Quote from: crofrihosl on September 24, 2019, 02:31:44 PM

Code:

...
for firstline in firstfile:
if firstline in secondfile:
print >>f1, (firstline)

For a small number of lines, that might be ok. But for a large number of lines, it would be faster to sort the files first, and then compare. It's O(n log n) vs, O(n²).

Like this:

Quote from: LoyceV on September 24, 2019, 03:57:19 PM

Does it have to be python? Bash command comm does exactly what you need:

Code:

comm -12 <(sort file1) <(sort file2)

I don't know how fast a Python loop would be, but the above code takes about 0.05 seconds for 2 files with 50,000 lines each.

Comparison psuedo code looks like this:

Code:

e1 = file1.begin()
e2 = file2.begin()
while e1 ≠ file1.end() and e2 ≠ file2.end()
   if *e1 < *e2
   ++e1
   else if *e1 > *e2
   ++e2
   else
   print *e1
   ++e1
   ++e2

Computer science FTW.