Implement sliding window on file lines in Python

Question

I'm trying to implement a sliding/moving window approach on lines of a csv file using Python. Each line can have a column with a binary value yes or no. Basically, I want to rare yes noises. That means if say we have 3 yes lines in a window of 5 (max of 5), keep them. But if there is 1 or 2, let's change them to no. How can I do that?

For instance, the following yes should both become no.

...
1,a1,b1,no,0.75
2,a2,b2,no,0.45
3,a3,b3,yes,0.98
4,a4,b4,yes,0.22
5,a5,b5,no,0.46
6,a6,b6,no,0.20
...

But in the followings, we keep as is (there can be a window of 5 where 3 of them are yes):

...
1,a1,b1,no,0.75
2,a2,b2,no,0.45
3,a3,b3,yes,0.98
4,a4,b4,yes,0.22
5,a5,b5,no,0.46
6,a6,b6,yes,0.20
...

I attempted writing something, having a window of 5, but got stuck (it is not complete):

        window_size = 5 
        filename='C:\\Users\\username\\v3\\And-'+v3file.split("\\")[5]
        with open(filename) as fin:
            with open('C:\\Users\\username\\v4\\And2-'+v3file.split("\\")[5],'w') as finalout:
                line= fin.readline()
                index = 0
                sequence= []
                accs=[]
                while line:
                    print(line)
                    for i in range(window_size):
                        line = fin.readline()
                        sequence.append(line)
                    index = index + 1
                    fin.seek(index)

Answer 1

You can use collections.deque with the maxlen argument set to the desired window size to implement a sliding window that keeps track of the yes/no flags of the most recent 5 rows. Keep a count of yeses instead of calculating the sum of yeses in the sliding window in every iteration to be more efficient. When you have a full-size sliding window and the count of yeses is greater than 2, add the line indices of these yeses to a set where the yeses should be kept as-is. And the in the second pass after resetting the file pointer of the input, alter the yeses to noes if the line indices are not in the set:

from collections import deque

window_size = 5
with open(filename) as fin, open(output_filename, 'w') as finalout:
    yeses = 0
    window = deque(maxlen=5)
    preserved = set()
    for index, line in enumerate(fin):
        window.append('yes' in line)
        if window[-1]:
            yeses += 1
        if len(window) == window_size:
            if yeses > 2:
                preserved.update(i for i, f in enumerate(window, index - window_size + 1) if f)
            if window[0]:
                yeses -= 1
    fin.seek(0)
    for index, line in enumerate(fin):
        if index not in preserved:
            line = line.replace('yes', 'no')
        finalout.write(line)

Demo: https://repl.it/@blhsing/StripedCleanCopyrightinfringement

Answer 2

Here is a 5-liner solution based on building successive list comprehensions:

lines = [
'1,a1,b1,no,0.75',
'2,a2,b2,yes,0.45',
'3,a3,b3,yes,0.98',
'4,a4,b4,yes,0.22',
'5,a5,b5,no,0.46',
'6,a6,b6,no,0.98',
'7,a7,b7,yes,0.22',
'8,a8,b8,no,0.46',
'9,a9,b9,no,0.20']

n = len(lines)

# flag all lines containing 'yes' (add 2 empty lines at boundaries to avoid pbs)
flags = [line.count('yes') for line in ['', '']+lines+['', '']]
# count number of flags in sliding window [p-2,p+2]
counts = [sum(flags[p-2:p+3]) for p in range(2,n+2)]
# tag lines that need to be changed
tags = [flag > 0 and count < 3 for (flag,count) in zip(flags[2:],counts)]
# change tagged lines
for n in range(n):
  if tags[n]: lines[n] = lines[n].replace('yes','no')

print(lines)

Result:

['1,a1,b1,no,0.75',
 '2,a2,b2,yes,0.45',
 '3,a3,b3,yes,0.98',
 '4,a4,b4,yes,0.22',
 '5,a5,b5,no,0.46',
 '6,a6,b6,no,0.98',
 '7,a7,b7,no,0.22',
 '8,a8,b8,no,0.46',
 '9,a9,b9,no,0.20']

EDIT : As you read your data from a standard text file, all you have to do is:

with file(filename,'r') as f:
  lines = f.read().strip().split('\n')

(strip to remove potential blank lines at top or bottom on file, split(\n) to turn file content into a list of lines) then use the code above...