DAY 9: Functions¶
Today we will learn about functions in programming. The functions in programming is similar to mathematical functions which takes input and produce output. But the main idea of function in programming is inspired by one of guiding principle of programming known as don't repeat yourself, also known as DRY. So if some part of code is needs to be written again and again then it means that we can convert this into a function as use them.
1. General Theory¶
Functions are the foundation of higher level programming language. There are programming language which implements everything based on functions only those are called functional programming language. Although modern languages implement a more abstract thing known as object which is composed of data and methods(function associated with the data). Such programming language are called object oriented programming language, and python is one of them.
Recall what have you done till now, you realize everything is a function. For example
 Variables: If we assign a variable to some value say
name = "sandeep"
, then we make a functionname
and when we callname
we get its value, i.e.,"sandeep"
.  Operations: From mathematics we know that operations are actually functions.
 Control Structure: You can think of it as it takes one or more
boolean
and assign a value.
Two main usage of function is the following:

Maximizing Code Reuse and Minimizing Redundancy
As I have already mentioned if we use some block of code again and again, we can convert this into a function used it. It is not limited to one program, a complex function written for one program can be used in many programs. Libraries and modules are example of such application.

Procedural Decomposition
A complex task can be achieved by using a number of small task in a sequence. We can make each such small independent task as it's own function. When we combine all of them, we get our original task. This make the code easy to use, extend and debug.
2. Python Function¶
Now we will see how function is implemented in python language.
2.1 General Structure¶
A general python function has a name for reference. It takes 0 to some finite number of inputs or arguments. It executes some block of code using those arguments. Finally it will return only one value. Return is also optional, you may want to return something or just carry some task without saying anything.
The template of a general python function is the following
def <name> (arg1, arg2, ..., argN): # function with name takes N arguments
<statements> # block of code executed with arguments
return <value> # a value is returned at the end
2.2 Simple Functions¶
Now let's look at some concrete examples

Function with no arguments
It is quite possible to have a python function with no argument at all. It works a complex variable. For example
def light_year(): speed_of_light = 299792458 return speed_of_light * 365 * 24 * 60 * 60 print(light_year())
9454254955488000
Now we can use light year as a variable.

Function with few arguments
Now let's look at some typical python function
def greet(arg): return "Hello " + arg # whatever it takes add a hello to it print(greet("Sandeep Suman"))
Hello Sandeep Suman
def prod(x, y): return x * y print(prod(2, 5)) # product of number print(prod("Hi!", 3)) # product as string
10 Hi!Hi!Hi!
Note
You can see that function takes care of the datatypes.

Using one function inside other
We can also use one function inside other as following
def greet(arg): return "Hello " + arg + "." def reading(name, subject): return greet(name) + " Do you want to learn " + subject + "?" print(reading("Sandeep", "Python")) print(reading("Suman", "Mathematics"))
Hello Sandeep. Do you want to learn Python? Hello Suman. Do you want to learn Mathematics?

Using control structure and loops
You can combine all your knowledge till now to make more and more complex functions. A function for computing the absolute value of a number is the following:
def abs_number(num): if num >= 0: return num else: return (1) * num print(abs_number(53)) print(abs_number(0)) print(abs_number(3.14))
53 0 3.14
A function to compute the sum of all elements of a list can be written as
def sum_list(given_list): sum = 0 for val in given_list: sum = sum + val return sum print(sum([1, 2, 3])) # sum of all element of list print(sum(range(51))) # sum of first 50 natural no
6 1275
2.3 Recursive Function¶
Similar to mathematics recursive function are those function which call themselves. A simple example is factorial.
def fact(n):
if n == 0: # base case
return 1
else: # general case
return n * fact(n  1)
print(fact(0))
print(fact(1))
print(fact(3))
print(fact(10))
1
1
6
3628800
Ex: Make your own function of each type and implement them in python.