1 .. Objectives

     2 .. ----------

     3 

     4 .. 8.1 LO: getting started with functions (3)

     5 

     6 .. At the end of this tutorial, you will be able to 

     7 

     8 .. 1. define function

     9 .. #. define functions with arguments

    10 .. #. learn about docstrings

    11 .. #. learn about return values

    12 ..     can return multiple values

    13 .. #. read code

    14 

    15 

    16 .. Prerequisites

    17 .. -------------

    18 

    19 ..   1. should have ``ipython`` installed. 

    20 ..   #. getting started with ``ipython``.

    21 

    22      

    23 .. Author              : Anoop Jacob Thomas <anoop@fossee.in>

    24    Internal Reviewer   : 

    25    External Reviewer   :

    26    Checklist OK?       : <put date stamp here, if OK> [2010-10-05]

    27 

    28 

    29 ==============================

    30 Getting started with functions

    31 ==============================

    32 

    33 {{{ show welcome slide }}}

    34 

    35 Hello and welcome to the tutorial getting started with functions.

    36 

    37 {{{ switch to next slide, outline slide }}}

    38 

    39 In this tutorial we will learn about functions in python, how to

    40 define functions, arguments to functions, docstrings, and function

    41 return value.

    42 

    43 {{{ switch to next slide, Function }}}

    44 

    45 While writing code, we always want to reduce the number of lines of

    46 code and functions is a way of reusing the code. Thus the same lines

    47 of code can be used again and again. A function is a portion of code

    48 within a larger program that performs a specific task and is

    49 relatively independent of the remaining code. Now let us get more

    50 familiar with functions,

    51 

    52 {{{ switch to next slide, f(x) a mathematical function }}}

    53 

    54 Consider a mathematical function f(x) = x square. Here x is a variable

    55 and with different values of x the value of function will change. When

    56 x is one f(1) will return the value 1 and f(2) will return us the

    57 value 4. Let us now see how to define the function f(x) in python.

    58 

    59 {{{ switch to next slide, define f(x) in Python }}}

    60 

    61 In your Ipython interpreter type the following,

    62 ::

    63 

    64     def f(x):

    65     	return x*x

    66 

    67 Well that defined the function, so before learning what we did let us

    68 see if it returns the expected values, try,

    69 ::

    70 

    71     f(1)

    72     f(2)

    73 

    74 Yes, it returned 1 and 2 respectively. And now let us see what we did,

    75 we wrote two lines. The first line ``def f(x)`` is used to define the

    76 name and the parameters to the function. ``def`` is a keyword and

    77 ``f`` is the name of the function and ``x`` the parameter of the

    78 function.

    79 

    80 {{{ switch to next slide, problem statement 1 }}}

    81 

    82 %% 1 %% Write a python function named cube which computes the cube of

    83    a given number n.

    84 

    85 Pause here and try to solve the problem yourself.

    86 

    87 {{{ switch to next slide, solution }}}

    88 

    89 The problem can be solved as,

    90 ::

    91 

    92     def cube(n):

    93     	return n**3

    94 

    95 And now let us see how to write functions without arguments.

    96 

    97 {{{ switch to next slide, greet function }}}

    98 

    99 let us define a new function called ``greet`` which will print ``Hello

   100 World``.

   101 ::

   102 

   103     def greet():

   104     	print "Hello World!"

   105 

   106 now try calling the function,

   107 ::

   108 

   109     greet()

   110 

   111 Well that is a function which takes no arguments. Also note that it is

   112 not mandatory for a function to return values. The function ``greet``

   113 neither takes any argument nor returns any value.

   114 

   115 Now let us see how to write functions with more than one argument.

   116 

   117 {{{ switch to next slide, exercise 2 }}}

   118 

   119 %% 2 %% Write a python function named ``avg`` which computes the

   120    average of ``a`` and ``b``.

   121 

   122 Pause here and try to solve the problem yourself.

   123 

   124 {{{ switch to next slide, solution 2 }}}

   125 

   126 The problem can be solved as,

   127 ::

   128 

   129     def avg(a,b):

   130     	return (a + b)/2

   131 

   132 Thus if we want a function to accept more arguments, we just list them

   133 separated with a comma between the parenthesis after the function name

   134 in the ``def`` line.

   135 

   136 {{{ switch to next slide, docstring }}}

   137 

   138 It is always a good practice to document the code that we write, and

   139 for a function we define we should write an abstract of what the

   140 function does, and that is called a docstring. Let us modify the

   141 function ``avg`` and add docstring to it. Do the following,

   142 ::

   143 

   144     def avg(a,b):

   145         """ avg takes two numbers as input (a & b), and

   146 	returns the average of a and b"""

   147 	return (a+b)/2

   148 

   149 Note that docstrings are entered in the immediate line after the

   150 function definition and put as a triple quoted string. And here as far

   151 as the code functionality is concerned, we didn't do anything. We just

   152 added an abstract of what the function does.

   153 

   154 Now try this in the ipython interpreter.

   155 ::

   156 

   157     avg?

   158 

   159 It displays the docstring as we gave it. Thus docstring is a good way

   160 of documenting the function we write.

   161 

   162 Try to do this,

   163 ::

   164 

   165     f?

   166 

   167 It doesn't have a docstring associated with it. Also we cannot infer

   168 anything from the function name, and thus we are forced to read the

   169 code to understand anything about the function.

   170 

   171 {{{ switch to next slide, exercise 3 }}}

   172 

   173 %% 3 %% Add docstring to the function f.

   174 

   175 Pause here and try to do it yourself.

   176 

   177 {{{ switch to next slide, solution }}}

   178 

   179 We need to define the function again to add docstring to the function

   180 ``f`` and we do it as,

   181 ::

   182 

   183     def f(x):

   184     	"""Accepts a number x as argument and,

   185 	returns the square of the number x."""

   186 	return x*x

   187 

   188 {{{ switch to next slide, exercise 4 }}}

   189 

   190 %% 4 %% Write a python function named ``circle`` which returns the

   191    area and perimeter of a circle given radius ``r``.

   192 

   193 Pause here and try to solve the problem yourself.

   194 

   195 {{{ switch to next slide, solution 4 }}}

   196 

   197 The problem requires us to return two values instead of one which we

   198 were doing till now. We can solve the problem as,

   199 ::

   200 

   201     def circle(r):

   202     	"""returns area and perimeter of a circle given radius r"""

   203 	pi = 3.14

   204 	area = pi * r * r

   205 	perimeter = 2 * pi * r

   206 	return area, perimeter

   207 

   208 A python function can return any number of values. There is no

   209 restriction for it.

   210 

   211 Let us call the function ``circle`` as,

   212 ::

   213 

   214     a, p = circle(6)

   215     print a

   216     print p

   217 

   218 Now we have done enough coding, let us do some code reading exercise,

   219 

   220 {{{ switch to next slide, what }}}

   221 

   222 What does the function ``what`` do?

   223 

   224 .. def what( n ):

   225 ..     if n < 0: n = -n

   226 ..     while n > 0:

   227 ..         if n % 2 == 1:

   228 ..             return False

   229 ..         n /= 10

   230 ..     return True

   231 

   232 Pause here and try to figure out what the function ``what`` does.

   233 

   234 {{{ switch to next slide, even_digits }}}

   235 

   236 .. def even_digits( n ):

   237 ..    """returns True if all the digits of number n is even

   238 ..    returns False if all the digits of number n is not even"""

   239 ..     if n < 0: n = -n

   240 ..     while n > 0:

   241 ..         if n % 2 == 1:

   242 ..             return False

   243 ..         n /= 10

   244 ..     return True

   245 

   246 The function returns ``True`` if all the digits of the number ``n``

   247 are even, otherwise it returns ``False``.

   248 

   249 Now one more code reading exercise,

   250 

   251 {{{ switch to next slide, what }}}

   252 

   253 What does the function ``what`` do?

   254 

   255 .. def what( n ):

   256 ..     i = 1

   257 ..     while i * i < n:

   258 ..         i += 1

   259 ..     return i * i == n, i

   260 

   261 Pause here and try to figure out what the function ``what`` does.

   262 

   263 {{{ switch to next slide, is_perfect_square }}}

   264 

   265 .. def is_perfect_square( n ):

   266 ..     """returns True and square root of n, if n is a perfect square,

   267 ..     otherwise returns False and the square root of the 

   268 ..     next perfect square"""

   269 ..     i = 1

   270 ..     while i * i < n:

   271 ..         i += 1

   272 ..     return i * i == n, i

   273 

   274 

   275 The function returns ``True`` and the square root of ``n`` if n is a

   276 perfect square, otherwise it returns ``False`` and the square root of

   277 the next perfect square.

   278 

   279 This brings us to the end of this tutorial, in this tutorial we covered

   280 

   281 {{{ switch to next slide, summary }}}

   282 

   283 - Functions in Python

   284 - Passing parameters to a function

   285 - Returning values from a function

   286 

   287 We also did few code reading exercises.

   288 

   289 {{{ switch to next slide, Thank you }}}

   290 

   291 Thank you!