Fastest way to count number of lines

[alfrednewman]

Hello,

Before processing a giant txt file, I need to know in advance how many lines 
that file has. Since I will have to process multiple files it would be 
important to perform this line counting operation as quickly as possible.

What is the fastest way to know how many lines a txt file has?

I am currently using the following: 
    
    
    for line in lines "input.txt":
        inc(i)
    

For some reason I think the code is very simple and should have some way to do 
it in a faster way...