巧妙地替换字符串

Question

我正在尝试将变量替换为文本字符串中的值，这是一个数学表达式。

math_Expresion = d + dft-t

到当前值：

 1 + 32 - 4

然后用一个DataTable()计算出来

var result = new DataTable().Compute(math_Expresion, null);

但如果名称中的变量包含另一个变量则不起作用，例如

a = 1
aa = 22
d = a + aa
d.Replace ("a", a)
d.Replace ("aa", aa)
return 1 + 11 not 1 + 22

混淆了名字。有没有一种方法可以用它们的值替换名称而不跳过它们、一些 ReplaceExacly() 函数或类似的东西来避免这种不适？

实际代码：

    for (int i = 0; i < variables_cache.Count; i++)
    {
        if (math_expresion.Contains(variables_cache[i].name))
        {
            math_expresion = math_expresion.Replace(variables_cache[i].name, variables_cache[i].value);
        }
    }

    var result = new DataTable().Compute(math_expresion, null);

Answer 1

您不必对公式做任何事情，您可以将变量作为 DataColumns:

private static object Compute(string formula, 
                              IDictionary<string, object> variables = null) {
  // using - don't forget to Dispose table which is IDisposable
  using (DataTable table = new DataTable()) {
    // If we have variables... 
    if (variables != null)
      foreach (var pair in variables) // ... we create columns 
        table.Columns.Add(pair.Key, pair.Value.GetType()).DefaultValue = pair.Value;

    // last column is computation result
    table.Columns.Add().Expression = formula;

    // result is the value of the last (computed) column
    return table.Rows.Add()[table.Columns.Count - 1];
  }
}

用法：

var variables = new Dictionary<string, object> {
  { "d", 1},
  { "dft", 32},
  { "t", 4},
};

var result = Compute("d + dft-t", variables);

Console.Write(result);

结果：

如果你坚持字符串处理你可以试试正则表达式来巧妙地替换：

  using System.Text.RegularExpressions;

  ...

  var variables = new Dictionary<string, object> {
    { "d", 1},
    { "dft", 32},
    { "t", 4},
  };

  var formula = "d + dft-t";

  formula = Regex.Replace(
      formula,
    @"\b\p{L}[\p{L}\d_]*\b",
      m => variables.TryGetValue(m.Value, out var value) 
           ? value?.ToString() 
           : m.Value);

  Console.Write(formula);

结果：

1 + 32-4

模式解释：

\b          - word border
\p{L}       - letter (unicode one, we can use, say, cyrillic letters as well)
[\p{L}\d_]* - zero or more letters, digits, or _
\b          - word border

编辑： 要从 variables_cache 获取字典，您可以使用 Linq:

IDictionary<string, object> dict = variables_cache
  .ToDictionary(item => item.name, (object) (item.value));

从技术上讲，您不需要字典，可以轻松使用自己的结构：

// Assuming that MyVariable has Name and Value properties or fields
private static object Compute(string formula, 
                              IEnumerable<MyVariable> variables = null) {
  // using - don't forget to Dispose table which is IDisposable
  using (DataTable table = new DataTable()) {
    // If we have variables... 
    if (variables != null)
      foreach (var pair in variables) // ... we create columns 
        table.Columns.Add(pair.Name, pair.Value.GetType()).DefaultValue = pair.Value;

    // last column is computation result
    table.Columns.Add().Expression = formula;

    // result is the value of the last (computed) column
    return table.Rows.Add()[table.Columns.Count - 1];
  }
}

巧妙地替换字符串

Replace string smartly

c#

string

variables

replace