---
id: io-speed
title: Input / Output Speed
author: Benjamin Qi / Nathan Chen
description: ""
---

Generally, input and output speed isn't an issue. However, some platinum tasks have relatively large input files. The [USACO Instructions Page](http://www.usaco.org/index.php?page=instructions) mentions some ways of speeding up I/O; let's check whether these actually make a difference.

## Fast Input

The largest USACO input file I know of is test case 11 of [USACO Platinum - Robotic Cow Herd](http://www.usaco.org/index.php?page=viewproblem2&cpid=674), which is 10.3 megabytes! The answer to this test case is $10^{18}$ (with $N=K=10^5$ and all microcontrollers costing $10^8$).

### C++

`freopen` with `cin` was the slowest method. 

<spoiler title="973ms">

```cpp
#include <bits/stdc++.h>
using namespace std;

vector<int> P[100000];

int main() {
	freopen("roboherd.in","r",stdin);
	freopen("roboherd.out","w",stdout);
	int N,K; cin >> N >> K;
	for (int i = 0; i < N; ++i) {
		int M; cin >> M; P[i].resize(M);
		for (int j = 0; j < M; ++j) cin >> P[i][j];
	}
	if (N == 3) cout << 61;
	else cout << 1000000000000000000;
}
```

</spoiler>

However, adding `ios_base::sync_with_stdio(0); cin.tie(0);` as the first line of `main()` reduced the runtime to 254 ms.  See [this CodeForces blog](https://codeforces.com/blog/entry/5217) and [StackOverflow](https://stackoverflow.com/questions/31162367/significance-of-ios-basesync-with-stdiofalse-cin-tienull) for more information. 

 - Actually, the former link says that it is supposedly prohibited to use `freopen` to redirect `cin` and `cout` if `ios_base::sync_with_stdio(0); cin.tie(0);` is included, but it works properly as far as I know.

`freopen` with `scanf` is about as fast.

<spoiler title="281ms">

```cpp
#include <bits/stdc++.h> // 281 ms
using namespace std;

vector<int> P[100000];
int main() {
	freopen("roboherd.in","r",stdin);
	freopen("roboherd.out","w",stdout);
	int N,K; scanf("%d%d",&N,&K);
	for (int i = 0; i < N; ++i) {
		int M; scanf("%d",&M); P[i].resize(M);
		for (int j = 0; j < M; ++j) scanf("%d",&P[i][j]);
	}
	if (N == 3) printf("%d",61);
	else printf("%lld",1000000000000000000LL);
}
```
</spoiler>

Using `ifstream` and `ofstream` is also about as fast.

<spoiler title="258ms">

```cpp
#include <bits/stdc++.h>
using namespace std;

vector<int> P[100000];

int main() { // 258 ms
	ifstream fin("roboherd.in");
	ofstream fout("roboherd.out");
	int N,K; fin >> N >> K;
	for (int i = 0; i < N; ++i) {
		int M; fin >> M; P[i].resize(M);
		for (int j = 0; j < M; ++j) fin >> P[i][j];
	}
	if (N == 3) fout << 61;
	else fout << 1000000000000000000;
}
```

</spoiler>

Of course, there are faster methods. For example, if we use FastIO from [here](https://github.com/bqi343/USACO/blob/master/Implementations/content/various/FastIO.h) then the runtime is further reduced.

<spoiler title="91ms">

```cpp
using namespace FastIO;

vector<int> P[100000];

int main() {
	freopen("roboherd.in","r",stdin);
	freopen("roboherd.out","w",stdout);
	int N,K; ri(N,K);
	for (int i = 0; i < N; ++i) {
		int M; ri(M); P[i].resize(M);
		for (int j = 0; j < M; ++j) ri(P[i][j]);
	}
	if (N == 3) cout << 61;
	else cout << 1000000000000000000;
}
```

</spoiler>

### Java

The Java `Scanner` is probably the easiest way to read input in Java, though it is also extremely slow.

*Reading input with `Scanner`*

<spoiler title="3188ms">

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

//3188ms
public class roboherd_scanner {
	
	static int P[][] = new int[100000][];
	
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(new File("roboherd.in"));
		PrintWriter pw = new PrintWriter(new BufferedWriter(new FileWriter("roboherd.out")));
		
		int N = sc.nextInt();
		int K = sc.nextInt();
		for(int i = 0; i < N; ++i) {
			int M = sc.nextInt(); P[i] = new int[M];
			for(int j = 0; j < M; ++j) P[i][j] = sc.nextInt();
		}
		
		if(N == 3) pw.println(61);
		else pw.println(1000000000000000000L);
		pw.close();
	}
}
```

</spoiler>

A common alternative to reading input for programming contests is `BufferedReader`, which reads faster from a file than `Scanner`. `BufferedReader` reads line-by-line with the `.readLine()` method, which returns a string. Methods like `Integer.parseInt()` are used to convert strings into primitives, and they can directly convert a line into a number, like in `Integer.parseInt(br.readLine())`.

Reading input is more complicated when multiple, space-separated values are placed in a single line. In order to individually read the values in each line, the programmer usually uses the `.split()` method in `String` or the `.nextToken()` in `StringTokenizer`. Notice that `StringTokenizer` for splitting strings is slightly faster than `.split()`.

*Reading input with `BufferedReader` and `.split()`*
<spoiler title="1209ms">

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

//1209ms
public class roboherd_buffered_reader_string_split {
	
	static int P[][] = new int[100000][];
	
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new FileReader("roboherd.in"));
		PrintWriter pw = new PrintWriter(new BufferedWriter(new FileWriter("roboherd.out")));
		
		String[] tokens = br.readLine().split(" ");
		int N = Integer.parseInt(tokens[0]);
		int K = Integer.parseInt(tokens[1]);
		for(int i = 0; i < N; ++i) {
			tokens = br.readLine().split(" ");
			int M = Integer.parseInt(tokens[0]); P[i] = new int[M];
			for(int j = 0; j < M; ++j) P[i][j] = Integer.parseInt(tokens[j+1]);
		}
		
		if(N == 3) pw.println(61);
		else pw.println(1000000000000000000L);
		pw.close();
	}
}
```
</spoiler>

*Reading input with `BufferedReader` and `StringTokenizer`*
<spoiler title="986ms">

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

//986ms
public class roboherd_buffered_reader_string_tokenizer {
	
	static int P[][] = new int[100000][];
	
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new FileReader("roboherd.in"));
		PrintWriter pw = new PrintWriter(new BufferedWriter(new FileWriter("roboherd.out")));
		
		StringTokenizer st = new StringTokenizer(br.readLine());
		int N = Integer.parseInt(st.nextToken());
		int K = Integer.parseInt(st.nextToken());
		for(int i = 0; i < N; ++i) {
			st = new StringTokenizer(br.readLine());
			int M = Integer.parseInt(st.nextToken()); P[i] = new int[M];
			for(int j = 0; j < M; ++j) P[i][j] = Integer.parseInt(st.nextToken());
		}
		
		if(N == 3) pw.println(61);
		else pw.println(1000000000000000000L);
		pw.close();
	}
}
```
</spoiler>

Faster methods of reading input exist too - Even faster than `BufferedReader` is a custom-written Fast I/O class that uses `InputStream`. Note that this custom class is similar to the custom-written `FastIO.h` in the C++ section, as both read input through a byte buffer.

<spoiler title="320ms">

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

//320ms
public class roboherd_is {
	
	static int P[][] = new int[100000][];
	
	public static void main(String[] args) throws Exception {
		FastIO sc = new FastIO("roboherd.in");
		PrintWriter pw = new PrintWriter(new BufferedWriter(new FileWriter("roboherd.out")));
		
		int N = sc.nextInt();
		int K = sc.nextInt();
		for(int i = 0; i < N; ++i) {
			int M = sc.nextInt(); P[i] = new int[M];
			for(int j = 0; j < M; ++j) P[i][j] = sc.nextInt();
		}
		
		if(N == 3) pw.println(61);
		else pw.println(1000000000000000000L);
		pw.close();
	}
	
	// new FastIO("file_name") to read a file
	// new FastIO(System.in) to read from stdin
	// has similar syntax to Scanner, though much faster :)
	static class FastIO {
		
		InputStream dis;
		byte[] buffer = new byte[1 << 17];
		int pointer = 0;

		public FastIO(String fileName) throws Exception {
			dis = new FileInputStream(fileName);
		}

		public FastIO(InputStream is) throws Exception {
			dis = is;
		}

		int nextInt() throws Exception {
			int ret = 0;

			byte b;
			do {
				b = nextByte();
			} while (b <= ' ');
			boolean negative = false;
			if (b == '-') {
				negative = true;
				b = nextByte();
			}
			while (b >= '0' && b <= '9') {
				ret = 10 * ret + b - '0';
				b = nextByte();
			}

			return (negative) ? -ret : ret;
		}

		long nextLong() throws Exception {
			long ret = 0;

			byte b;
			do {
				b = nextByte();
			} while (b <= ' ');
			boolean negative = false;
			if (b == '-') {
				negative = true;
				b = nextByte();
			}
			while (b >= '0' && b <= '9') {
				ret = 10 * ret + b - '0';
				b = nextByte();
			}

			return (negative) ? -ret : ret;
		}

		byte nextByte() throws Exception {
			if (pointer == buffer.length) {
				dis.read(buffer, 0, buffer.length);
				pointer = 0;
			}
			return buffer[pointer++];
		}

		String next() throws Exception {
			StringBuffer ret = new StringBuffer();

			byte b;
			do {
				b = nextByte();
			} while (b <= ' ');
			while (b > ' ') {
				ret.appendCodePoint(b);
				b = nextByte();
			}

			return ret.toString();
		}

	}
}
```
</spoiler>

The most realistic input method to implement in contest is `BufferedReader` and `StringTokenizer`, as it is relatively fast for the amount of code necessary.

### Python

Faster than the first C++ method! Significantly less if $P$ does not need to be stored.

<spoiler title="853ms">

```py
fin = open("roboherd.in","r")
fout = open("roboherd.out","w")
N,K = map(int,fin.readline().split())
P = [[] for i in range(N)]
for i in range(N):
	P[i] = map(int,fin.readline().split())
if N == 3:
	fout.write(str(61))
else:
	fout.write(str(1000000000000000000))
```

</spoiler>


## Fast Output

In general, it may be faster to store the answer all in a single `string` (C++) or `StringBuffer` (Java) and outputting it with a single function call. This method avoids the overhead of calling an output method many times, especially if the output is generated in many parts.

### C++
When printing many lines in C++, it may be faster to use the newline character `\n` in place of `endl`. Output streams in C++ (such as `cout` and `ofstream`) are buffered, meaning that they don't immediately print their output, but store some of it. At some point, the buffer's contents are written (i.e. "flushed") to the output device, whether it be the standard output stream or a file. Buffering the output helps with efficiency if accessing the output device (like a file) is slow. Because `endl` flushes the output, it may be faster to use `\n` instead and avoid unnecessary flushes.

### Java
When printing to the standard output stream in Java, it is faster to use `new PrintWriter(System.out)` than `System.out`. The syntax for `PrintWriter` is equivalent to that of `System.out` so it should be easy to use. The only difference is that one has to call `.flush()` or `.close()` on the `PrintWriter` at the very end of the program in order to write the output.
