从具有 readline 偏移量的文件中读取行

Question

I want to read a file line by line, but I want to move the line pointer on every two reads. The file looks like

100
200
300
400

So, if I write

line_1 = f.readline()  # 100
line_2 = f.readline()  # 200

Then upon the third readline, I will get 300. I want to get 100 with a readline and the I want to get 200 with an incremental statement. Then I will put those into a loop and finally I want to get the lines in this manner:

iteration #1: 100 and 200
iteration #2: 200 and 300
iteration #3: 300 and 400

How can I do that?

Answer 1

You can create a generator (it removes the EOL character too, you can get rid of rstrip if you want something different):

def readpairsoflines(f):
    l1 = f.readline().rstrip('\n')
    for l2 in f:
        l2 = l2.rstrip('\n')
        yield l1, l2
        l1 = l2

And use it like this:

with open(filename) as f:
    for l1, l2 in readpairsoflines(f):
        # Do something with your pair of lines, for example print them
        print(f'{l1} and {l2}')

Result:

100 and 200
200 and 300
300 and 400

With this approach only two lines are read and kept in memory. Therefore, it works also with large files where memory is a possible concern.

Answer 2

I'm always a fan of simple and readable solutions (though sometimes less "pythonic" ).

with open("example.txt") as f:
    old = f.readline().rstrip()
    
    for line in f:
        line = line.rstrip()
        print("{} and  {}".format(old, line))
        old = line

• A first read is performed before looping through the remaining lines
• Then, the desired output is printed, and the old string is updated
• The rstrip() call is required ion order to remove the undesired trailing '\\n'
• I assumed that nothing had to be printed in case of files with less than two lines; the code can be easily modified to manage any need in that special case

The output:

100 and  200
200 and  300
300 and  400

Answer 3

now I would suggest splitting the document in newlines like this

with open('params.txt') as file:
    data = file.read()
data = data.split('\n')
for index, item in enumerate(data):
    try:
        print(str(item) + ' ' + str(data[index + 1]))
    except IndexError:
        print(str(item))

and using some list logic print what you need so what this code does is creates a list of needed values(not efficient for verrry large files) and gets their index so when it prints the item it also prints the next item in the list and it does it for every item index error is because last item wont have next item but you can also work around it by using if else statements