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    copied!<p>This is my "reference implementation" in C# (somewhat unwieldy).</p> <pre><code> static int RevIndexOf(string S, char Ch, int StartPos) { for (int P = StartPos; P &gt;= 0; P--) if (S[P] == Ch) return P; return -1; } static bool IsDigit(char Ch) { return (((Ch &gt;= '0') &amp;&amp; (Ch &lt;= '9')) || (Ch == '.')); } static int GetNextOperator(List&lt;string&gt; Tokens) { int R = Tokens.IndexOf("^"); if (R != -1) return R; int P1 = Tokens.IndexOf("*"); int P2 = Tokens.IndexOf("/"); if ((P1 == -1) &amp;&amp; (P2 != -1)) return P2; if ((P1 != -1) &amp;&amp; (P2 == -1)) return P1; if ((P1 != -1) &amp;&amp; (P2 != -1)) return Math.Min(P1, P2); P1 = Tokens.IndexOf("+"); P2 = Tokens.IndexOf("-"); if ((P1 == -1) &amp;&amp; (P2 != -1)) return P2; if ((P1 != -1) &amp;&amp; (P2 == -1)) return P1; if ((P1 != -1) &amp;&amp; (P2 != -1)) return Math.Min(P1, P2); return -1; } static string ParseSubExpression(string SubExpression) { string[] AA = new string[] { "--", "++", "+-", "-+" }; string[] BB = new string[] { "+", "+", "-", "-" }; for (int I = 0; I &lt; 4; I++) while (SubExpression.IndexOf(AA[I]) != -1) SubExpression = SubExpression.Replace(AA[I], BB[I]); const string Operators = "^*/+-"; List&lt;string&gt; Tokens = new List&lt;string&gt;(); string Token = ""; foreach (char Ch in SubExpression) if (IsDigit(Ch) || (("+-".IndexOf(Ch) != -1) &amp;&amp; (Token == ""))) Token += Ch; else if (Operators.IndexOf(Ch) != -1) { Tokens.Add(Token); Tokens.Add(Ch + ""); Token = ""; } else throw new Exception("Unhandled error: invalid expression."); Tokens.Add(Token); int P1 = GetNextOperator(Tokens); while (P1 != -1) { double A = double.Parse(Tokens[P1 - 1]); double B = double.Parse(Tokens[P1 + 1]); double R = 0; switch (Tokens[P1][0]) { case '^': R = Math.Pow(A, B); break; case '*': R = A * B; break; case '/': R = A / B; break; case '+': R = A + B; break; case '-': R = A - B; break; } Tokens[P1] = R.ToString(); Tokens.RemoveAt(P1 + 1); Tokens.RemoveAt(P1 - 1); P1 = GetNextOperator(Tokens); } if (Tokens.Count == 1) return Tokens[0]; else throw new Exception("Unhandled error."); } static bool FindSubExpression(string Expression, out string Left, out string Middle, out string Right) { int P2 = Expression.IndexOf(')'); if (P2 == -1) { Left = ""; Middle = ""; Right = ""; return false; } else { int P1 = RevIndexOf(Expression, '(', P2); if (P1 == -1) throw new Exception("Unhandled error: unbalanced parentheses."); Left = Expression.Substring(0, P1); Middle = Expression.Substring(P1 + 1, P2 - P1 - 1); Right = Expression.Remove(0, P2 + 1); return true; } } static string ParseExpression(string Expression) { Expression = Expression.Replace(" ", ""); string Left, Middle, Right; while (FindSubExpression(Expression, out Left, out Middle, out Right)) Expression = Left + ParseSubExpression(Middle) + Right; return ParseSubExpression(Expression); } </code></pre>
 

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