Old post, but I liked the solution source posted and thought I'd post a fixed version. It uses the source posted above, but now correctly calculates the result with operator precedence and negative numbers. Eg. 10 + 12*10 - -47 % 30.0 = 147 or -2 + -4 * (-2 - -2*10) = -74
public static class StringToFormula
{
private static readonly string[] operators = { "+", "-", "/", "%", "*", "^" };
private static readonly Func<double, double, double>[] operations = {
(a1, a2) => a1 + a2,
(a1, a2) => a1 - a2,
(a1, a2) => a1 / a2,
(a1, a2) => a1 % a2,
(a1, a2) => a1 * a2,
(a1, a2) => Math.Pow(a1, a2)
};
public static bool TryEval(string expression, out double value)
{
try
{
value = Eval(expression);
return true;
}
catch
{
value = 0.0;
return false;
}
}
public static double Eval(string expression)
{
if (string.IsNullOrEmpty(expression))
return 0.0;
if (double.TryParse(expression, NumberStyles.Any, CultureInfo.InvariantCulture, out double value))
return value;
List<string> tokens = GetTokens(expression);
tokens.Add("$"); // Append end of expression token
Stack<double> operandStack = new Stack<double>();
Stack<string> operatorStack = new Stack<string>();
int tokenIndex = 0;
while (tokenIndex < tokens.Count - 1)
{
string token = tokens[tokenIndex];
string nextToken = tokens[tokenIndex + 1];
switch (token)
{
case "(":
{
string subExpr = GetSubExpression(tokens, ref tokenIndex);
operandStack.Push(Eval(subExpr));
continue;
}
case ")":
throw new ArgumentException("Mis-matched parentheses in expression");
// Handle unary ops
case "-":
case "+":
{
if (!IsOperator(nextToken) && operatorStack.Count == operandStack.Count)
{
operandStack.Push(double.Parse($"{token}{nextToken}", CultureInfo.InvariantCulture));
tokenIndex += 2;
continue;
}
}
break;
}
if (IsOperator(token))
{
while (operatorStack.Count > 0 && OperatorPrecedence(token) <= OperatorPrecedence(operatorStack.Peek()))
{
if (!ResolveOperation()) {
throw new ArgumentException(BuildOpError());
}
}
operatorStack.Push(token);
}
else
{
operandStack.Push(double.Parse(token, CultureInfo.InvariantCulture));
}
tokenIndex += 1;
}
while (operatorStack.Count > 0)
{
if (!ResolveOperation())
throw new ArgumentException(BuildOpError());
}
return operandStack.Pop();
bool IsOperator(string token)
{
return Array.IndexOf(operators, token) >= 0;
}
int OperatorPrecedence(string op)
{
switch (op)
{
case "^":
return 3;
case "*":
case "/":
case "%":
return 2;
case "+":
case "-":
return 1;
default:
return 0;
}
}
string BuildOpError() {
string op = operatorStack.Pop();
string rhs = operandStack.Any() ? operandStack.Pop().ToString() : "null";
string lhs = operandStack.Any() ? operandStack.Pop().ToString() : "null";
return $"Operation not supported: {lhs} {op} {rhs}";
}
bool ResolveOperation()
{
if (operandStack.Count < 2)
{
return false;
}
string op = operatorStack.Pop();
double rhs = operandStack.Pop();
double lhs = operandStack.Pop();
operandStack.Push(operations[Array.IndexOf(operators, op)](lhs, rhs));
Console.WriteLine($"Resolve {lhs} {op} {rhs} = {operandStack.Peek()}");
return true;
}
}
private static 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];
switch (token) {
case "(": parenlevels += 1; break;
case ")": parenlevels -= 1; break;
}
if (parenlevels > 0)
subExpr.Append(token);
index += 1;
}
if (parenlevels > 0)
throw new ArgumentException("Mis-matched parentheses in expression");
return subExpr.ToString();
}
private static 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.ToString());
}
else
{
sb.Append(c);
}
}
if ((sb.Length > 0))
{
tokens.Add(sb.ToString());
}
return tokens;
}
}
22-10-2024 update:
This should probably be updated to use spans, so it doesn't make (string) copies of parts of the expression
random formula fieldandanother UI?ExpressionTree