需求
指定一个String表达式,表达式符合给出的运算符规范,比如:2!=2 and 2>=3 or 4<=4,计算出表达式的结果(true or false)。
支持的操作符:(,),and,or,mod,+,-,*,/,>,>=,<,<=,=,!=
思路
1. 首先要用Java运算符替换表达式中的部分操作符,如and替换为&&,or替换为||,具体如下:
```
operatorsMap.put("\s+and\s+", "&&");
operatorsMap.put("\s+or\s+", "||");
operatorsMap.put("\s+mod\s+", "%");
operatorsMap.put("(?|<|\!|=)=(?!=|>|<|\!)", "==");
operatorsMap.put("\)and\s+", ")&&");
operatorsMap.put("\s+and\(", "&&(");
operatorsMap.put("\)and\(", ")&&(");
operatorsMap.put("\)or\s+", ")||");
operatorsMap.put("\s+or\(", "||(");
operatorsMap.put("\)or\(", ")||(");
operatorsMap.put("\)mod\s+", ")%");
operatorsMap.put("\s+mod\(", "%(");
operatorsMap.put("\)mod\(", ")%(");
operatorsMap.put("\!\s+\=", "!=");
operatorsMap.put("\>\s+\=", ">=");
operatorsMap.put("\<\s+\=", "<=");
```
2. 将替换后的表达式,转化了一个```List
```,Segment定义为:
```java
public Segment(String word,int type){
this.word = word;//词
this.type = type;//类型,如DIGIT = 1;LETTER = 2;
}
```
比如(ab+cd)/2 >= 3,解析后的Segment列表为:
```
segment1: (
segment2: ab
segment3: +
segment4: cd
segment5: )
segment6: /
segment7: 2
segment8: space
segment9: >=
segment10: space
segment11: 3
```
3. 将```List```转化为后缀表达式```List```,其中过滤掉空格(space)```Segment```
```java
public void doConvert(Segment segment) {
int type = segment.getType();
if (isBarcket(type)) { //括号的处理
dealBracket(segment);
} else if (isOperator(type)) {
dealOperator(segment);//运算符的处理
} else {
list.add(segment.getWord());//操作数的处理
}
}
```
4. 自定义各种运算符的计算规则
```java
operationMap.put("+", new PlusOperator());
operationMap.put("-", new MinusOperator());
operationMap.put("*", new MultipliedOperator());
operationMap.put("/", new DivideOperator());
operationMap.put("%", new ModOperator());
operationMap.put("^", new PowerOperator());
operationMap.put(">", new GtOperator());
operationMap.put("<", new LtOperator());
operationMap.put(">=", new GeOperator());
operationMap.put("<=", new LeOperator());
operationMap.put("==", new EqOperator());
operationMap.put("!=", new NeOperator());
operationMap.put("&&", new AndOperator());
operationMap.put("||", new OrOperator());
//比如加法运算符,PlusOperator:
public void operator(Deque<String> stack) {
//操作数出栈,完成运算
BigDecimal b = new BigDecimal(stack.pop());
BigDecimal a = new BigDecimal(stack.pop());
stack.push(a.add(b).stripTrailingZeros().toPlainString());
}
5. 计算后缀表达式的值。如果后缀表达式中操作数都是变量名,那么计算之前需要完成值的替换。
```java
public String compute(List<String> postfix) {
try {
Deque<String> stack = new ArrayDeque<>();
for (String item : postfix) {
Operator op = operationMap.get(item);
if (null == op) {
stack.push(item);
} else {
op.operator(stack);
}
}
return stack.pop();
} catch (Exception e) {
logger.info(e.getMessage(), e);
return "ERROR";
}
}
- 返回布尔值
//List<String> mustList后缀表达式
//Map<String, Object> value表达式中变量的值
public boolean getResult(List<String> mustList, Map<String, Object> value) {
return Boolean.parseBoolean(compute(replace(mustList, value)));
}
总结
将表达式处理为后缀表达式,通过栈完成操作数的运算,是个比较经典的小程序,比较考验计算机功底和细节处理。