Input & Output

C++

Java

Python

C++

Method 1: <iostream>

#include <iostream>
using namespace std;

int main() {
    int a, b, c; cin >> a >> b >> c;
    cout << "sum is " << a+b+c << "\n";
}

Method 2: <cstdio>

This library includes the scanf and printf functions, which are slightly more complicated to use, but are significantly faster (generally only an issue with large input sizes):

#include <cstdio>
using namespace std;

int main() {
    int a, b, c;
    // %d specifies that a value of type int is being input.
    // Use %lld (a few judging platforms might need %I64d)
    // to input a long long (64-bit) integer.
    // Many other specifiers are also available; see link for more details.
    // Be sure to add a & character (address-of operator) when using

The first method can be sped up so that the difference in speed is not significant; see Fast I/O for details.

Java

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. However, BufferedReader is harder to use than Scanner.

Here is a Java template for input and output that which combines BufferedReader and PrintWriter, based off Kattis's Kattio.java. Note that we import the entire util and io libraries for ease of use.

/** Simple yet moderately fast I/O routines.
 *
 * Example usage:
 *
 * Kattio io = new Kattio();
 *
 * while (io.hasMoreTokens()) {
 *    int n = io.nextInt();
 *    double d = io.nextDouble();
 *    double ans = d*n;

The input methods in our Kattio class mimic those of Scanner. Given an instance io:

PrintWriter pw = new PrintWriter(new BufferedWriter(new FileWriter("problemname.out")));

PrintWriter pw = new PrintWriter(new FileWriter("problemname.out"));

Python

Python

The most intuitive way to do input/output is using the built in input() and print() methods. The input() method will return the next line, and can be processed using different python methods. The print() method takes in a string and an optional string end (defaults to '\n'). Below is an annotated demonstration on different input/output scenarios.

# read in a string
myStr = input()
# prints the string on its own line
print(myStr)

# take in an integer n on a single line
n = int(input())
# prints integer n with " test" after it
print(n, end=" test")

We can also split along with map or a list comprehension to read in multiple integers on the same line (separated by whitespace).

# read in a series of numbers on one line into a list
nums = [int(x) for x in input().split()]
# does the same thing
nums = list(map(int, input().split()))

We can use something similar to the above if we are unpacking a fixed number of integers.

# read in integers n and m, both on the same line
n, m = [int(x) for x in input().split()]
# does the same thing
n, m = map(int, input().split())

So taking three integers as input and printing their sum is quite simple:

a,b,c = map(int, input().split())
print("sum is",a+b+c)

C++

As noted in the resource above, this problem requires 64-bit integers.

Java

As noted in the resource above, this problem requires 64-bit integers.

Method 1 - Scanner and System.out.print

Method 2 - BufferedReader and PrintWriter

Method 3 - Kattio

Essentially the same as method 2.

Python

C++

Method 1: freopen

You will need the <cstdio> library.

#include <cstdio>
using namespace std;

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

Method 2: <fstream>

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

Java

Again, BufferedReader and PrintWriter should be used. The provided constructor public Kattio(String problemName) handles file input / output for you.

Python

Python

See here for documentation about file I/O.

The most intuitive way to do file I/O in Python is by redirecting the system input and output to files. After doing this, you can then use the above input() and print() methods as usual.

import sys

sys.stdin = open("problemname.in", "r")
sys.stdout = open("problemname.out", "w")

C++

Method 1

Use freopen. If you comment out both of the lines containing freopen then the program reads from standard in and writes to standard out as usual.

Method 2

Use ifstream & ofstream.

Java

Method 1 - Scanner and PrintWriter

Method 2 - BufferedReader and PrintWriter

Method 3 - Kattio

Essentially the same as method 2. Note how static initialization of io is different compared to standard input / output since we need to catch IOExceptions.

Python

Method 1

Method 2

Redirecting file input using sys, as mentioned above.

C++

C++

For method 1:

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

Python

Python

For method 2:

print(ans) # OK, newline
print(ans,end='') # OK, no newline
print(str(ans)+'\n') # NOT OK, extra newline