usaco-guide/content/1_Intro/Input_Output.md

id	title	author	description
io	Input & Output	Darren Yao	Demonstrates how to read input and print output for USACO contests.

Additional Reading

• CSES 1.2
• PAPC 2.4

Standard I/O

In most websites (such as CodeForces and CSES), input and output are standard.

C++

The <iostream> library suffices.

#include <iostream>
using namespace std;

int main() {
    int x; cin >> x;
    cout << "FOUND " << x << "\n";
}

Java

In your CS classes, you've probably implemented input and output using standard input and standard output, or using Scanner to read input and System.out.print to print output. These methods work, but Scanner and System.out.print are slow when we have to handle inputting and outputting tens of thousands of lines. Thus, we use BufferedReader and PrintWriter instead, which are faster because they buffer the input and output and handle it all at once as opposed to parsing each line individually.

Here is a Java template for input and output, which is effectively a faster Scanner. We import the entire util and io libraries for ease of use.

import java.io.*;
import java.util.*;

public class template {
    static class InputReader {
        BufferedReader reader;
        StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        String next() { // reads in the next string
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() { // reads in the next int
            return Integer.parseInt(next());
        }

        public long nextLong() { // reads in the next long
            return Long.parseLong(next());
        }

        public double nextDouble() { // reads in the next double
            return Double.parseDouble(next());
        }
    }

    static InputReader r = new InputReader(System.in);
    static PrintWriter pw = new PrintWriter(System.out);

    public static void main(String[] args) {

        // YOUR CODE HERE

        pw.close(); // flushes the output once printing is done
    }
}

Here's a brief description of the methods in our InputReader class, with an instance r, and PrintWriter with an instance pw.

Method	Description
r.next()	Reads the next token (up to a whitespace) and returns a String
r.nextInt()	Reads the next token (up to a whitespace) and returns as an int
r.nextLong()	Reads the next token (up to a whitespace) and returns as a long
r.nextDouble()	Reads the next token (up to a whitespace) and returns as a double
pw.println()	Prints the argument to designated output stream and adds newline
pw.print()	Prints the argument to designated output stream

File I/O

In USACO, input is read from a file called problemname.in, and output is printed to a file called problemname.out. Note that you'll have to rename the .in and .out files. For example, for this problem, you would replace problemname with socdist1 to get socdist1.in and socdist1.out.

In order to test a program, create a file called problemname.in, and then run the program. The output will be printed to problemname.out.

C++

You will need the <cstdio> or the <fstream> library. Essentially, replace every instance of the word problemname in the word below with the input/output file name, which should be given in the problem.

Below, we have included C++ templates for input and output. We use using namespace std; so that we don't have to preface standard library functions with std:: each time we use them.

If <cstdio> is used:

#include <cstdio>
using namespace std;

int main() {
    freopen("problemname.in", "r", stdin);
    freopen("problemname.out", "w", stdout);
    // rest of your code ...
}

If <fstream> is used: (Note that you cannot use C-style I/O (scanf, printf) with this method):

#include <fstream>
using namespace std;

int main() {
    ifstream fin("problemname.in");
    ofstream fout("problemname.out");
    // rest of your code ...
}

Java

We can modify the template above to support file I/O.

import java.util.*;
import java.io.*;

public class template {
    static class InputReader {
        BufferedReader reader;
        StringTokenizer tokenizer;

        public InputReader() throws FileNotFoundException {
            reader = new BufferedReader(new FileReader("template.in"));
            tokenizer = null;
        }

        String next() { // reads in the next String
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() { // reads in the next int
            return Integer.parseInt(next());
        }

        public long nextLong() { // reads in the next long
            return Long.parseLong(next());
        }

        public double nextDouble() { // reads in the next double
            return Double.parseDouble(next());
        }
    }

    public static void main(String[] args) throws FileNotFoundException, IOException {

        InputReader r = new InputReader();
        PrintWriter pw = new PrintWriter(new BufferedWriter(new FileWriter("template.out")));

        // YOUR CODE HERE
        
        pw.close(); // flushes the output once printing is done
    }
}

Here's an example to show how input/output works. Let's say we want to write a program that takes three numbers as input and prints their sum.

// InputReader template code above
static InputReader r = new InputReader(System.in);
static PrintWriter pw = new PrintWriter(System.out);

public static void main(String[] args) {
    int a = r.nextInt();
    int b = r.nextInt();
    int c = r.nextInt()
    pw.println(a + b + c);
    pw.close();
}