Python Input Methods for Competitive Programming

Python is an amazingly user-friendly language with the only flaw of being slow. In comparison to C, C++, and Java, it is quite slower. In online coding platforms, if the C/C++ limit provided is x. Usually, in Java time provided is 2x, and in Python, it’s 5x. To improve the speed of code execution for input/output intensive problems, languages have various input and output procedures. In this article, we are going to see various Python input methods for competitive programming.

Example Problem 

Consider a question of finding the sum of N numbers inputted from the user. 
Input a number N. 
Input N numbers are separated by a single space in a line. 

Example

Input: 
5
1 2 3 4 5
Output:
15

Input Methods for Competitive Programming in Python

Below are the methods by which we can take faster input in Python:

• Normal Method
• Faster Method using Inbuilt Function
• Taking User Input Given in a Single Line in Separate Variables
• Taking User Inputs of List of Integers
• Taking String Input from the User
• Adding a buffered pipe io (Python 2.7) 

Normal Method Python (Python 2.7)

• raw_input() takes an optional prompt argument. It also strips the trailing newline character from the string it returns. 
• print is just a thin wrapper that formats the inputs (space between args and newline at the end) and calls the write function of a given object. 

Faster Method Using Inbuilt stdin, stdout (Python 2.7)

• sys.stdin on the other hand is a File Object. It is like creating any other file object one could create to read input from the file. In this case, the file will be a standard input buffer. 
• stdout.write(‘D\n’) is faster than print ‘D’. 
• Even faster is to write all once by stdout.write(“”.join(list-comprehension)) but this makes memory usage dependent on the size of the input.  

Difference in Time

Timing summary (100k lines each) 
——————————– 
Print : 6.040 s 
Write to file : 0.122 s 
Print with Stdout : 0.121 s

As we have seen till now that taking input from the standard system and giving output to the standard system is always a good idea to improve the efficiency of the code which is always a need in Competitive programming. But wait! would you like to write these long lines every time when you need them? Then, what’s the benefit of using Python. 
Let’s discuss the solution to this problem. What we can do is let’s create separate functions for taking inputs of various types and just call them whenever you need them. 

Taking User Input Given in a Single Line in Separate Variables

Suppose the input is of the following form 

5 7 19 20

We want separate variables to reference them. What we want is given below

a = 5
b = 7
c = 19
d = 20

To do this, we can create a function named as get_ints() as given below in the code

Now we don’t need to write this line again and again. You just have to call the get_ints() function in order to take input in this form. In the function get_ints we are using the map function.

Taking User Inputs of List of Integers

Suppose the input is of the following form 

1 2 3 4 5 6 7 8

Now, we want that a single variable will hold the whole list of integers. What we want is given below.

Arr = [1, 2, 3, 4, 5, 6, 7, 8]

Here we will create a function named get_list() as given below.

Now you don’t have to write this line again and again. You just have to call the get_ints() function in order to take input in this form 

Taking String Input from the User

Suppose the input is of the following form  

GeeksforGeeks is the best platform to practice Coding.

Now, we want that a single reference variable will hold this string. What we want is given below

string = "GeeksforGeeks if the best platform to practice coding."

Here we will create a function named get_string() as given below in the code. 

Now you don’t have to write this line again and again. You just have to call the get_string() function in order to take input in this form

Adding a buffered pipe io (Python 2.7) 

• Simply, adding the buffered IO code before your submission code to make the output faster. 
• The benefit of io.BytesIO objects is that they implement a common interface (commonly known as a ‘file-like’ object). BytesIO objects have an internal pointer and for every call to read(n) the pointer advances. 
• The atexit module provides a simple interface to register functions to be called when a program closes down normally. The sys module also provides a hook, sys.exitfunc, but only one function can be registered there. The atexit registry can be used by multiple modules and libraries simultaneously.  

Summary

While handling a large amount of data usually, the normal method fails to execute within the time limit. Method 2 helps in maintaining a large amount of I/O data. Method 3 is the fastest. Usually, handling of input data files greater than 2 or 3 MBs is helped via methods 2 and 3.
Note: above mention codes are in Python 2.7, to use in Python 3.X versions. Simply replace the raw_input() with Python 3.X’s input() syntax. Rest should work fine.

References

• More About Input in Python 2.7 
• Output via sys library and other commands. 
• Input via sys library and other commands. 
• Python atexit Module docs.