Java字符串型方程两位数值问题 [英] Java string-type equation double digit value issue
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问题描述
我已被分配去创建一个Java程序，该程序读取一个字符串方程式并计算其值.但是到目前为止，我设法创建的代码仅能计算一位整数.而且我不知道如何使我的长整数工作.为了使用两位甚至三位数的整数，我的函数应该是什么样的?
I've been assigned to create a Java program which reads a string equation and calculates its value. But the code I've managed to create this far is only able to calculate single-digit integers. And I have no idea how to make my longer length integers work. What should my function look like in order to work with double or even triple digit integers?
public class Ex8 {
/**
* @param args the command line arguments
*/
public static void main(String[] args) {
// TODO code application logic here
String e = "1 - 3 * 18 / 4 + 2";
double t = executeStringEquation(e);
System.out.println(e + " = "+t);
}
public static double executeStringEquation(String inputEquation)
{
if("".equals(inputEquation))
{
return 0;
}
else if(inputEquation.length() == 1)
{
return Integer.valueOf(inputEquation);
}
else
{
String _a = String.valueOf(inputEquation.charAt(0));
String _b = String.valueOf(inputEquation.charAt(1));
if("+".equals(_b) || "-".equals(_b) )
{
if("+".equals(_b))
{
return Integer.valueOf(_a) + executeStringEquation(inputEquation.substring(2));
}
else
{// "-"
return Integer.valueOf(_a) - executeStringEquation(inputEquation.substring(2));
}
}
else
{// "*" or "/"
boolean isMulti = ("*".equals(_b));
String _c = String.valueOf(inputEquation.charAt(2));
Integer tmp = 0;
if(isMulti)
{
tmp = Integer.valueOf(_a) * Integer.valueOf(_c);
}
else
{
tmp = Integer.valueOf(_a) / Integer.valueOf(_c);
}
String newStr = String.valueOf(tmp) + inputEquation.substring(3);
return executeStringEquation(newStr);
}
}
}
}
推荐答案
我正在开发一个函数，该函数使用后缀求值法对多位数进行求值.这样做的想法是可以在此处阅读的Postfix和Infix表示法: http ://interactivepython.org/runestone/static/pythonds/BasicDS/InfixPrefixandPostfixExpressions.html
I'm developing a function to evaluate your expressions with postfix evaluation for multidigit numbers. The idea to do it is the Postfix and Infix notation that you can read here: http://interactivepython.org/runestone/static/pythonds/BasicDS/InfixPrefixandPostfixExpressions.html
这是此任务的代码段. 我更正了表达式评估器上的错误，现在评估是正确的，请检查一下:
Here is the piece of code for this task. I corrected the bug on the expression evaluator and now the evaluation is correct, please check it out:
package com.stackoverflow.infix;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Stack;
import java.util.StringTokenizer;
public class InfixPostfixEvaluator {
private static final String operators = "-+/*";
private int getPrecedence(String input) {
char operator = input.charAt(0);
int ret = 0;
if (operator == '-' || operator == '+') {
ret = 1;
} else if (operator == '*' || operator == '/') {
ret = 2;
}
return ret;
}
private boolean operatorGreaterOrEqual(String op1, String op2) {
return getPrecedence(op1) >= getPrecedence(op2);
}
private boolean isOperator(String operator) {
return operators.indexOf(operator) >= 0;
}
private boolean isOperand(String val) {
try {
Double.parseDouble(val);
} catch (NumberFormatException nfe) {
return false;
}
return true;
}
public Stack<String> convert2Postfix(List<String> expressionInTokens) {
Stack<String> stack = new Stack<String>();
Stack<String> postExpression = new Stack<String>();
for (String c : expressionInTokens) {
if (isOperator(c)) {
while (!stack.isEmpty()) {
if (operatorGreaterOrEqual(stack.peek(), c)) {
postExpression.push(stack.pop());
} else {
break;
}
}
stack.push(c);
} else if (isOperand(c)) {
postExpression.push(c);
}
}
while(!stack.isEmpty()){
postExpression.push(stack.pop());
}
Collections.reverse(postExpression);
return postExpression;
}
public int evaluatePostfix(Stack<String> expression) {
Stack<Integer> stack = new Stack<Integer>();
while (!expression.isEmpty()) {
String c = expression.pop();
if (isOperand(String.valueOf(c))) {
stack.push(Integer.parseInt(c));
} else if (isOperator(String.valueOf(c))) {
int op1 = stack.pop();
int op2 = stack.pop();
int result;
switch (c) {
case "*":
result = op1 * op2;
stack.push(result);
break;
case "/":
result = op2 / op1;
stack.push(result);
break;
case "+":
result = op1 + op2;
stack.push(result);
break;
case "-":
result = op2 - op1;
stack.push(result);
break;
}
}
}
return stack.pop();
}
/**
* Takes an expression and parses it, returning a list with every element of
* the expression.The expression must not have white spaces, or characters
* different from numbers or the operands + - / *
*
*/
public List<String> split(String expression) {
List<String> a = new LinkedList<String>();
StringTokenizer st = new StringTokenizer(expression, "+*/-", true);
while (st.hasMoreTokens())
a.add(st.nextToken());
return a;
}
}
和测试
package com.stackoverflow.infix;
import static org.junit.Assert.assertEquals;
import java.util.LinkedList;
import java.util.List;
import java.util.Stack;
import java.util.StringTokenizer;
import org.junit.Test;
public class TestInfixPostfixEvaluator {
@Test
public void testWhole() {
InfixPostfixEvaluator eval = new InfixPostfixEvaluator();
String infixExpr = "5+50-10";
List<String> expression = eval.split(infixExpr);
Stack<String> stack = eval.convert2Postfix(expression);
assertEquals(45, eval.evaluatePostfix(stack));
infixExpr = "2-3+4";
expression = eval.split(infixExpr);
stack = eval.convert2Postfix(expression);
assertEquals(3, eval.evaluatePostfix(stack));
infixExpr = "305-303+4";
expression = eval.split(infixExpr);
stack = eval.convert2Postfix(expression);
assertEquals(6, eval.evaluatePostfix(stack));
infixExpr = "2+3";
expression = eval.split(infixExpr);
stack = eval.convert2Postfix(expression);
assertEquals(5, eval.evaluatePostfix(stack));
infixExpr = "2*5+5";
expression = eval.split(infixExpr);
stack = eval.convert2Postfix(expression);
assertEquals(15, eval.evaluatePostfix(stack));
infixExpr = "2*5/5*5";
expression = eval.split(infixExpr);
stack = eval.convert2Postfix(expression);
assertEquals(10, eval.evaluatePostfix(stack));
infixExpr = "4+2*16/4";
expression = eval.split(infixExpr);
stack = eval.convert2Postfix(expression);
assertEquals(12, eval.evaluatePostfix(stack));
infixExpr = "16*3/4+2";
expression = eval.split(infixExpr);
stack = eval.convert2Postfix(expression);
assertEquals(14, eval.evaluatePostfix(stack));
}
@Test
public void testSplit() {
String exp = "15+20+84/205-1";
LinkedList<String> a = new LinkedList<String>();
StringTokenizer st = new StringTokenizer(exp, "+*/-", true);
while (st.hasMoreTokens())
a.add(st.nextToken());
System.out.println(a.toString());
}
}
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