how can I limit the length of a variable??thanks

I want to read a file as input.
I have to do this:
I have to read a file then to take the 2,4,6,8,.... rows and put them in onother file and then the same thing whit 1,3,5,...rows.

hy! How can I do so that my input to be a file.To read a file as input.thanks

hy!
I would like to know how can i ckeck the input if its a number or if its a text?