20155330 第十一周课堂练习（20170503）

一、代码检查-P74

修改教材P74一行代码NineNineTable.java，让执行结果是个三角形：

码云链接

public class Nine {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\t",i,j,i*j);}System.out.println();}}
}

二、迷你直流

提交测试截图和码云练习项目链接，实现Linux下dc的功能，计算后缀表达式的值

码云链接

import java.util.StringTokenizer;
import java.util.Stack;public class MyDC {/*** constant for addition symbol*/private final char ADD = '+';/*** constant for subtraction symbol*/private final char SUBTRACT = '-';/*** constant for multiplication symbol*/private final char MULTIPLY = '*';/*** constant for division symbol*/private final char DIVIDE = '/';/*** the stack*/private Stack<Integer> stack;public MyDC() {stack = new Stack<Integer>();}public int evaluate(String expr) {int op1, op2, result = 0;String token;StringTokenizer tokenizer = new StringTokenizer(expr);while (tokenizer.hasMoreTokens()) {token = tokenizer.nextToken();//如果是运算符，调用isOperatorif (isOperator(token)) {op2 = (stack.pop()).intValue();op1 = (stack.pop()).intValue();result = evalSingleOp(token.charAt(0), op1, op2);stack.push(new Integer(result));//从栈中弹出操作数2//从栈中弹出操作数1//根据运算符和两个操作数调用evalSingleOp计算result;//计算result入栈;} elsestack.push(new Integer(Integer.parseInt(token)));//如果是操作数//操作数入栈;}return result;}private boolean isOperator(String token) {return (token.equals("+") || token.equals("-") ||token.equals("*") || token.equals("/"));}private int evalSingleOp(char operation, int op1, int op2) {int result = 0;switch (operation) {case ADD:result = op1 + op2;break;case SUBTRACT:result = op1 - op2;break;case MULTIPLY:result = op1 * op2;break;case DIVIDE:result = op1 / op2;break;}return result;}
}

import java.util.Scanner;
public class MyDCTest {public static void main (String[] args) {String expression, again;int result;try{Scanner in = new Scanner(System.in);do{MyDC evaluator = new MyDC();System.out.println ("Enter a valid postfix expression: ");expression = in.nextLine();result = evaluator.evaluate (expression);System.out.println();System.out.println ("That expression equals " + result);System.out.print ("Evaluate another expression [Y/N]? ");again = in.nextLine();System.out.println();}while (again.equalsIgnoreCase("y"));}catch (Exception IOException){System.out.println("Input exception reported");}}
}