Find all the numbers in one file that are not in another file in python

Question

There are two files, say FileA and FileB and we need to find all the numbers that are in FileA which is not there in FileB. All the numbers in the FileA are sorted and all t

Answer 1

As files are sorted you can just iterate through each line at a time, if the line of file A is less than the line of file B then you know that A is not in B so you then increment file A only and then check again. If the line in A is greater than the line in B then you know that B is not in A so you increment file B only. If A and B are equal then you know line is in both so increment both files. while in your original question you stated you were interested in entries which are in A but not B, this answer will extend that and also give entries in B not A. This extends the flexability but still allows you so print just those in A not B.

def strip_read(file):
    return file.readline().rstrip()

in_a_not_b = []
in_b_not_a = []
with open("fileA") as A:
    with open("fileB") as B:
        Aline = strip_read(A)
        Bline = strip_read(B)
        while Aline or Bline:
            if Aline < Bline and Aline:
                in_a_not_b.append(Aline)
                Aline = strip_read(A)
            elif Aline > Bline and Bline:
                in_b_not_a.append(Bline)
                Bline = strip_read(B)
            else:
                Aline = strip_read(A)
                Bline = strip_read(B)

print("in A not in B", in_a_not_b, "\nin B not in A", in_b_not_a)

OUTPUT for my sample Files

in A not in B ['2', '5', '7'] 
in B not in A ['6']