title: Writing python code to convert linux read, write and execute octal format to the string
body: <p>The permissions of a file in a Linux system are split into three sets of three permissions: read, write, and execute for the owner, group, and others. Each of the three values can be expressed as an octal number summing each permission, with 4 corresponding to read, 2 to write, and 1 to execute. Or it can be written with a string using the letters r, w, and x or - when the permission is not granted. For example: 640 is read/write for the owner, read for the group, and no permissions for the others; converted to a string, it would be: "rw-r-----" 755 is read/write/execute for the owner, and read/execute for group and others; converted to a string, it would be: "rwxr-xr-x" Fill in the blanks to make the code convert a permission in octal format into a string format.</p>
  
  <pre><code>def octal_to_string(octal):
    result = ""
    value_letters = [(4,"r"),(2,"w"),(1,"x")]
    # Iterate over each of the digits in octal
    for ___ in [int(n) for n in str(octal)]:
        # Check for each of the permissions values
        for value, letter in value_letters:
            if ___ &gt;= value:
                result += ___
                ___ -= value
            else:
                ___
    return result
  
  print(octal_to_string(755)) # Should be rwxr-xr-x
  print(octal_to_string(644)) # Should be rw-r--r--
  print(octal_to_string(750)) # Should be rwxr-x---
  print(octal_to_string(600)) # Should be rw-------
  </code></pre>
  
  <p>Please help me to fix this problem</p>