Question

This has to be a common question that all programmers have from time to time.
How do I read a line from a text file? Then the next question is always how do i write it back.

Of course most of you use a high level framework in day to day programming (which are fine to use in answers) but sometimes it’s nice to know how to do it at a low level too.

I myself know how to do it in C, C++ and Objective-C, but it sure would be handy to see how it’s done in all of the popular languages, if only to help us make a better decision about what language to do our file io in. In particular I think it would be interesting to see how its done in the string manipulation languages, like: python, ruby and of course perl.

So I figure here we can create a community resource that we can all star to our profiles and refer to when we need to do file I/O in some new language. Not to mention the exposure we will all get to languages that we don’t deal with on a day to day basis.

This is how you need to answer:

1. Create a new text file called “fileio.txt“
2. Write the first line “hello” to the text file.
3. Append the second line “world” to the text file.
4. Read the second line “world” into an input string.
5. Print the input string to the console.

Clarification:

• You should show how to do this in one programming language per answer only.
• Assume that the text file doesn’t exist beforehand
• You don’t need to reopen the text file after writing the first line

No particular limit on the language.
C, C++, C#, Java, Objective-C are all great.

If you know how to do it in Prolog, Haskell, Fortran, Lisp, or Basic then please go right ahead.

Answer 1

Python 3

with open('fileio.txt', 'w') as f:
   f.write('hello')
with open('fileio.txt', 'a') as f:
   f.write('\nworld')
with open('fileio.txt') as f:
   s = f.readlines()[1]
print(s)

Clarifications

• readlines() returns a list of all the lines in the file.
  Therefore, the invokation of readlines() results in reading each and every line of the file.
  In that particular case it’s fine to use readlines() because we have to read the entire file anyway (we want its last line).
  But if our file contains many lines and we just want to print its nth line, it’s unnecessary to read the entire file.
  Here are some better ways to get the nth line of a file in Python: What substitutes xreadlines() in Python 3?.

• What is this with statement?
  The with statement starts a code block where you can use the variable f as a stream object returned from the call to open().
  When the with block ends, python calls f.close() automatically.
  This guarantees the file will be closed when you exit the with block no matter how or when you exit the block
  (even if you exit it via an unhandled exception). You could call f.close() explicitly, but what if your code raises an exception and you don’t get to the f.close() call? That’s why the with statement is useful.

• You don’t need to reopen the file before each operation. You can write the whole code inside one with block.

  with open('fileio.txt', 'w+') as f:
      f.write('hello')
      f.write('\nworld')
      s = f.readlines()[1]
  print(s)
  

  I used three with blocks to emphsize the difference between the three operations:
  write (mode ‘w’), append (mode ‘a’), read (mode ‘r’, the default).