Help me to understand this c# code

this code in Beginning C# 3.0: An Introduction to Object Oriented Programming

this is a program that has the user enter a couple of sentences in a multi - line textbox and then count how many times each letter occurs in that text

  private const int MAXLETTERS = 26;            // Symbolic constants
  private const int MAXCHARS = MAXLETTERS - 1;
  private co开发者_JS百科nst int LETTERA = 65;

.........

private void btnCalc_Click(object sender, EventArgs e)
  {
    char oneLetter;
    int index;
    int i;
    int length;
    int[] count = new int[MAXLETTERS];
    string input;
    string buff;

    length = txtInput.Text.Length;
    if (length == 0)    // Anything to count??
    {
      MessageBox.Show("You need to enter some text.", "Missing Input");
      txtInput.Focus();
      return;
    }
    input = txtInput.Text;
    input = input.ToUpper();


    for (i = 0; i < input.Length; i++)    // Examine all letters.
    {
      oneLetter = input[i];               // Get a character
      index = oneLetter - LETTERA;        // Make into an index
      if (index < 0 || index > MAXCHARS)  // A letter??
        continue;                         // Nope.
      count[index]++;                     // Yep.
    }

    for (i = 0; i < MAXLETTERS; i++)
    {
      buff = string.Format("{0, 4} {1,20}[{2}]", (char)(i + LETTERA)," ",count[i]);
      lstOutput.Items.Add(buff);
    }
  }

I do not understand this line

 count[index]++;

and this line of code

buff = string.Format("{0, 4} {1,20}[{2}]", (char)(i + LETTERA)," ",count[i]);

count[index]++; means "add 1 to the value in count at index index". The ++ is specifically known as incrementing. What the code is doing is tallying the number of occurrences of a letter.

buff = string.Format("{0, 4} {1,20}[{2}]", (char)(i + LETTERA)," ",count[i]); is formatting a line of output. With string.Format, you first pass in a format specifier that works like a template or form letter. The parts between { and } specify how the additional arguments passed into string.Format are used. Let me break down the format specification:

{0, 4}       The first (index 0) argument (which is the letter, in this case).
             The ,4 part means that when it is output, it should occupy 4 columns
             of text.

{1,20}       The second (index 1) argument (which is a space in this case).
             The ,20 is used to force the output to be 20 spaces instead of 1.

{2}          The third (index 2) argument (which is the count, in this case).

So when string.Format runs, (char)(i + LETTERA) is used as the first argument and is plugged into the {0} portion of the format. " " is plugged into {1}, and count[i] is plugged into {2}.

count[index]++;

That's a post-increment. If you were to save the return of that it would be count[index] prior to the increment, but all it basically does is increment the value and return the value prior to the increment. As for the reason why there is a variable inside square brackets, it is referencing a value in the index of an array. In other words, if you wanted to talk about the fifth car on the street, you may consider something like StreetCars(5). Well, in C# we use square brackets and zero-indexing, so we would have something like StreetCars[4]. If you had a Car array call StreetCars you could reference the 5th Car by using the indexed value.

As for the string.Format() method, check out this article.