trouble appending select elements of an array to a string then printing in RichTextBo

[TheGreatCthulhu]

I didn't try and run the code, but I think I know where's the problem.

In C++ you're doing this:

codon.append(RNA.substr(beginIndex,endIndex-beginIndex));

// and then later...

protein.append(aminoAcids[k]);

So, in the code above, you are appending the new string to the existing cumulative string.
In C#, though, you're doing this:

codon=(RNA.Substring(beginIndex, endIndex - beginIndex));

// and later...

protein = (aminoAcids[k]);

This simply replaces the entire string with a new one, which is why you end up with only one codon and one amino acid - the one which was the last in the sequence entered (right?).
You can use the += operator, which will append the string instead.

An expression of the form

someString += otherString;

is simply a shorthand for:

someString = someString + otherString;

So you can try this (BTW, the outer "(" and ")" are not required):

codon += RNA.Substring(beginIndex, endIndex - beginIndex);

// and later...

protein += aminoAcids[k];

Or, if you need some kind of a separator in between:

codon += RNA.Substring(beginIndex, endIndex - beginIndex) + " ";

// and later...

protein += aminoAcids[k] + " ";

Also, make sure that there aren't any more errors of the same sort in other parts of the program, since I may have missed them.