.. 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 .. ------------- .. none .. Author : Nishanth Amuluru Internal Reviewer : Puneeth External Reviewer : Language Reviewer : Bhanukiran Checklist OK? : [2010-10-05] Script ------ .. L1 {{{ Show the first slide containing title, name of the production team along with the logo of MHRD }}} .. R1 Hello friends and welcome to this tutorial on 'Input/Output'. .. L2 {{{ Show the slide containing the objectives }}} .. R2 At the end of this tutorial,you will be able to, 1. Print some value. #. Print using modifiers. #. Take input from user. #. Display a prompt to the user before taking the input. .. R3 Let us first start ipython on our teminal .. L3 :: ipython .. R4 Let us start this tutorial by typing a string .. L4 :: a = "This is a string" a print a .. R5 ``print a``, obviously, prints 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. Typing just ``a`` displays the content of ``a`` whereas the statement ``print a`` prints the string itself.This difference becomes more evident when we use strings with newlines in them. .. L5 :: b = "A line \n New line" b print b .. R6 As you can see, just typing ``b`` shows that b contains a newline character but While typing ``print b``,it 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. We shall look at different ways of outputting the data. print statement in python supports string formatting. Various arguments can be passed to print using modifiers. type .. L6 :: x = 1.5 y = 2 z = "red" print "x is %2.1f, y is %d, z is %s"%(x,y,z) .. R7 As you can see, the values of x, y and z are substituted in place of the modifiers ``%2.1f``, ``%d`` and ``%s`` respectively. Pause the video here, try out the following exercise and resume the video. .. L7 .. L8 {{{ Show slide with exercise 1 }}} .. R8 What happens when you do ``print "x is %d, y is %f" %(x, y)`` .. R9 Switch to the terminal for solution. .. L9 {{{continue from paused state}}} {{{ Switch to the terminal }}} :: print "x is %d, y is %f" %(x, y) .. R10 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 have seen that ``print`` statement prints a new line character everytime it is called. This can be suppressed by using a "," at the end of the ``print`` statement. Let us see this by typing out following code on an editor as print_example.py .. L10 {{{ open an editor }}} :: print "Hello" print "World" print "Hello", print "World" .. R11 Save the script as 'print_example.py' and run it 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 .. L11 :: ip = raw_input() .. R12 The cursor is blinking indicating that it is waiting for input type something and hit enter. .. L12 :: an input .. R13 Now let us see what is the value of ip by typing it. .. L13 :: ip .. R14 We can see that it contains the string "an input" Pause the video here, try out the following exercise and resume the video. .. L14 .. L15 {{{ Show slide with exercise 2 }}} .. R15 Enter the number 5.6 as input and store it in a variable called c. .. R16 Switch to the terminal for solution. .. L16 {{{continue from paused state}}} {{{ Switch to the terminal }}} .. R17 We have to use the raw_input command with variable c. type .. L17 :: c = raw_input() 5.6 c .. R18 Now let us see the type of c. .. L18 :: type(c) .. R19 We see that c is a string. This implies that anything you enter as input, it will be taken as a string no matter what you enter. Pause the video here, try out the following exercise and resume the video. .. L19 .. L20 {{{ Show slide with exercise 3 }}} .. R20 What happens when you do not enter anything and hit enter. .. R21 Switch to the terminal for solution. .. L21 {{{continue from paused state}}} {{{ Switch to the terminal }}} :: d = raw_input() d .. R22 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. .. L22 :: name = raw_input("Please enter your name: ") .. R23 It prints the string given as argument and then waits for the user input. Let us do one more exercise. Pause the video here, try out the following exercise and resume the video. .. L23 .. L24 {{{ Show slide with exercise 3 }}} .. R24 How do you display a prompt and let the user enter input in next line. .. R25 Switch to the terminal for solution. The trick is to include a newline character at the end of the prompt string. .. L25 {{{continue from paused state}}} {{{ Switch to the terminal }}} :: ip = raw_input("Please enter a number in the next line\n> ") .. R26 It prints the newline character and hence the user enters input in the next line .. L26 .. L27 {{{ Show summary slide }}} .. R27 This brings us to the end of the tutorial. In this totorial, we have learnt to, 1. Use the print statement. #. Use the modifiers %d, %f, %s in the print statement. #. Take input from user by using ``raw_input()``. #. Display a prompt to the user before taking the input by passing a string as an argument to ``raw_input``. .. L28 {{{Show self assessment questions slide}}} .. R28 Here are some self assessment questions for you to solve 1. ``a = raw_input()`` and user enters ``2.5``. What is the type of a? - str - int - float - char 2. ``a = 2`` and ``b = 4.5``. What does ``print "a is %d and b is %2.1f" %(b, a)`` print? - a is 2 and b is 4.5 - a is 4 and b is 2 - a is 4 and b is 2.0 - a is 4.5 and b is 2 .. L29 {{{solution of self assessment questions on slide}}} .. R29 And the answers, 1. No matter what you enter, it will be taken as a string.Hence 2.5 is a string. 2. Since 'b' is called first, It will display integer value of 'a' because the modifier used is %d. Similarly, 'b' will get the float value of 'a' due to it's modifier %2.1f. Hence 'a' will be 4 and 'b' 2.0 . .. L30 {{{ Show the Thankyou slide }}} .. R30 Hope you have enjoyed this tutorial and found it useful. Thank You!