Python-3.4.3/C2/Using-plot-command-interactively/English-timed

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Time Narration
00:01 Hello friends! Welcome to the tutorial on using plot command interactively in IPython.
00:08 At the end of this tutorial, you will be able to:

create simple plots of mathematical functions,

use the Plot window to study plots better.

00:20 To record this tutorial, I am using:

Ubuntu Linux 14.04 operating system,

Python 3.4.3 and

IPython 5.1.0

00:34 Pre-requisites to practise this tutorial are -

you should know how to run basic Python commands on the ipython console.

00:44 If not, for relevant Python tutorials, please visit this website.
00:50 Let us first open the Terminal by pressing Ctrl+Alt+T keys simultaneously.
00:58 Now, type ipython3 and press Enter.
01:05 Let us initialise the pylab package. Type: percentage pylab and press Enter.
01:16 Let's first understand what is Pylab.
01:20 Pylab is a convenient Python module

which provides- plotting functionality and has mathematical and scientific functions.

01:32 After running percentage pylab in the iPython console, you will see a message-

Using matplotlib backend: TkAgg ”.

01:41 This means matplotlib is running.
01:45 But sometimes you may get an error that says-

“ImportError: No module named matplotlib”.

01:55 In such cases, you have to install matplotlib and run this command again.
02:02 Let's come back to the ipython console. Type 'linspace' followed by a question mark in ipython console. Press Enter.
02:14 Please note that the command is linspace and not linespace.
02:20 The displayed information says that

linspace returns evenly spaced numbers which are calculated over the interval start and stop.

02:34 Press q to exit the documentation and return to the console.
02:41 Let's try to generate 100 points from 1 to 100. Type: linspace brackets 1 comma 100 comma 100
02:58 Here, 1 is the start , 100 is the stop and the next 100 is the number of points. Now, press Enter.
03:09 As you can see, a sequence of numbers from 1 to 100 is displayed.
03:15 Now let's try to generate 200 points between 0 and 1.
03:21 We do that by typing linspace brackets 0 comma 1 comma 200 and press Enter.
03:36 Here is the expected sequence of numbers.
03:40 In linspace, the start and stop points can be integers, decimals or constants.
03:48 Let us now learn about the len function.
03:52 First we will generate 100 points between minus pi and pi.
03:58 So, type: linspace brackets minus pi comma pi comma 100 and press Enter.
04:10 Here 'pi' is a constant defined by pylab.
04:15 Now, let us save this to a variable, say t. Press Enter.
04:22 If we now type: len bracket t and press Enter, we will get the number of points between minus pi and pi.
04:32 len function gives the no. of elements present in a given sequence.
04:37 Next, let's try and plot a cosine curve between minus pi and pi.
04:43 For this, we use the plot command. Type: plot brackets t comma cos(t) and press Enter.
04:59 As you can see from the cosine plot, cos(t) gets the cosine value at every point corresponding to point t.
05:09 We can also assign the value of cos(t) to a variable cosine by typing cosine equals to cos(t). Press Enter.
05:21 And then plot by typing plot(t comma cosine) and press Enter.
05:31 To clear the plot, we have to use the clf() function. This avoids overlapping of new plots over older plots.
05:42 In the console, type: clf() and press Enter. The previous plot is cleared and a blank plot window is displayed.
05:56 Now, let's try to plot a sine plot.
06:00 Type: plot brackets t comma sin(t) and press Enter. A sine plot is displayed.
06:14 To study the plot better on the plot window, we can use various options that are available on it.

Let us have a look at these options.

06:25 Moving the mouse pointer along the plot gives us the location of each point on the plot.
06:33 Notice here, at the bottom left of the window, there are a few buttons.
06:39 The right-most among them is for saving the file. Just click on it and type the file name.
06:47 Let us save the plot by the name sin underscore curve in pdf format.
06:54 Click on the dropdown here. As you can see, there are many formats in which to save the file.

Formats like png, eps, pdf, and ps are available.

07:08 We have to specify the format that we prefer at the time of saving.
07:14 To the left of the save button, is the slider button. Using this button, we can specify the margins of the plot window.
07:24 To the left of the slider button is the zoom button . It is used to zoom into the plot. Just specify the region to zoom into.
07:37 The button to the left of zoom, can be used to move the axes of the plot.
07:41 The next two buttons with left and right arrow icons, change the state of the plot.
07:48 It takes us to the previous or next state of the plot. It acts like the back and forward button in a browser.
07:57 The last one is 'home', referring to the initial plot.
08:03 Pause the video here, try out the following exercise and then resume the video.
08:09 Plot (sin(x) multiplied by sin(x)) divided by x.

1. Save the plot as sinsquarebyx dot pdf.

2. Zoom and find the maxima.

3. Bring it back to initial position.

08:26 This brings us to the end of this tutorial. In this tutorial, we have learnt to:

start IPython with pylab,

use the linspace function to create equally spaced points in a region,

08:42 find the length of sequences using len function,

plot mathematical functions using plot,

clear drawing area using clf.

08:53 usage of buttons in the UI of the plot window such as -
save, zoom, move axis, back and forward, Home.
09:04 Here are some self assessment questions for you to solve -

1. Create 100 equally spaced points between minus pi by 2 and pi by 2.

09:15 2. How can we find the length of a sequence ?
09:19 3. What will the command linspace(minus pi comma pi comma 100) do?
09:26 And the answers are-

We use the command linspace(minus pi by 2 comma pi by 2 comma 100) to create 100 equally spaced lines between the points minus pi by 2 and pi by 2.

09:43 len(sequence underscore name) is the function used to find out the length of a sequence.
09:50 The command linspace(minus pi comma pi comma 100) will return 100 evenly spaced samples from -pi to pi including both minus pi and pi.
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11:07 This is Usha from IIT Bombay, signing off. Thanks for watching.

Contributors and Content Editors

Pratik kamble, Sandhya.np14