简单的数学表达式解析 [英] simple mathematical expression parsing

问题描述

我尝试编写 equals 覆盖函数.我想我写得对，但问题在于解析表达式.我有一个 ArrayList 数组类型，它从键盘获取输入而不是评估结果.我可以与另一个 ArrayList 变量进行比较，但如何将 ArrayList 与 String 进行比较.例如，

I try to write equals override function. I think I have written right but the problem is that parsing the expression. I have an array type of ArrayList<String> it takes inputs from keyboard than evaluate the result. I could compare with another ArrayList<String> variable but how can I compare the ArrayList<String> to String. For example,

String expr = "(5 + 3) * 12 / 3";
ArrayList<String> userInput = new ArrayList<>();
userInput.add("(");
userInput.add("5");
userInput.add(" ");
userInput.add("+");
userInput.add(" ");
userInput.add("3");
.
.
userInput.add("3");
userInput.add(")");

然后将 userInput 转换为 String 然后使用 equals 进行比较如您所见，申请测试时间太长了.我曾经拆分过，但它也拆分了组合数字.像 12 到 1 和 2

then convert userInput to String then compare using equals As you see it is too long when a test is wanted to apply. I have used to split but It splits combined numbers as well. like 12 to 1 and 2

public fooConstructor(String str) 
{
   // ArrayList<String> holdAllInputs; it is private member in class
   holdAllInputs = new ArrayList<>();

    String arr[] = str.split("");

    for (String s : arr) {
        holdAllInputs.add(s);
    }
}

正如您所料，它没有给出正确的结果.如何修复?或者有人可以帮助编写正则表达式以根据需要正确解析它吗?作为输出，我得到:

As you expect it doesn't give the right result. How can it be fixed? Or can someone help to writing regular expression to parse it properly as wanted? As output I get:

(,5, ,+, ,3,), ,*, ,1,2, ,/, ,3 

而不是

(,5, ,+, ,3,), ,*, ,12, ,/, ,3

推荐答案

帮助您的正则表达式是

"(?<=[-+*/()])|(?=[-+*/()])"

当然，您需要避免不需要的空格.

and of course, you need to avoid unwanted spaces.

我们开始，

String expr = "(5 + 3) * 12 / 3";
.
. // Your inputs
.
String arr[] = expr.replaceAll("\\s+", "").split("(?<=[-+*/()])|(?=[-+*/()])");
for (String s : arr) 
{
    System.out.println("Element : " + s);
} 

请看我的实验:http://rextester.com/YOEQ4863

希望有帮助.

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