Reading Input and Writing Output Like a Judge

18 min · Intermediate

A judge feeds your program text and compares what it prints, character by character. Reading and printing correctly is half of the marks, and fast reading often decides large tests.

input() reads one line

Standard input is the stream of text a program reads from. A judge fills it with the test data instead of a keyboard. Both input() and sys.stdin read from it.

A judge gives your program text on standard input. The first line often holds a count, and the next line holds the values. Read each line with input() and convert it.

Python

n = int(input())prices = list(map(int, input().split()))print(sum(prices))

Example 1 · Fake input to test locally

Python

1import io, sys2
3sys.stdin = io.StringIO("3\n10 20 30\n")4
5n = int(input())6prices = list(map(int, input().split()))7print(n, sum(prices))

Output

3 60

What this code does

Replacing sys.stdin with io.StringIO lets you test input code without a judge.
Delete the lines that fake the input before you submit, so the program reads the real input.

input() returns one line as text. split() cuts it at spaces, and map(int, ...) turns each piece into a number.

sys.stdin for big inputs

Token is one piece of input text separated from the next by spaces or line breaks. split() with no argument turns the whole input into a list of tokens.

With 100,000 lines, calling input() each time is slow. Read the whole input once and split it into tokens, then walk through the list.

Python

1import sys2
3data = sys.stdin.read().split()4n = int(data[0])5prices = list(map(int, data[1:1 + n]))6print(sum(prices))

Example 2 · Read all tokens at once

Python

1import io, sys2
3sys.stdin = io.StringIO("5\n3 1 4\n1 5\n")4
5data = sys.stdin.read().split()6n = int(data[0])7values = list(map(int, data[1:1 + n]))8print(values, max(values))

Output

[3, 1, 4, 1, 5] 5

What this code does

The values may be spread over several lines, and read().split() does not care about line breaks.
This is the safest way to read when the layout of the lines is not guaranteed.

sys.stdin.read() returns all the text at once. The index 1 + n stops the slice after the n prices.

Several test cases and unknown length

Many problems start with the number of test cases. Others give no count, and you read until the input ends. Both patterns are short with sys.stdin.

Python

1import sys2
3cases = int(sys.stdin.readline())4for _ in range(cases):5    sys.stdin.readline()6    row = list(map(int, sys.stdin.readline().split()))7    print(max(row))8
9for line in sys.stdin:10    print(line.strip())

Example 3 · Several test cases

Python

1import io, sys2
3sys.stdin = io.StringIO("2\n3\n4 9 2\n2\n7 1\n")4
5t = int(sys.stdin.readline())6for case in range(1, t + 1):7    n = int(sys.stdin.readline())8    row = list(map(int, sys.stdin.readline().split()))9    print(f"Case {case}: {max(row)}")

Output

Case 1: 9
Case 2: 7

What this code does

The first line is the number of cases. Each case has a size line and a values line.
The output format Case 1: 9 comes from the statement, so copy it exactly.

Example 4 · Read until the input ends

Python

1import io, sys2
3sys.stdin = io.StringIO("laptop 2\nmouse 5\nlaptop 4\n")4
5totals = {}6for line in sys.stdin:7    name, quantity = line.split()8    totals[name] = totals.get(name, 0) + int(quantity)9print(totals)

Output

{'laptop': 6, 'mouse': 5}

What this code does

The loop stops by itself when the input has no more lines.
line.split() gives two tokens that unpack into a name and a quantity.

readline() reads one line. Looping over sys.stdin gives lines until the end of the input, with no count needed.

Print once and match the format exactly

Judges compare your output text with the expected text. Build the answers in a list, join them, and print once. Match spaces, line breaks and decimal places exactly.

Python

results = [n * n for n in range(1, 6)]print("\n".join(map(str, results)))print(*results)print(f"{2 / 3:.2f}")

Example 5 · Output formats

Python

1row = [3, 14, 7]2print(*row)3print(*row, sep=", ")4print(*row, sep="\n")5print(f"{sum(row) / len(row):.2f}")6print("YES" if sum(row) > 20 else "NO")

Output

What this code does

sep changes the text between items, and a separator of a line break prints one item per line.
Print words such as YES and NO exactly as the statement writes them, including capital letters.

print(*results) separates the items by one space. The format .2f always shows two decimal places, so 0.6666 prints as 0.67.

Next step

Check your understanding

10 quick questions

input() reads one line

Standard input is the stream of text a program reads from. A judge fills it with the test data instead of a keyboard. Both input() and sys.stdin read from it.

A judge gives your program text on standard input. The first line often holds a count, and the next line holds the values. Read each line with input() and convert it.

1import sys2 3cases = int(sys.stdin.readline())4for _ in range(cases):5 sys.stdin.readline()6 row = list(map(int, sys.stdin.readline().split()))7 print(max(row))8 9for line in sys.stdin:10 print(line.strip())

1import io, sys2 3sys.stdin = io.StringIO("2\n3\n4 9 2\n2\n7 1\n")4 5t = int(sys.stdin.readline())6for case in range(1, t + 1):7 n = int(sys.stdin.readline())8 row = list(map(int, sys.stdin.readline().split()))9 print(f"Case {case}: {max(row)}")

1import io, sys2 3sys.stdin = io.StringIO("laptop 2\nmouse 5\nlaptop 4\n")4 5totals = {}6for line in sys.stdin:7 name, quantity = line.split()8 totals[name] = totals.get(name, 0) + int(quantity)9print(totals)