字典初始化问题

Question

Why does it work

Dictionary<string, Func<double, Func<double, double>>> functions = new Dictionary<string, Func<double, Func<double, double>>>
        {
            {"+", a => b => a + b },
            {"-", a => b => a - b },
            {"*", a => b => a * b },
            {"/", a => b => a / b },
            {"^", a => b => Math.Pow(a, b) }
        };
functions.Add("Square", a => b => functions["^"](a)(2));
functions.Add("Sqrt", a => b => functions["^"](a)(0.5));

and this doesn't

Dictionary<string, Func<double, Func<double, double>>> functions = new Dictionary<string, Func<double, Func<double, double>>>()
        {
            {"+", a => b => a + b },
            {"-", a => b => a - b },
            {"*", a => b => a * b },
            {"/", a => b => a / b },
            {"^", a => b => Math.Pow(a, b) },
            {"Square", a => b => functions["^"](a)(2) },
            {"Sqrt", a => b => functions["^"](a)(0.5) }
        };

I am getting confused because if i declare "functions" like class field the second way would work.

Answer 1

The issue is, that by the time of defining the member of the Dictionary:

{"Square", a => b => functions["^"](a)(2) },

the Dictionary object is not yet assigned to variable functions .

But defining the last members as:

{"Square", a => b => Math.Pow(a, 2) },
{"Sqrt", a => b => Math.Pow(a, 0.5) }

Will definitely work.

Answer 2

You can do this instead:

 Dictionary<string, Func<double, Func<double, double>>> functions  = null;

 functions = new Dictionary<string, Func<double, Func<double, double>>>()
    {
        {"+", a => b => a + b },
        {"-", a => b => a - b },
        {"*", a => b => a * b },
        {"/", a => b => a / b },
        {"^", a => b => Math.Pow(a, b) },
        {"Square", a => b => functions["^"](a)(2) },
        {"Sqrt", a => b => functions["^"](a)(0.5) }
    };

Your problem is that you cannot use a variable in its own declaration. For example:

 int a = 2 * a;

Will also give you a compile error.

You can get around your issue by simply making sure the compiler knows that functions has a value before it's used. It is safe to do this in your case, because your values are delegates, and will not be called until after the variable is declared.

It works in a class declaration because when you declare a member in a class:

 public class Foo { public int a; }
 public static void Main() 
 {
      Console.WriteLine((new Foo()).a.ToString()); //"0"
      int a = 2 * a; //Compile error
 }

The compiler will automatically initialize a to be 0 when declared in a class. However, in your own methods, the compiler does not act the same way.

Answer 3

Because functions is not assigned to anything at the point of initialization. For example this sample:

var dict = new Dictionary<int, int> {
    { 1, 2 },
    { 3, dict[1] }       // CS0841  Cannot use local variable 'dict' before it is declared
};                       // CS0165  Use of unassigned local variable 'dict'

is the same as:

Dictionary<int, int> dict;               // dict is not assigned to anything and is null

var tempDictionary = new Dictionary<int, int>();
tempDictionary.Add(1, 2);
tempDictionary.Add(3, dict[1]);          // CS0165  Use of unassigned local variable 'dict'

dict = tempDictionary;                   // now dict is assigned and can be used

As a side note, square and sqrt functions that accept 2 parameters seems quite counterintuitive.

---

Alternative could be using temporary variable, or declaring the power function before the dictionary.

// using FdFdd = System.Func<double, System.Func<double, double>>; // at the top of the file

FdFdd temp;
var functions = new Dictionary<string, FdFdd>()
{
    { "+", a => b => a + b },
    { "-", a => b => a - b },
    { "*", a => b => a * b },
    { "/", a => b => a / b },
    { "^", temp = a => b => Math.Pow(a, b) },
    { "Square", a => b => temp(a)(2) },
    { "Sqrt", a => b => temp(a)(0.5) }
};