{{{ Screen shows welcome slide }}}

We often require to plot points obtained from experimental

observations. In this tutorial we shall learn to read data from files

and save it into sequences that can later be used to plot.

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We shall use the ``loadtxt`` command to load data from files. We will

be looking at how to read a file with multiple columns of data and

load each column of data into a sequence. 

{{{ switch back to the terminal }}}

As usual, let us start IPython, using 

::

  ipython -pylab 

Now, Let us begin with reading the file primes.txt, which contains

just a list of primes listed in a column, using the loadtxt command.

The file, in our case, is present in ``/home/fossee/primes.txt``. 

{{{ Navigate to the path in the OS, open the file and show it }}}

.. #[punch: do we need a slide for showing the path?]

.. We use the ``cat`` command to see the contents of this file. 

.. #[punch: should we show the cat command here? seems like a good place

   to do it] ::

     cat /home/fossee/primes.txt

.. #[Nishanth]: A problem for windows users.

                Should we simply open the file and show them the data

                so that we can be fine with GNU/Linux ;) and windows?

Now let us read this list into the variable ``primes``.

::

  primes = loadtxt('/home/fossee/primes.txt')

``primes`` is now a sequence of primes, that was listed in the file,

``primes.txt``.

We now type, ``print primes`` to see the sequence printed.

We observe that all of the numbers end with a period. This is so,

because these numbers are actually read as ``floats``. We shall learn

about them, later.

Now, let us use the ``loadtxt`` command to read a file that contains

two columns of data, ``pendulum.txt``. This file contains the length

of the pendulum in the first column and the corresponding time period

in the second.

This is how we look at the contents of the file, ``pendulum.txt``

::

  cat /home/fossee/pendulum.txt

.. #[Nishanth]: The first column is L values and second is T values

                from a simle pelculum experiment.

                Since you are using the variable names later in the

                script.

                Not necessary but can be included also.

Let us, now, read the data into the variable ``pend``. Again, it is

assumed that the file is in ``/home/fossee/``

::

  pend = loadtxt('/home/fossee/pendulum.txt')

Let us now print the variable ``pend`` and see what's in it. 

::

  print pend

Notice that ``pend`` is not a simple sequence like ``primes``. It has

two sequences, containing both the columns of the data file. Let us

use an additional argument of the ``loadtxt`` command, to read it into

two separate, simple sequences.

::

  L, T = loadtxt('/home/fossee/pendulum.txt', unpack=True)

.. #[Nishanth]: It has a sequence of items in which each item contains

                two values. first is l and second is t

Let us now, print the variables L and T, to see what they contain.

::

  print L

  print T

.. #[Nishanth]: Stress on ``unpack=True`` ??

Notice, that L and T now contain the first and second columns of data

from the data file, ``pendulum.txt``, and they are both simple

sequences. ``unpack=True`` has given us the two columns in to two

separate sequences instead of one complex sequence. 

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In this tutorial, we have learnt the basic use of the ``loadtxt``

command, which is capable of doing a lot more than we have used it for

until now, for example

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::

  L, T = loadtxt('/home/fossee/pendulum_semicolon.txt', unpack=True, delimiter=';')

  print L

  print T

This brings us to the end of this tutorial. 

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You should now be able to do the following, comfortably. 

  + Read data from files, containing a single column of data using the

    ``loadtxt`` command.

  + Read multiple columns of data, separated by spaces or other

    delimiters.