Because you have done sonething wrong!

There is absolutely no more that we can say if you don't even tell us what the error message says!!

Please, read the message carefully, and ask your self "what is this telling me?"
Take the words literally - if it says something is "undefined" then you need to supply a definition for it.

Look in your compiler Manual for descriptions of its error messages.

If you can't see anything wrong on the line where the error is actually reported (this is quite common) look carefully at the immediately preceding code - have you forgotten something like a semicolon, closing bracket, etc?

If you're still stuck, then please post the full text of the message, the line of code where it occurs, and a few lines of context before.

Don't forget the < pre > and < /pre > tags around the code!