Input & Output

The code snippets below will read in three integers as part of a single line and output their sum. For example, given the input

1 2 3

the output will be as follows:

The sum of these three numbers is 6

Feel free to test them out at ide.usaco.guide.

Standard I/O

In most websites (such as Codeforces and CSES), and in USACO problems after December 2020, input and output are standard.

Method 1 - <iostream>

More straightforward to use. Calling the extraction operator operator>> on cin reads whitespace-separated data from standard input. Similarly, calling the insertion operator operator<< on cout writes to standard output. The escape sequence \n represents a new line.

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#include <iostream>
using namespace std;

int main() {
	int a;
	int b;
	int c;
	cin >> a >> b >> c;
	// "\n" can be replaced with endl as well
	cout << "The sum of these three numbers is " << a + b + c << "\n";
}

Method 2 - <cstdio>

This library includes the scanf and printf functions, which are slightly more complicated to use.

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#include <cstdio>
using namespace std;

int main() {
	int a;
	int b;
	int c;
	/*
	 * %d specifies that a value of type int is being input.
	 * To input a 64-bit (long long) number,

Example Problem - Weird Algorithm

Try to implement this yourself!

Solution - Weird Algorithm

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#include <iostream>
using namespace std;

int main() {
	long long x;
	cin >> x;
	while (x != 1) {
		cout << x << " ";
		if (x % 2 == 0) {
			x /= 2;

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#include <iostream>
using namespace std;

int main() {
	int x;
	cin >> x;
	while (x != 1) {
		cout << x << " ";
		if (x % 2 == 0) {
			x /= 2;

How to Submit Your Solution

Do the following to submit on CSES. Submitting on other platforms (such as USACO) is similar.

1. Run your solution code with the sample input, and make sure that it produces the sample output.*
2. Download your solution code into a file. The file extension should be one of .cpp, .java, .py or their equivalents, depending on your programming language.
3. Open the problem statement. Log in if you aren't already logged in.
4. Press the submit tab (for USACO, scroll to the bottom of the page).
5. Upload your solution file. For CSES, assuming the file had the proper extension, the language should be automatically detected (for USACO, select the language yourself). Some platforms (such as Codeforces) allow you to paste your code into a text box instead of uploading a file.
6. Submit your solution. For CSES, you will be redirected to the results page (for USACO, results will appear at the top of the page). If your solution is correct on all the test cases, you're done! Otherwise, fix your code and start again from step 1.

File I/O

In older USACO problems, the input and output file names are given and follow the convention problemname.in. After the program is run, output must be printed to a file called problemname.out.

You must use the correct file names when opening the .in and .out files, depending on the problem. The file names are given on USACO problems which require file opening. For example, you would open paint.in and paint.out in the above problem.

Method 1 - freopen

You will need the <cstdio> library. The freopen statements reuse standard I/O for file I/O. Afterwards, you can simply use cin and cout (or scanf and printf) to read and write data.

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#include <cstdio>
#include <iostream>
using namespace std;

int main() {
	freopen("problemname.in", "r", stdin);
	// the following line creates/overwrites the output file
	freopen("problemname.out", "w", stdout);

	// cin now reads from the input file instead of standard input

To test your solution locally without file I/O, just comment out the lines with freopen.

For convenience, we can define a function that will redirect stdin and stdout based on the problem name:

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#include <cstdio>
#include <iostream>
using namespace std;

// the argument is the input filename without the extension
void setIO(string s) {
	freopen((s + ".in").c_str(), "r", stdin);
	freopen((s + ".out").c_str(), "w", stdout);
}

Method 2 - <fstream>

You cannot use C-style I/O (scanf, printf) with this method.

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#include <fstream>
using namespace std;

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

	int a;
	int b;
	int c;
	fin >> a >> b >> c;
	fout << "The sum of these three numbers is " << a + b + c << "\n";
}

Example Solution - Fence Painting

For an explanation of the solutions below, check the Rectangle Geometry module.

Method 1 - freopen

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#include <iostream>
#include <vector>
using namespace std;

int main() {
	// Use standard input to read from "paint.in"
	freopen("paint.in", "r", stdin);
	// Use standard output to write to "paint.out"
	freopen("paint.out", "w", stdout);

Method 2 - <fstream>

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#include <fstream>
#include <vector>
using namespace std;

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

	vector<bool> cover(100);
	int a, b, c, d;

USACO Note - Extra Whitespace

Importantly, USACO will automatically add a newline to the end of your file if it does not end with one.

Make sure not to output trailing spaces, or you will get an error such as the following:

Here are some examples of what is allowed and what isn't when the intended output consists of a single integer ans:

C++

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cout << ans;            // OK, no newline
cout << ans << endl;    // OK, newline
cout << ans << "\n";    // OK, newline
cout << ans << " ";     // NOT OK, extra space
cout << ans << "\n\n";  // NOT OK, extra newline