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

id	title	author	order
io	Input and Output	Darren Yao	5

Demonstrates how to read in input and print output for USACO.

C++

In CodeForces and CSES, input and output are standard, meaning that using the library suffices.

However, in USACO, input is read from a file called problemname.in, and printing output to a file called problemname.out. Note that you'll have to rename the .in and .out files. You will need the or the library. Essentially, replace every instance of the word template in the word below with the input/output file name, which should be given in the problem.

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.

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.

For USACO:

If <cstdio> is used:

#include <cstdio>

using namespace std;

int main() {
    freopen("template.in", "r", stdin);
    freopen("template.out", "w", stdout);
}

If <fstream> is used (note that if you use <fstream>, you must replace cin and cout with fin and fout):

#include <fstream>

using namespace std;

int main() {
    ifstream fin("template.in");
    ofstream fout("template.out");
}

For CodeForces, CSES, and other contests that use standard input and output, simply use the standard input / output from <iostream>.

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.

In CodeForces and CSES, input and output are standard, and the above methods work. However, 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.

However, in USACO, input is read from a file called problemname.in, and printing output to a file called problemname.out. Note that you'll have to rename the .in and .out files. Essentially, replace every instance of the word template in the word below with the input/output file name, which should be given in the problem.

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.

Below, we have included Java templates for input and output, which are effectively faster Scanners. We import the entire util and io libraries for ease of use.

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
    }
}

For CodeForces, CSES, and other contests that use standard input and output, the template is as follows:

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.

(insert table)

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();
}