User Interfaces - Game Programming Algorithms and Techniques - page 253

The first step to allow typing is to start with an empty string. Every time the player
types in a letter, we can then append the appropriate character to said string. But
in order to do this, we need to determine the correct character. Recall that Chapter
5 also discussed the idea of virtual keys (how every key on the keyboard corres-
ponds to an index in an enum ). So, for example, K_A might correspond to the “A”
key on the keyboard. Conveniently, in a typical system, the letter virtual keys are
sequential within the enum . This means that that K_B would be one index after
K_A , K_C would be an index after K_B , and so on. It just so happens that the
ASCII character “B” is also sequentially after the ASCII character “A.” Taking
advantage of this parallel allows us to implement a function that converts from a
letter key code to a particular character:
Click here to view code image
function KeyCodeToChar( int keyCode )
// Make sure this is a letter key
if keyCode >= K_A && keyCode <= K_Z
// For now, assume upper case.
// Depending on language, may have to cast to
a char
return ('A' + ( char )( keyCode - K_A))
else if keyCode == K_SPACE
return ' '
else
return ''
end
end
Let's test out this code with a couple examples. If keyCode is K_A , the result of
the subtraction should be 0, which means the letter returned is simply “A.” If in-
stead keyCode is K_C , the subtraction would yield 2, which means the function
would return A + 2, or the letter “C.” These examples prove that the function gives
the results we expect.
Once we've implemented this conversion function, the only other code that needs
to be written is the code that checks whether any key from K_A to K_Z was
“just pressed.” If this happens, we convert that key to its appropriate code and ap-
pend said character to our string. If we wanted to, we could also further extend
KeyCodeToChar to support upper- and lowercase by defaulting to lowercase