/**
* 计算一个正确的字符串形式运算式
* @author HLP
*
*/
public class MathUtil {
/**
* 数据栈
*/
Stacknumber;
/**
* 符号栈
*/
Stackoperator;
/**
* 计算除法时，设置的结果标度
*/
int length;
public MathUtil(int length) {
this.length = length;
}
/**
* 获得计算结果的字符串表示形式
* @param exp
* @return
*/
public StringgetResult(String exp) {
this.number =new Stack();
this.operator =new Stack();
calculate(exp);
return this.number.peek().toString();
}
/**
* 对运算式exp进行解析，并计算
* @param exp
*/
public void calculate(String exp) {
int index =0;
int sign =0;
while (index < exp.length()) {
char op = exp.charAt(index);
if ((op <='9' && op >='0') || op =='.') {
index++;
}else {
if (index >0) {
char opp = exp.charAt(index-1);
if (op =='-' && !((opp <='9' && opp >='0') || opp =='.')) {
index++;
continue;
}
}
if (sign != index) {
String numStr = exp.substring(sign, index);
BigDecimal num =new BigDecimal(numStr);
number.push(num);
}
analysis(op);
sign = ++index;
}
}
}
/**
* 对得到的数据和符号进行分析处理
* @param op
*/
public void analysis(char op) {
if (operator.empty() || comparePriority(op, operator.peek())) {
operator.push(op);
}else {
makeNum(op);
}
}
/**
* 计算
* @param op
*/
private void makeNum(char op) {
BigDecimal result =null;
while (!operator.empty() && !comparePriority(op, operator.peek())) {
char op2 =operator.pop();
if (op ==')' && op2 =='(') {
return;
}
BigDecimal num1 =null;
BigDecimal num2 =number.pop();
if (number.empty()) {
num1 =new BigDecimal(0);
}else {
num1 =number.pop();
}
switch (op2) {
case '+':
result = num1.add(num2);
break;
case '-':
result = num1.subtract(num2);
break;
case '*':
result = num1.multiply(num2);
break;
case '/':
result = num1.divide(num2, new MathContext(length));
break;
default:
operator.pop();
break;
}
number.push(result);
}
if (op !=')') {
operator.push(op);
}
}
/**
* 判断优先级：true(c1优先级高于c2)、false(c1优先级低于c2)
* @param c1
* @param c2
* @return
*/
public boolean comparePriority(char c1, char c2) {
if (c1 =='=') {
return false;
}else if (c1 ==')') {
return false;
}else if (c1 =='(') {
return true;
}else if (c2 =='(') {
return true;
}else if ((c1 =='*' || c1 =='/') && (c2 =='-' || c2 =='+')) {
return true;
}else {
return false;
}
}
}
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