How to create this kind of loop?

Question

I am having a bit of trouble figuring the following out:

I have a file with 100 lines for example, let's call it file A

I also have another file with 100 lines for example, let's call it file B

Now I need the first loop to read 10 lines from file A and do it's thing and then go to the other loop that reads 10 lines from file B, does it thing and then goes back to the first loop to do 11-20 lines from file A and then back to second loop that does 11-20 lines from file B.

I need both loops to remember from which line to read.

How should I approach this?

Thanks!

EDIT:

Could something like this work?

a=0
b=10
x=0
y=10

    for 1000 times:
        read a-b rows:
            do its thing
        a += 10
        b += 10
            
        read x-y rows:
             do its thing
        x += 10
        y += 10

Answer 1

You can iterate over 10 lines at a time using this approach.

class File:
    def __init__(self, filename):
        self.f = open(filename, 'r')

    def line(self):
        yield self.f.readline()

    def next(self, limit):
        for each in range(limit):
            yield self.f.readline()

    def lines(self, limit=10):
        return [x for x in self.next(limit=limit)]


file1 = File('C:\\Temp\\test.csv')
file2 = File('C:\\Temp\\test2.csv')
print(file1.lines(10)
print(file2.lines(10)
print(file1.lines(10)
print(file2.lines(10)

Now you can jump back and forth between files iterating over the next 10 lines.

Answer 2

Here is another solution using a generator and a context manager:

class SwitchFileReader():
    
    def __init__(self, file_paths, lines = 10):
        self.file_paths = file_paths
        self.file_objects = []
        self.lines = 1 if lines < 1 else lines

    def __enter__(self):
        for file in self.file_paths:
            file_object = open(file, "r")
            self.file_objects.append(file_object)
        return self
    
    def __exit__(self, type, value, traceback):
        for file in self.file_objects:
            file.close()
    
    def __iter__(self):

        while True:

            next_lines = [
                [file.readline() for _ in range(self.lines)] 
                for file in self.file_objects
            ]
            
            if any(not all(lines) for lines in next_lines):
                break

            for lines in next_lines:
                yield lines

file_a = r"D:\projects\playground\python\stackgis\data\TestA.txt"
file_b = r"D:\projects\playground\python\stackgis\data\TestB.txt"

with SwitchFileReader([file_a, file_b], 10) as file_changer:
    for next_lines in file_changer:
        print(next_lines , end="")  # do your thing

The iteration will stop as soon as there are less remaining lines in any of the files.

Assuming file_a has 12 lines and file_b has 13 lines. Line 11 and 12 from file_a and line 11 to 13 from file_b would be ignored.

Answer 3

For simplicity I'm going to work with list. You can read the file into a list.

Let's split the problem. We need

1. group each list by any number. In your case 10
2. Loop in each 10 bunches for both arrays.

Grouping

Here an answer: https://stackoverflow.com/a/4998460/2681662

def group_by_each(lst, N):
    return [lst[n:n+N] for n in range(0, len(lst), N)]

Loop in two list at the same time:

You can use zip for this.

lst1 = list(range(100)) # <- Your data
lst2 = list(range(100, 200)) # <-- Your second data

def group_by_each(lst, N):
    return [lst[n:n+N] for n in range(0, len(lst), N)]

for ten1, ten2 in zip(group_by_each(lst1, 10), group_by_each(lst2, 10)):
    print(ten1)
    print(ten2)

Answer 4

When you iterate over a file object, it yields lines in the associated file. You just need a single loop that grabs the next ten lines from both files each iteration. In this example, the loop will end as soon as either file is exhausted:

from itertools import islice

lines_per_iter = 10

file_a = open("file_a.txt", "r")
file_b = open("file_b.txt", "r")

while (a := list(islice(file_a, lines_per_iter))) and (b := list(islice(file_b, lines_per_iter))):
    print(f"Next {lines_per_iter} lines from A: {a}")
    print(f"Next {lines_per_iter} lines from B: {b}")

file_a.close()
file_b.close()

Answer 5

Ok, thank you for all the answers, I found a working solution to my project like this:

a=0
b=10
x=0
y=10

while True:

    for list1 in range(a, b):
        #read the lines from file A
    a += 10
    b += 10

    for list2 in range(x, y):
        #read the lines from file B
    if y == 100:
        break
    x += 10
    y += 10