## Day 9

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.

## 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

1. Variables: If we assign a variable to some value say name = "sandeep", then we make a function name and when we call name we get its value, i.e., "sandeep".
2. Operations: From mathematics we know that operations are actually functions.
3. 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:

1. 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.

2. 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.

## Python Function

Now we will see how function is implemented in python language.

### 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


### Simple Functions

Now let’s look at some concrete examples

1. 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.

2. 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!

3. Using one function inside other

We can also use one function inside other as following

def greet(arg):
return "Hello " + arg + "."

return greet(name) + " Do you want to learn " + subject + "?"


Hello Sandeep. Do you want to learn Python?
Hello Suman. Do you want to learn Mathematics?

4. 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


### 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.