How to print specific lines of a file in Python

Question

I have a .txt file and I would like to print lines 3, 7, 11, 15,...

So, after printing the third line, I would like to print every 4th line afterward.

I began by looking at the modulus operator:

#Open the file
with open('file.txt') as file:

  #Iterate through lines
  for i, line in enumerate(file):

      #Choose every third line in a file
      if i % 3 == 0:
          print(line)

  #Close the file when you're done
  file.close()

but that approach prints every third line. If i % 3 == 1 that prints lines 1, 4, 7, 10, 13 etc.

Answer 1

Instead of using modulo, simply just use addition, start it with the first line you want to show, and then add 4 to it

next_line = 2  # Line 3 is index 2
for i, line in enumerate(file):

    if i == next_line:
        print(line)
        next_line = next_line + 4

Answer 2

Your code is almost fine, except for the modulo: you want the remainder of the division by 4 to be 3.

with open('file.txt') as file:
  for i, line in enumerate(file):
      if i % 4 == 3:
          print(line)

Note that you don't need to explicitely close your file at the end: that's what with is intended for, it makes sure that your file gets closed whatever happens.

Answer 3

So you want to something to happen every fourth time, that means modulo 4. Try changing your if to if i % 4 == N: with a good number for N .

By the way, when using the with statement you have don't have to call close() , it does so automatically.

Answer 4

How about:

# Fetch all lines from the file
lines = open('20 - Modular OS - lang_en_vs2.srt').readlines()

# Print the 3rd line
print(lines[2])

# throw away the first 3 lines, so the modulo (below) works ok
for i in range(3):
    del(lines[0])

# print every 4th line after that
for (i in range(len(lines)):
    if (i > 0 and i % 4 == 0):
        print(lines[i])

Read every line into an array. Output the 3rd line. We then need every fourth line, so by deleteing the first 3 elements, it's easy to simply test against modulo 4 (the "% 4") and output the line.

Answer 5

x = 0
with open('file.txt') as file:

  #Iterate through lines
  for i, line in enumerate(file):
      x += 1
      #Choose every third line in a file
      if x == 4:
          print(line)
          x = 0

  #Close the file when you're done
  file.close()

Result

>>> i = 0
>>> for x in range(0, 100):
...     i += 1
...     if i is 4:
...         print(x)
...         i = 0

3 7 11 15 19 23 27 31 35 39 43 47 51 55 59 63 67 71 75 79 83 87 91 95 99

Answer 6

file = open('file.txt')
print(file[2])
#Iterate through lines
for i in file:
  #Choose every third line in a file, beginning with 4
  if i % 4 == 0:
      print(i+3)
  elif i % 4 == 0:
      print(i)

This works, but isn't super elegant.