Python Input Methods for Competitive Programming?

In competitive programming, efficient input/output methods can significantly improve your solution's performance. Python offers several approaches for reading input, each with different speed characteristics.

Let's explore various I/O methods using a simple example: reading four numbers a, b, c, d and printing their product.

Basic Input Methods

Using List Comprehension

This method uses list comprehension to convert input strings to integers ?

a, b, c, d = [int(x) for x in input().split()]
print(a * b * c * d)

Using map() Function

The map() function provides a cleaner syntax for type conversion ?

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

Fast I/O Using sys Module

For large inputs, using sys.stdin and sys.stdout provides better performance ?

from sys import stdin, stdout

a, b, c, d = [int(x) for x in stdin.readline().rstrip().split()]
stdout.write(str(a * b * c * d) + "\n")

Practical Example: SPOJ INTEST Problem

Let's solve the "Enormous Input Test" problem to compare different approaches. The problem requires counting how many numbers are divisible by k.

Problem Statement

Input: First line contains two integers n and k (n, k ? 10⁷). Next n lines contain one integer each (? 10⁹).

Output: Count of numbers divisible by k.

Example

Input:
7 3
1
51
966369
7
9
999996
11

Output:
4

Solution Approaches

Method 1: Basic Input

Standard approach using input() function ?

def main():
    n, k = [int(c) for c in input().split()]
    cnt = 0
    for _ in range(n):
        t = int(input())
        if t % k == 0:
            cnt += 1
    print(cnt)

if __name__ == "__main__":
    main()

Method 2: Using stdin/stdout

Faster approach using sys module ?

from sys import stdin, stdout

def main():
    n, k = map(int, stdin.readline().split())
    cnt = 0
    for _ in range(n):
        t = int(stdin.readline())
        if t % k == 0:
            cnt += 1
    stdout.write(str(cnt) + "\n")

if __name__ == "__main__":
    main()

Method 3: Bulk Reading

Most efficient approach by reading all input at once ?

from sys import stdin, stdout

def main():
    lines = stdin.read().strip().split('\n')
    n, k = map(int, lines[0].split())
    
    cnt = 0
    for i in range(1, n + 1):
        if int(lines[i]) % k == 0:
            cnt += 1
    stdout.write(str(cnt) + "\n")

if __name__ == "__main__":
    main()

Performance Comparison

Key Tips

• Use stdin.readline().rstrip() to remove trailing newlines
• For multiple test cases, read all input at once when possible
• Use stdout.write() instead of print() for faster output
• Avoid repeated int() conversions in loops

Conclusion

For competitive programming, use sys.stdin and sys.stdout for large inputs. The bulk reading approach with stdin.read() provides the best performance for time-critical problems.