I think the problem lies here:
string item_num;

if you declare it like this, item_num now is an empty string with 0 places to put characters in. This means that if you want to add text to it, the text is simply discarted because there is no place.

string item_num = "#5";

This however gives you a string with room for 5 characters. If you now put something in it (in your case, str_for_num puts something in it), the text is not discarted anymore because there is room for it. The first 5 characters of your text will now be displayed. Anything longer than 5 characters will still be cut off.