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Welcome to this tutorial on loading data from files. 

{{{ Screen shows welcome slide }}}

Until now, all the plots we have made use analytic functions. We have

been using analytic functions to generate a sequence of points and

plotting them, against another sequence of points. But, this is not

what we do most often. We often require to plot points obtained from

experimental observations.

#[punch: the initial part of the paragraph may be removed, to make

this a more generic LO?]

In this tutorial we shall learn to read data from files and save it

into sequences that can later be used to plot.

{{{ Show the outline for this tutorial }}} 

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

be looking at how to get multiple columns of data into multiple

sequences.

{{{ 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``.

#[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

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.

%%1%% Pause the video here, and use the ``cat`` command to view the

contents of this file and then resume the video.

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

::

  cat /home/fossee/pendulum.txt

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)

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

::

  print L

  print T

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.

{{{ show the slide with loadtxt --- other features }}}

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

%%2%% Pause the video here, and read the file

``pendulum_semicolon.txt`` which contains the same data as

``pendulum.txt``, but the columns are separated by semi-colons instead

of spaces. Use the IPython help to see how to do this. Once you have

finished, resume the video to look at the solution.

{{{ switch back to the terminal }}}

::

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

  print L

  print T

This brings us to the end of this tutorial. 

{{{ show the summary slide }}}

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.

Thank you!