What's wrong with assigning an element from a string array to a string variable?

Question

C++ / C#

string Str2 = Str[(f + l) /2 ];
after this action Str2 is always empty. Str has lines

Answer 1

Show the full example, not what you think about it ;-)...
What is Str, and what is there in Str, and what is f, etc.

Answer 2

Try this first and draw your own conclusions.
The snag is most likely in (f + l) /2.