Using for loop to iterate through an array using C#

C# strings can also be used like arrays:

for (int i = 0; i < str.Length; i++) 
    //str[i] is a char

The simplest way to iterate over the characters in a string is to use foreach:

foreach (char c in yourString)
{
    // ...
}

This will also work on an array.

If you just want to iterate over the letters A to Z in order you don't even need to store a string containing all the letters. You can instead use this:

for (char c = 'A'; c <= 'Z'; c++)
{
    // ...
}

to access nth character of a string use

myArray[i]

that will do

It's just about the same if you want it to be:

var myArray = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; //I think ToCharArray();
for (var i=0; i < myArray.Length; i++) {
    char letter = myArray[0];
    if (somecondition) {
        write some html;
    } else {
        write some other html;
    }
}

var characters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
foreach(char ch in characters) 
{
    if (somecondition) 
    {
        Response.Write("some html");
    } 
    else 
    {
        Repsonse.Write("some other html");
    }
}

You can either use:

Char c = myArray[i];

or something like this:

StringBuilder html = new StringBuilder();
foreach(Char c in myArray)
{
    if (c = something)
        html.Append(somehtml);
    else
        html.Append(otherhtml);
}

I hope that this will help you.

The other answers are reasonable, but personally I would be inclined to simply write

foreach(char c in "ABCDEFGHIJKLMNOPQRSTUVWXYZ")
{ ... }

and skip the indexer, local variable, blah blah blah.

More generally: if your problem solving technique every time you don't know the syntax of C# is going to be to ask on StackOverflow, you're not going to be very efficient at work. Why not learn basic C#? If you already understand JavaScript it shouldn't be too hard to pick up the basic idioms and syntax of procedural code. The type system is rather different, but the basic control flows are quite similar.

I actually did something like this a few years ago so I pulled up my code and modified it to try and show it as an example based on what you've shown:

            <div id="azindex">
            <ul id="index">
            <%
            var myArray = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray();
            var letters = Model.State.Cities.Select(city => city.Name[0]).Distinct(); //assume array setup based on same database values
            var letterArray = myArray.Where(chr => letters.Any(letter => letter == chr));
            foreach (var letter in letters) 
            {
            %>
                <li><a href='#<%= letter %>'><%= letter %></a></li>
            <%
            } 
            %>
           </ul>
        </div>

Assuming using your example where you want to write out some other html when the condition doesn't match I'd modify my code somewhat similiar to the following uncompilable code:

for (var i=0; i < myArray.Length; i++)
{
var letter =  myArray[i];
if (letters.Any(p => p == letter)
{
    <span class="no-link">{letter}</span>
} 
else
{
    <a href="#to-{letter}">{letter}</a>
}
}