concatenate a string with an integer

If you truly have a string (ie. in a variable), then you can simply do:

sprintf(f, "%s%d", string, k);

where 'string' is the variable and 'k' is the int value.

If the string is a constant value (eg. "N" as in your example) you can simplify it to:

sprintf(f, "N%d", k);

where 'k' is the int value.

... and really, both of the above instances of sprintf() will really be sprintf_s() if you write code for me.

Just for the record - unless you've got a good reason to be fiddling about with sprintf and other relics of C++'s C heritage the way to the do this sort of string fiddling is to use a std::stringstream:

std::string stick_an_N_in_front_of_an_integer( int k )
{
    std::ostringstream formatter;
    formatter << "N" << k;
    return formatter.str();
}

If you've really got to have everything tucked into a character buffer have a look at std::strstream instead. It's similar to std::stringstream but does all its output to a raw character buffer instead.