.. Objectives

.. ----------

.. #. How to print some value

.. #. How to print using modifiers

.. #. How to take input from user

.. #. How to display a prompt to the user before taking the input

.. Prerequisites

.. -------------

..   1. Loops

.. Author              : Nishanth Amuluru

   Internal Reviewer   : Puneeth 

   External Reviewer   :

   Language Reviewer   : Bhanukiran

Script

------

Hello friends and welcome to this tutorial on Input/Output

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Input and Output are used in almost every program we use.

In this tutorial, we shall learn how to 

 * Output data

 * Take input from the user

type

::

    a = "This is a string"

    a

    print a

``print a``, obviously, is printing the value of ``a``.

As you can see, even when you type just a, the value of a is shown.

But there is a difference.

.. #[Amit: The next sentence does seem to be clear enough]

Typing a shows the value of a while print a prints the string. This difference

becomes more evident when we use strings with newlines in them.

type

::

    b = "A line \n New line"

    b

    print b

As you can see, just typing b shows that b contains a newline character.

While typing print b prints the string and hence the newline.

Moreover when we type just a, the value a is shown only in interactive mode and

does not have any effect on the program while running it as a script.

.. #[punch: I think we could show that?]

We shall look at different ways of outputting the data.

.. #[Amit: C's printf syntax ?? i think its better to elaborate the

   idea]

print statement  in python supports string formatting.

Various arguments can be passed to print using modifiers.

type

::

    x = 1.5

    y = 2

    z = "zed"

    print "x is %2.1f y is %d z is %s"%(x,y)

As you can see, the values of x and y are substituted in place of

``%2.1f`` and ``%d`` 

{{{ Pause here and try out the following exercises }}}

%% 1 %% What happens when you do ``print "x is %d y is %f" %(x, y)``

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We see that the ``int`` value of x and ``float`` value of y are

printed corresponding to the modifiers used in the print statement.

We can also see that ``print`` statement prints a new line character

at the end of the line, everytime it is called. This can be suppressed

by using a "," at the end ``print`` statement.

Let us see this by typing out following code on an editor as print_example.py

{{{ open an editor }}}

type

::

    print "Hello"

    print "World"

    print "Hello",

    print "World"

Now we run the script using %run /home/fossee/print_example.py

As we can see, the print statement when used with comma in the end, prints a

space instead of a new line.

Now we shall look at taking input from the user.

We will use the ~~raw_input~~ for this.

type

::

    ip = raw_input()

The cursor is blinking indicating that it is waiting for input    

type

::

    an input

and hit enter.

Now let us see what is the value of ip by typing.

::

    ip

We can see that it contains the string "an input"

{{{ Pause here and try out the following exercises }}}

%% 2 %% enter the number 5.6 as input and store it in a variable called c.

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We have to use the raw_input command with variable c.

type

::

    c = raw_input()

    5.6

    c

Now let us see the type of c.

::

    type(c)

We see that c is a string. This implies that anything you enter as input, will

be taken as a string no matter what you enter.

{{{ Pause here and try out the following exercises }}}

%% 3 %% What happens when you do not enter anything and hit enter

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::

    d = raw_input()

    <RET>

    d

We see that when nothing is entered, an empty string is considered as input.

raw_input also can display a prompt to assist the user.

::

    name = raw_input("Please enter your name: ")

prints the string given as argument and then waits for the user input.

{{{ Pause here and try out the following exercises }}}

%% 4 %% How do you display a prompt and let the user enter input in next line

{{{ continue from paused state }}}

.. #[Puneeth: We didn't talk of new-line character till now, did we?]

.. #[Puneeth: non-programmers might not know?]

.. #[Amit: Well there is a discussion earlier about new lines, I think its good

.. as a slight trick question. But may be next line is a more easier lexicon]

The trick is to include a newline character at the end of the prompt string.

::

    ip = raw_input("Please enter a number in the next line\n> ")

prints the newline character and hence the user enters input in the next line

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This brings us to the end of the tutorial.

In this totorial we have learnt

 * How to print some value

 * How to print using modifiers

 * How to take input from user

 * How to display a prompt to the user before taking the input

{{{ Show the "sponsored by FOSSEE" slide }}}

This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India

Hope you have enjoyed and found it useful.

Thank You.