====== Computer Lab 07, Files ======
  * Q/A
  * Practical work

===== Practical work =====

==== Count characters, words, and lines in a file ====
In module ''texttools.py'', create a function ''count()'' that will return a triple consisting of the number of characters (including white space, but excluding new-line characters), words, and lines in a text file, given to the function as argument.

<code python>
def count(filepath):
    """Return number of characters, words, and lines in the given file
    
    :param filepath: string, path to a text file
    :return: 3-tuple with number of chars, words, and lines in the file
    """
</code>

If we had a file named ''example.txt'' in the current directory with the following contents

<code>
One two three,
dee dah dee.
Four, five, six,
what a mix!
</code>

we could execute the code and get results like this:

<code python>
>>> print(count('example.txt'))
(53, 12, 4)
</code>

==== Inserting line numbers ====
In module ''texttools.py'', create function ''insert_line_numbers()'' which shall read the input text file line by line, aend write them into an output text file such that each line will start with an ''n''-digit long line number and a space, starting from 1. The default value for ''n'' is 3.

<code python>
def insert_line_numbers(in_filepath, out_filepath, num_len=3):
    """Create a copy of the input file with numbered lines
    
    :param in_filepath: string, path to the input text file
    :param out_filepath: string, path to the output text file
    :param num_len: int, the length of the string containg the line number
    :return: None
    """
</code>

Running the function on the text file from the previous exercise like this:

<code python>
>>> insert_line_numbers('example.txt', 'numbered_example.txt', num_len=2)
</code>

will create file ''numbered_example.txt'' in the current directory with the following contents:

<code>
 1 One two three,
 2 dee dah dee.
 3 Four, five, six,
 4 what a mix!
</code>

Try to apply this function to some of your ''.py'' files!

==== Removing the line beginnings ====
In module ''texttools.py'', create function ''snip_line_beginnings()'' that can be used to undo the line numbering (but can be used in more general cases).

<code python>
def snip_line_beginnings(in_filepath, out_filepath, n=4):
    """Create a copy of the input file with the first n chars of each line snipped off.
    
    :param in_filepath: string, path to the input text file
    :param out_filepath: string, path to the output text file
    :param n: int, the number of chars to snip from the beginning of each line
    :return: None
    """
</code>

==== Chemical elements database ====
Let's have a file with a simple database of chemical elements, named e.g. ''elements.txt'', with the following contents:

<code>
berryllium 4 9.012
magnesium 12 24.305
calcium 20 20.078
strontium 38 87.62
barium 56 137.327
radium 88 226
</code>

For each stored chemical element, its respective line in a file contains the element name, atomic number and atomic weight.

Write function ''read_elements()'' that will accept a path to the file with element database, will read the information therein, and will produce a list of lists, where each inner list will contain data for individual element, i.e. its name (string), its atomic number (int), and atomic weight (float).

<code python>
def read_elements(filepath):
    """Return a list of lists with info about chem. elements read from the given file
    
    :param filepath: string, path to a file with the elements database
    :return: list of lists of , each inner list containing name (string), number (int),
             and weight (float) for each element.
</code>

When the function is used to read in the above file, it should produce the following:

<code python>
>>> read_elements('elements.txt')
[['berryllium', 4, 9.012], ['magnesium', 12, 24.305], ['calcium', 20, 20.078], ...]
</code>



===== Homework =====

  * Finish all the above tasks.
  * Read [[http://openbookproject.net/thinkcs/python/english3e/files.html|chapter 13 - Files]] of {[a4b99rph:Wenthworth]}.