Functions in Python

Last Updated: 11-09-2018

A function is a set of statements that take inputs, do some specific computation and produces output. The idea is to put some commonly or repeatedly done task together and make a function, so that instead of writing the same code again and again for different inputs, we can call the function.
Python provides built-in functions like print(), etc. but we can also create your own functions. These functions are called user-defined functions.

# A simple Python function to check 
# whether x is even or odd 
def evenOdd( x ): 
if (x % 2 == 0): 
print "even"
else: 
print "odd"
 
# Driver code 
evenOdd(2) 
evenOdd(3) 

Output:

even
odd

Pass by Reference or pass by value?
One important thing to note is, in Python every variable name is a reference. When we pass a variable to a function, a new reference to the object is created. Parameter passing in Python is same as reference passing in Java.

# Here x is a new reference to same list lst 
def myFun(x): 
x[0] = 20
 
# Driver Code (Note that lst is modified 
# after function call. 
lst = [10, 11, 12, 13, 14, 15]  
myFun(lst); 
print(lst) 

Output:

[20, 11, 12, 13, 14, 15]

In C++, files are mainly dealt by using three classes fstream, ifstream, ofstream available in fstream headerfile.
ofstream: Stream class to write on files
ifstream: Stream class to read from files
fstream: Stream class to both read and write from/to files.

Now the first step to open the particular file for read or write operation. We can open file by
1. passing file name in constructor at the time of object creation
2. using the open method
For e.g.

Open File by using constructor
ifstream (const char* filename, ios_base::openmode mode = ios_base::in);
ifstream fin(filename, openmode) by default openmode = ios::in
ifstream fin(“filename”);

Open File by using open method
Calling of default constructor
ifstream fin;

fin.open(filename, openmode)
fin.open(“filename”);

Modes :

Default Open Modes :

Below is the implementation by using fstream class.:

/* File Handling with C++ using ifstream & ofstream class object*/
/* To write the Content in File*/
/* Then to read the content of file*
#include <iostream>
/* fstream header file for ifstream, ofstream,  
fstream classes */
#include <fstream> 
using namespace std;

// Creation of ofstream class object
int main()
string line; 
ofstream fout;
// by default ios::out mode, automatically deletes 
// the content of file. To append the content, open in ios:app 
// fout.open("sample.txt", ios::app) 
fout.open("sample.txt"); 

// Execute a loop If file successfully opened 
while (fout) { 

    // Read a Line from standard input 
    getline(cin, line); 

    // Press -1 to exit 
    if (line == "-1") 
        break; 

    // Write line in file 
    fout << line << endl; 
} 

// Close the File 
fout.close(); 

// Creation of ifstream class object to read the file 
ifstream fin; 

// by default open mode = ios::in mode 
fin.open("sample.txt"); 

// Execute a loop until EOF (End of File) 
while (fin) { 

    // Read a Line from File 
    getline(fin, line); 

    // Print line in Console 
    cout << line << endl; 
} 

// Close the file 
fin.close(); 

return 0; 

The Standard Template Library (STL) is a set of C++ template classes to provide common programming data structures and functions such as lists, stacks, arrays, etc. It is a library of container classes, algorithms, and iterators. It is a generalized library and so, its components are parameterized. A working knowledge of template classes is a prerequisite for working with STL.

STL has four components

• Algorithms
• Containers
• Functions
• Iterators

Algorithms

The header algorithm defines a collection of functions especially designed to be used on ranges of elements.They act on containers and provide means for various operations for the contents of the containers.

• Algorithm
  • Sorting
  • Searching
  • Important STL Algorithms
  • Useful Array algorithms
  • Partition Operations
• Numeric
  • valarray class

Containers

Containers or container classes store objects and data. There are in total seven standard “first-class” container classes and three container adaptor classes and only seven header files that provide access to these containers or container adaptors.

• Sequence Containers: implement data structures which can be accessed in a sequential manner.
  • vector
  • list
  • deque
  • arrays
  • forward_list( Introduced in C++11)
• Container Adaptors : provide a different interface for sequential containers.
  • queue
  • priority_queue
  • stack
• Associative Containers : implement sorted data structures that can be quickly searched (O(log n) complexity).
  • set
  • multiset
  • map
  • multimap
• Unordered Associative Containers : implement unordered data structures that can be quickly searched
  • unordered_set (Introduced in C++11)
  • unordered_multiset (Introduced in C++11)
  • unordered_map (Introduced in C++11)
  • unordered_multimap (Introduced in C++11)

Functions

The STL includes classes that overload the function call operator. Instances of such classes are called function objects or functors. Functors allow the working of the associated function to be customized with the help of parameters to be passed.

• Functors

Iterators

As the name suggests, iterators are used for working upon a sequence of values. They are the major feature that allow generality in STL.

• Iterators

Pointers store address of variables or a memory location.

// General syntax
datatype *var_name; 

// An example pointer "ptr" that holds
// address of an integer variable or holds
// address of a memory whose value(s) can
// be accessed as integer values through "ptr"
int *ptr;  

Using a Pointer:

To use pointers in C, we must understand below two operators.

• To access address of a variable to a pointer, we use the unary operator & (ampersand) that returns the address of that variable. For example &x gives us address of variable x.

// The output of this program can be different 
// in different runs. Note that the program 
// prints address of a variable and a variable 
// can be assigned different address in different 
// runs. 
#include <stdio.h> 

int main() 
// Prints address of x 
printf("%p", &x); 

return 0; 
}

• One more operator is unary * (Asterisk) which is used for two things :
  • To declare a pointer variable: When a pointer variable is declared in C/C++, there must be a * before its name.

// C program to demonstrate declaration of 
// pointer variables. 
#include <stdio.h> 
int main() 
{ 
    int x = 10; 
// 1) Since there is * in declaration, ptr 
// becomes a pointer varaible (a variable 
// that stores address of another variable) 
// 2) Since there is int before *, ptr is 
// pointer to an integer type variable 
int *ptr; 

// & operator before x is used to get address 
// of x. The address of x is assigned to ptr. 
ptr = &x; 

return 0; 
}

• To access the value stored in the address we use the unary operator (*) that returns the value of the variable located at the address specified by its operand. This is also called Dereferencing.

What are the main principles of Object-Oriented Programming?

The four principles of object-oriented programming are encapsulation, abstraction, inheritance,and polymorphism.

These words may sound scary for a junior developer. And the complex, excessively long explanations in Wikipedia sometimes double the confusion.

That’s why I want to give a simple, short, and clear explanation for each of these concepts. It may sound like something you explain to a child, but I would actually love to hear these answers when I conduct an interview.

Encapsulation

Say we have a program. It has a few logically different objects which communicate with each other — according to the rules defined in the program.

Encapsulation is achieved when each object keeps its state private, inside a class. Other objects don’t have direct access to this state. Instead, they can only call a list of public functions — called methods.

So, the object manages its own state via methods — and no other class can touch it unless explicitly allowed. If you want to communicate with the object, you should use the methods provided. But (by default), you can’t change the state.

Let’s say we’re building a tiny Sims game. There are people and there is a cat. They communicate with each other. We want to apply encapsulation, so we encapsulate all “cat” logic into a Catclass. It may look like this:

Here the “state” of the cat is the private variables mood, hungry and energy. It also has a private method meow(). It can call it whenever it wants, the other classes can’t tell the cat when to meow.

What they can do is defined in the public methods sleep(), play() and feed(). Each of them modifies the internal state somehow and may invoke meow(). Thus, the binding between the private state and public methods is made.

This is encapsulation.

Abstraction

Abstraction can be thought of as a natural extension of encapsulation.

In object-oriented design, programs are often extremely large. And separate objects communicate with each other a lot. So maintaining a large codebase like this for years — with changes along the way — is difficult.

Abstraction is a concept aiming to ease this problem.

Applying abstraction means that each object should only expose a high-level mechanism for using it.

This mechanism should hide internal implementation details. It should only reveal operations relevant for the other objects.

Think — a coffee machine. It does a lot of stuff and makes quirky noises under the hood. But all you have to do is put in coffee and press a button.

Preferably, this mechanism should be easy to use and should rarely change over time. Think of it as a small set of public methods which any other class can call without “knowing” how they work.

Another real-life example of abstraction?
Think about how you use your phone:

You interact with your phone by using only a few buttons. What’s going on under the hood? You don’t have to know — implementation details are hidden. You only need to know a short set of actions.

Implementation changes — for example, a software update — rarely affect the abstraction you use.

Inheritance

OK, we saw how encapsulation and abstraction can help us develop and maintain a big codebase.

But do you know what is another common problem in OOP design?

Objects are often very similar. They share common logic. But they’re not entirely the same. Ugh…

So how do we reuse the common logic and extract the unique logic into a separate class? One way to achieve this is inheritance.

It means that you create a (child) class by deriving from another (parent) class. This way, we form a hierarchy.

The child class reuses all fields and methods of the parent class (common part) and can implement its own (unique part).

For example:

If our program needs to manage public and private teachers, but also other types of people like students, we can implement this class hierarchy.

This way, each class adds only what is necessary for it while reusing common logic with the parent classes.

Polymorphism

We’re down to the most complex word! Polymorphism means “many shapes” in Greek.

So we already know the power of inheritance and happily use it. But there comes this problem.

Say we have a parent class and a few child classes which inherit from it. Sometimes we want to use a collection — for example a list — which contains a mix of all these classes. Or we have a method implemented for the parent class — but we’d like to use it for the children, too.

This can be solved by using polymorphism.

Simply put, polymorphism gives a way to use a class exactly like its parent so there’s no confusion with mixing types.But each child class keeps its own methods as they are.

This typically happens by defining a (parent) interface to be reused. It outlines a bunch of common methods. Then, each child class implements its own version of these methods.

Any time a collection (such as a list) or a method expects an instance of the parent (where common methods are outlined), the language takes care of evaluating the right implementation of the common method — regardless of which child is passed.

Take a look at a sketch of geometric figures implementation. They reuse a common interface for calculating surface area and perimeter:

Having these three figures inheriting the parent Figure Interface lets you create a list of mixed triangles, circles, and rectangles. And treat them like the same type of object.

Then, if this list attempts to calculate the surface for an element, the correct method is found and executed. If the element is a triangle, triangle’s CalculateSurface()is called. If it’s a circle — then cirlce’s CalculateSurface()is called. And so on.

If you have a function which operates with a figure by using its parameter, you don’t have to define it three times — once for a triangle, a circle, and a rectangle.

You can define it once and accept a Figureas an argument. Whether you pass a triangle, circle or a rectangle — as long as they implement CalculateParamter(), their type doesn’t matter.

I hope this helped. You can directly use these exact same explanations at job interviews.

If you find something still difficult to understand — don’t hesitate to ask in the comments below.

What’s next?

Being prepared to answer one of the all-time interview question classics is great — but sometimes you never get called for an interview.

Next, I’ll focus on what employers want to see in a junior developer and how to stand out from the crowd when job hunting.

Stay tuned.