Chandrika: Created page with '{| border=1 !Visual Cue !Narration |- | Show Slide 1 Containing title, name of the production team along with the logo of MHRD | Hello Friends and Welcome to this tutorial on …'

2012-11-29T05:57:19Z

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{| border=1
!Visual Cue
!Narration
|-
| Show Slide 1

Containing title, name of the production team along with the logo of MHRD
| Hello Friends and Welcome to this tutorial on "loading data from files".

|-
| Show Slide 2

Learning objectives
| At the end of this tutorial, you will be able to,

# Read data from files, containing a single column of data
# Read multiple columns of data, separated by spaces or other delimiters.

|-
| Switch to the terminal

ipython -pylab
| Let us switch to the terminal and start IPython, using ipython -pylab

|-
| Navigate to the path in the OS, open the file and show it
| Now, Let us begin with reading the file primes.txt, which contains a list of prime numbers listed in a column, using the loadtxt command. Please make sure that you provide the correct path of the file, 'primes.txt'. The file, in our case, is present in <tt>/home/fossee/primes.txt</tt>.

|-
| cat /home/fossee/primes.txt
| Otherwise we can use the <tt>cat</tt> command to locate the file and read the contents of it.

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| primes = loadtxt('/home/fossee/primes.txt')
| Now let us read this list into the variable <tt>primes</tt>.

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| print primes
| <tt>primes</tt> is now a sequence of prime numbers, that was listed in the file,``primes.txt``.

We now type, <tt>print primes</tt> to see the sequence printed.

|-
| Highlight the output on the terminal
| We observe that all the numbers end with a period. This is so, because these numbers are actually read as <tt>floats</tt>.

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| cat /home/fossee/pendulum.txt
| Now, let us use the <tt>loadtxt</tt> command to read a file <tt>pendulum.txt</tt> that contains two columns of data. This file contains the length of the pendulum in the first column and the corresponding time period in the second. Note that here <tt>loadtxt</tt> needs both the columns to have equal number of rows.

We use the <tt>cat</tt> command to view the contents of this file.

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| pend = loadtxt('/home/fossee/pendulum.txt')
| Let us, now, read the data into the variable <tt>pend</tt>. Again, it is assumed that the file is in <tt>/home/fossee/</tt>

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| print pend
| Let us now print the variable <tt>pend</tt> and see what it contains.

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| L, T = loadtxt('/home/fossee/pendulum.txt', unpack=True)
| Notice that <tt>pend</tt> is not a simple sequence like <tt>primes</tt>. It has two sequences, containing both the columns of the data file. Let us use an additional argument of the <tt>loadtxt</tt> command, to read it into two separate, simple sequences.

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

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|
| Notice, that L and T now contain the first and second columns of data from the data file, <tt>pendulum.txt</tt>, and they are both simple sequences. <tt>unpack=True</tt> has given us the two columns into two separate sequences instead of one complex sequence.

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| Show Slide 3

Assignment 1
| Till now, we have learnt the basic use of the <tt>loadtxt</tt> command. Let us try an example.

Pause the video here, try out the following exercise and resume the video.

Read the file <tt>pendulum_semicolon.txt</tt> which contains the same data as <tt>pendulum.txt</tt>, but the columns are separated by semi-colons instead of spaces. Use the IPython help to see how to do this.

|-
| Switch back to the terminal

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

print L

print T
|

|-
| Show Slide 4

Summary slide
| This brings us to the end of this tutorial. In this tutorial, we have learnt to,

# To Read data from files, containing a single column of data using the <tt>loadtxt</tt> command.
# To Read multiple columns of data, separated by spaces or other delimiters.

|-
| Show Slide 5

Self assessment questions slide
| Here are some self assessment questions for you to solve

# <tt>loadtxt</tt> can read data from a file with one column only. True or False?
# Given a file <tt>data.txt</tt> with three columns of data separated by spaces, read it into 3 separate simple sequences.
# Given a file <tt>data.txt</tt> with three columns of data separated by ":", read it into 3 separate simple sequences.

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| Show Slide 6

Solution of self assessment questions on slide
| And the answers,

# False. <tt>loadtxt</tt> command can read data from files having both single columns as well as multiple columns.
# A file with three columns of data separated by spaces to be read into 3 separate sequences, we use the loadtxt command as,

Enumerated list ends without a blank line; unexpected unindent.

x = loadtxt("data.txt", unpack=True)

# If a file with three columns of data separated by delimiters,we read it into three separate sequences by using an additional argument of delimiter in the loadtxt command

Enumerated list ends without a blank line; unexpected unindented.

x = loadtxt("data.txt", unpack=True, delimiter=":")

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| Show Slide 7

Acknowledgment slide
| Hope you have enjoyed this tutorial and found it useful. Thank you!

Python/C2/loading-data-from-files/English - Revision history

Chandrika: Created page with '{| border=1 !Visual Cue !Narration |- | Show Slide 1 Containing title, name of the production team along with the logo of MHRD | Hello Friends and Welcome to this tutorial on …'