue的例子：求最大公约数,import javax.swing.JOptionPane; public class GreatestCommonDivisor3 public static void main(String args) String s1 = JOptionPane.showInputDialog(Enter first integer); int n1 = Integer.parseInt(s1); String s2 = JOptionPane.showInputDialog(Enter second inte