验证字符串数学表达式 [英] Validating a string mathematical expression

问题描述

伙计们，我正在研究一个评估字符串数学表达式的系统.

Guys I am working on a system that evaluates string mathematical expression.

我的班级进行计算

public Double Calculate(string argExpression)
{
    //get the user passed string
    string ExpressionToEvaluate = argExpression;

    //pass string in the evaluation object declaration.
    Expression z = new Expression(ExpressionToEvaluate);

    //command to evaluate the value of the **************string expression
    var result = z.Evaluate();
    Double results = Convert.ToDouble(result.ToString());

    return results;
}

还有我的电话代码.

Double Finalstat = calculator.Calculate(UserQuery); 

直到现在我的表情都是

4 + 5 + 69 * (100*3)

但是，在测试过程中，我发现表达式也可能会失真(因为它是用户构建的). 像

However during testing I found that the expression may also be distorted(as it is user built). To things like

45+99abs - 778anv

所以我想知道是否有一种方法可以在将用户构建的(表达式)发送给班级中的用户进行评估之前?

So I wanted to know if there is a way of validating the user-built(expression) before sending it to be evaluated in the class ?

推荐答案

使用正则表达式，由于必须验证括号这一简单事实，您将遇到问题:

With regexes you will have a problem due to simple fact you have to validate parentheses:

abs(sin(log(...)))

您应该解析表达式，而不仅仅是将其与某种模式匹配.编写解析器规则也更加简洁和易于理解.作为奖励，您不仅可以进行验证，还可以对表达式进行评估.我的套件 NLT 包含了C#所需的一切(已经包含了简单的计算器)，并且说您会喜欢通过添加abs来增强它.

You should parse the expression, not just match it against some pattern. Writing parser rules is also much cleaner and easier to understand. As a bonus you will get not only validation but also evaluation of the expression. My suite NLT contains everything you need for C# (there is already simple calculator included), and say you would like to enhance it by adding abs.

您可以省略词法分析器部分，然后直接进入解析器:

You can omit lexer section, and go right to parser:

expr -> "abs" "(" e:expr ")"  // pattern
        { Math.Abs(e) };      // action

假设expr已通过求和，减法等定义.

assuming expr is already defined with sum, subtraction, and so on.

对于好奇的灵魂-不是"abs("，因为在abs ( 5 )这样的情况下会给出错误(无效的表达式).

For curious soul -- not "abs(" because in such case as abs ( 5 )would give error (invalid expression).

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