Efficiently Reading Input For Competitive Programming using Java 8

As we all know, while solving any CP problems, the very first step is collecting input or reading input. A common mistake we all make is spending too much time on writing code and compile-time as well. In Java, it is recommended to use BufferedReader over Scanner to accept input from the user. Why? It is discussed in one of our previous articles here. (Also, the issues associated with the java.util.Scanner is available) Yet for a better understanding, we will go through both the implementations in this article. Ways of Reading Inputs Using Scanner class Using BufferedReader class Using BufferedReader class with help of streams (More optimized) Now let us discuss ways of reading individually to depth by providing clean java programs and perceiving the output generated from the custom input. Way 1: Simple Scanner Input Reading The java.util.Scanner class provides inbuilt methods to read primitive data from the console along with the lines of text. In the below code snippet let's understand how it is done.

// Java Program Illustrating Reading Input 
// Using Scanner class 

// Importing Arrays and Scanner class 
// from java.util package 
import java.util.Arrays; 
import java.util.Scanner; 

// Main class 
class GFG { 

	// Main driver method 
	public static void main(String[] args) 
	{ 
		// Creating object of Scanner class 
		Scanner scan = new Scanner(System.in); 

		// Basic Input Reading 
		int a = scan.nextInt(); 
		float b = scan.nextFloat(); 

		System.out.println("Integer value: " + a); 
		System.out.println("Float value: " + b); 

		// Space Separated Input Reading 
		int[] arr = new int[5]; 

		for (int i = 0; i < arr.length; i++) { 
			arr[i] = scan.nextInt(); 
		} 

		System.out.println(Arrays.toString(arr)); 
	} 
}

The above example illustrates the most common approach used by the majority of programmers while solving competitive programming problems. But what if we can enhance our code a bit to make it faster and reliable? Method 2: Simple BufferedReader Input Reading java.io.BufferedReader class does not provide any method to read primitive data inputs. Java.io.BufferedReader class reads text from a character-input stream, buffering characters so as to provide for the efficient reading of the sequence of characters. Although it throws a checked exception known as IOException. Let us see how to handle that exception and read input from the user. Consider custom input as below as follows: Input: 4 5.6 1 2 3 4 5 Example

// Java Program Illustrating Reading Input 
// Using 

// Importing required classes 
import java.io.BufferedReader; 
import java.io.IOException; 
import java.io.InputStreamReader; 
import java.util.Arrays; 

// Main class 
class GFG { 

	// Main driver method 
	public static void main(String[] args) 
		throws IOException 
	{ 

		// Reading input via BufferedReader class 
		BufferedReader br = new BufferedReader( 
			new InputStreamReader(System.in)); 

		// Basic Input Reading 
		int a = Integer.parseInt(br.readLine()); 
		float b = Float.parseFloat(br.readLine()); 

		// Print above input values in console 
		System.out.println("Integer value: " + a); 
		System.out.println("Float value: " + b); 

		// Space Separated Input Reading 
		int[] arr = new int[5]; 
		String[] strArr = br.readLine().split(" "); 

		for (int i = 0; i < arr.length; i++) { 
			arr[i] = Integer.parseInt(strArr[i]); 
		} 

		// Printing the elements in array 
		// using toString() method 
		System.out.println(Arrays.toString(arr)); 
	} 
}

The above example illustrates another common approach used to read the data while solving competitive programming problems. So is this enough? What if we can enhance it even more? Yes. It is possible. Stay tuned.

Method 3: Enhanced way for reading separated data using BufferedReader via Streams In the previous examples, we have seen while reading space-separated data we stored it first in a String array, and then we iterated over elements and then used java typecasting to convert it to the required data type. How about a single line of code making this possible? Yes. Java 8's stream library provides a variety of functions to make it easy and optimized. Consider custom input as below as follows:

Input: 34 55 78 43 78 43 22 94 67 96 32 79 6 33

// Java Program to Read Separated Data 
// Using BufferedReader class voa enhanced for loopd 

// Importing required classes 
import java.io.BufferedReader; 
import java.io.IOException; 
import java.io.InputStreamReader; 
import java.util.Arrays; 
import java.util.List; 
import java.util.stream.Collectors; 
import java.util.stream.Stream; 

// Main class 
class GFG { 

	// Main driver method 
	public static void main(String[] args) 
		throws IOException 
	{ 

		// Reading input separated by space 
		BufferedReader br = new BufferedReader( 
			new InputStreamReader(System.in)); 

		// Storing in array 
		int[] arr = Stream.of(br.readLine().split(" ")) 
						.mapToInt(Integer::parseInt) 
						.toArray(); 
		System.out.println(Arrays.toString(arr)); 

		// Using streams concepts to parse map to integer 
		// later on collecting via Collectors via toList() 
		// method and storing it an integer list 
		List<Integer> arrayList 
			= Stream.of(br.readLine().split(" ")) 
				.mapToInt(Integer::parseInt) 
				.boxed() 
				.collect(Collectors.toList()); 

		// Print the above List as created 
		System.out.println(arrayList); 
	} 
}

Next TopicLength of the longest substring without repeating characters in Java

← prev next →