Script to read file and print first two and last two lines of it-CodePudding

I'm writing a program where the user inputs a file name and then the file and prints only the first 2 and the last 2 lines of the file. I have worked out how to print the first two lines and I have tried to print the last two lines but have hit a hiccup. Is anyone able to explain what I did wrong?

f1 = open(input("Source file name: "))

line1= f1.readline()
line2= f1.readline()
line12= f1.readline()
line13= f1.readline()
print("Output:",line1,line2,line12[-1],line13[-2], sep= "")
f1.close()

The file is 13 lines long so the output should be as follows:

output:
line 1
line 2
line 12
line 13

CodePudding user response：

You're reading the first 4 lines. You need to read through all of them and keep only the last two.

This code reads through all of them, saving the last two lines read:

line1 = f1.readline()
line2 = f1.readline()

last1, last2 = f1.readline(), f1.readline()
while True:
    line = f1.readline()
    if not line:  # eof
        break
    last1, last2 = line, last1

print("Output:",line1,line2,last2,last1, sep= "")

For example, with a file test.txt:

Line1
line2
Line3
line4
Line5
line6
last line, line 7

You get:

Output:Line1
line2
line6
last line, line 7

CodePudding user response：

You could use next() on the file object iterator to get the first 2 lines, then use a deque() as a circular buffer to capture the last 2 lines. This will work with files that are less than 4 lines or no lines also!

from collections import deque


with open("test.txt") as f_in:

    print(next(f_in, ""), end="")
    print(next(f_in, ""), end="")

    buffer = deque(maxlen=2)
    for line in f_in:
        buffer.append(line)

    for line in buffer:
        print(line, end="")

text.txt

Output: