Method 1 - input() and print()

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 various 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.

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# Read in a string
my_str = input()
# Prints the string on its own line
print(my_str)

# Take in an integer n on a single line
n = int(input())
# Prints n with " test" (no newline) after it
print(n, end=" test")

Method 2 - stdin and stdout

The first method of reading input can be far slower (sometimes hundreds of times slower!) than using stdin. Coupled with Python's relatively slow execution speed, reading input quickly becomes incredibly important.

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# Import the sys module to use stdin/stdout
import sys

# sys.stdin/stdout is similar to a file in that we read lines for input/output
my_str = sys.stdin.readline()
sys.stdout.write(str(myStr) + "\n")
# Renaming the read/write methods for convenience
input = sys.stdin.readline
print = sys.stdout.write

We can also use split, map or a list comprehension to read in multiple whitespace-separated integers on the same line.

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import sys

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

# stdin/stdout, just replace input() with sys.stdin.readline()
nums = list(map(int, sys.stdin.readline().split()))

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

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import sys

# Read in integers n and m on the same line with a list comprehension
n, m = [int(x) for x in input().split()]
# Do the same thing but with map instead
n, m = map(int, input().split())

# stdin and stdout
n, m = map(int, sys.stdin.readline().split())

So taking three integers as input and printing their sum is quite simple. On a larger scale (thousands of integers), using stdin and stdout becomes far more important for speed:

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import sys

a, b, c = map(int, input().split())
print("The sum of these three numbers is", a + b + c)

# stdin and stdout
a, b, c = map(int, sys.stdin.readline().split())
print("The sum of these three numbers is", a + b + c)

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x = int(input())
while x != 1:
	print(x, end=" ")
	if x % 2 == 0:
		x //= 2
	else:
		x = 3 * x + 1
print(x)

C++

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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;

Java

Python

C++

Java

As noted in the resource above, this problem requires 64-bit integers. The following solution, which uses int instead of long, does not pass all of the test cases.

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import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner r = new Scanner(System.in);
		int x = r.nextInt();
		while (x != 1) {
			System.out.print(x + " ");
			if (x % 2 == 0) {
				x /= 2;

This happens because some numbers in the sequence can exceed the maximum possible value for the int data type ($2^{31}-1$, as mentioned in the prerequisite module).

Python

C++

Java

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.

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import sys

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

A different approach to file I/O in Python is to still use the open() method, but use the built-in functions .readline() or .readlines():

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"""
Note: The second argument can be omitted in the open()
command for read-only files
"""
fin = open("problemname.in", "r")
fout = open("problemname.out", "w")

# One way to read the file using .readline()
line1 = fin.readline()
# readline() will pick up where you left off

• fin.readline() will return the next line as a string. This method is useful for problems where you only have to read a short amount of lines but you still need to map each value to a variable.

• fin.readlines() returns all of the file's content as a list, separated by newlines ("\n"). Combined with a for loop, this method provides a concise way to separate variables in the same line in a problem. Keep in mind that each line entry in the list will still have a "\n" at the end.

• fout.write(data) will write the variable data to the file. data must be a string, and you can convert non-string variables with str(my_var). The write() method will NOT write a new line at the end. You must also run fout.write("\n") if you wish to write a new line.

• f-strings were added in Python 3.6, and generally look nicer than concatenating (adding) strings. To define an f-string, simply add the letter f right before the start of the string, and any variables or expressions enclosed in curly braces ({}) will be put into the string. As an example, fout.write(f"{var1} {var2} {var3+var4}") looks much cleaner than fout.write(str(var1)+" "+str(var2)+" "+str(var3+var4))

After you read a line, you may wish to process it further. Python has many built-in string methods and functions:

• str.strip() removes any trailing or leading whitespace. You should always run this method after reading a line, to ensure that there is no extra whitespace: line = fin.readline().strip()

• map(func, iterable) will run a function (the func argument) against each element of an iterable (list) you pass it. This is useful for changing a list of strings into a list of ints: nums = list(map(int, ["1", "2", "3"])). Please note that map() returns a Map object, and you need to convert it into a list with list().

• str.split(delim) will split the string. If no argument is passed, it will split by space. This is useful if you want to separate a string of space-separated integers into ints: nums = list(map(int, line.split()))

Method 1

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with open("paint.in", "r") as inp:
	lines = [line for line in inp]
	a, b = map(int, lines[0].split())
	c, d = map(int, lines[1].split())

	cover = [0] * 100
	for i in range(a, b):
		cover[i] = 1
	for i in range(c, d):
		cover[i] = 1

Method 2

Redirecting file input using sys, as mentioned above.

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import sys

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

a, b = map(int, input().split())
c, d = map(int, input().split())

cover = [0] * 100
for i in range(a, b):

Python

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