hai
I am writing a program using lex & yacc.inspite of reading input from the standard input how can i take input to my program from a file say "dat"?

One easy way is to just feed the file into your program after it's compiled. Like so:

Actually what i want is the program must read the file name and open that file.How is it possible?

Probably what I'd do, if there doesn't need to be much in the way of a main() function, is to add a main() to the yacc/bison input file, and open the file from there. The Lex and YACC HOWTO shows an example of writing your own main() function.

For reading files, C has fopen; you can then get input with fscanf or fgets. C++ has ifstream, which lets you use the >> stream input operator.

There's probably also a way to add the file-opening part to the (f)lex token-expansion definitions. I've used this before to create objects, assign pointers and stuff; you could probably expand this to include opening a file.