如何以编程方式将字符串转换为数学表达式

问题描述

我是C#的初学者.将字符串转换为数学表达式时遇到问题.我有一个用户界面，用户可以在其中使用随机公式字段创建公式.然后，在另一个用户界面中，用户将输入这些公式字段.

I am a beginner at C#. I am facing a problem while converting a string to a mathematical expression. I have a UI where user can create formula using random formula field. And in another UI user will give input of those formula field.

例如，在第一次时，公式可以是(a+b)^n，而另一个公式可以是((a+b+c)^n+b).

like for example at first time the formula may be (a+b)^n and another the formula may be ((a+b+c)^n+b).

在我的计算UI中，用户将首次输入a，b，n，而第二个公式用户将输入a，b，c，n.任何人都可以帮助我以编程方式获取两个公式的结果吗?预先感谢

In my calculation UI for the first time user will give input for a,b,n and for 2nd formula user will give input for a,b,c,n. Can anyone please help me about how to get the result for both of the formula programmatic-ally? Thanks in advance

推荐答案

应这样做:

public class StringToFormula
{
    private string[] _operators = { "-", "+", "/", "*","^"};
    private  Func<double, double, double>[] _operations = {
        (a1, a2) => a1 - a2,
        (a1, a2) => a1 + a2,
        (a1, a2) => a1 / a2,
        (a1, a2) => a1 * a2,
        (a1, a2) => Math.Pow(a1, a2)
    };

    public double Eval(string expression)
    {
        List<string> tokens = getTokens(expression);
        Stack<double> operandStack = new Stack<double>();
        Stack<string> operatorStack = new Stack<string>();
        int tokenIndex = 0;

        while (tokenIndex < tokens.Count) {
            string token = tokens[tokenIndex];
            if (token == "(") {
                string subExpr = getSubExpression(tokens, ref tokenIndex);
                operandStack.Push(Eval(subExpr));
                continue;
            }
            if (token == ")") {
                throw new ArgumentException("Mis-matched parentheses in expression");
            }
            //If this is an operator
            if (Array.IndexOf(_operators, token) >= 0) {
                while (operatorStack.Count > 0 && Array.IndexOf(_operators, token) < Array.IndexOf(_operators, operatorStack.Peek())) {
                    string op = operatorStack.Pop();
                    double arg2 = operandStack.Pop();
                    double arg1 = operandStack.Pop();
                    operandStack.Push(_operations[Array.IndexOf(_operators, op)](arg1, arg2));
                }
                operatorStack.Push(token);
            } else {
                operandStack.Push(double.Parse(token));
            }
            tokenIndex += 1;
        }

        while (operatorStack.Count > 0) {
            string op = operatorStack.Pop();
            double arg2 = operandStack.Pop();
            double arg1 = operandStack.Pop();
            operandStack.Push(_operations[Array.IndexOf(_operators, op)](arg1, arg2));
        }
        return operandStack.Pop();
    }

    private string getSubExpression(List<string> tokens, ref int index)
    {
        StringBuilder subExpr = new StringBuilder();
        int parenlevels = 1;
        index += 1;
        while (index < tokens.Count && parenlevels > 0) {
            string token = tokens[index];
            if (tokens[index] == "(") {
                parenlevels += 1;
            }

            if (tokens[index] == ")") {
                parenlevels -= 1;
            }

            if (parenlevels > 0) {
                subExpr.Append(token);
            }

            index += 1;
        }

        if ((parenlevels > 0)) {
            throw new ArgumentException("Mis-matched parentheses in expression");
        }
        return subExpr.ToString();
    }

    private List<string> getTokens(string expression)
    {
        string operators = "()^*/+-";
        List<string> tokens = new List<string>();
        StringBuilder sb = new StringBuilder();

        foreach (char c in expression.Replace(" ", string.Empty)) {
            if (operators.IndexOf(c) >= 0) {
                if ((sb.Length > 0)) {
                    tokens.Add(sb.ToString());
                    sb.Length = 0;
                }
                tokens.Add(c);
            } else {
                sb.Append(c);
            }
        }

        if ((sb.Length > 0)) {
            tokens.Add(sb.ToString());
        }
        return tokens;
    }
}

像这样调用类和方法:

string formula = "type your formula here"; //or get it from DB
StringToFormula stf = new StringToFormula();
double result = stf.Eval(formula);

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