The round() function in Python is used to round a floating-point number to a specified number of decimal places, or to the nearest integer if no decimal places are specified.

Syntax

round(number, ndigits)

• number: The number you want to round.
• ndigits (optional): The number of decimal places to round to. If omitted, it defaults to 0, meaning the number will be rounded to the nearest integer.

Return Value

• If ndigits is provided, it returns the number rounded to the specified decimal places.
• If ndigits is omitted or 0, it returns the nearest integer.
• For negative numbers, it rounds towards zero (also known as rounding half to even).

Examples

1. Rounding to the nearest integer (default behavior):

   print(round(4.6))  # Output: 5
   print(round(4.3))  # Output: 4
   

2. Rounding to specific decimal places: You can specify the number of decimal places using the ndigits argument:

   print(round(3.14159, 2))  # Output: 3.14
   print(round(3.14159, 3))  # Output: 3.142
   

3. Rounding with ndigits = 0 (explicit integer rounding):

   print(round(9.99, 0))  # Output: 10.0
   

4. Rounding negative numbers:

   print(round(-4.6))  # Output: -5
   print(round(-4.3))  # Output: -4
   

5. Rounding to negative decimal places: You can also round numbers to negative decimal places, which rounds to the nearest tens, hundreds, etc.

   print(round(12345, -1))  # Output: 12350
   print(round(12345, -2))  # Output: 12300
   

How It Handles Halfway Cases (Banker's Rounding)

The round() function uses a technique called rounding half to even, also known as banker's rounding. This means that when a number is exactly halfway between two integers (like 2.5 or 3.5), it rounds towards the nearest even number.

Example:

print(round(2.5))  # Output: 2 (rounds to even)
print(round(3.5))  # Output: 4 (rounds to even)

Summary

• round() is used to round numbers to a specified number of decimal places.
• It uses rounding half to even for numbers that are exactly halfway between two values.
• Can handle rounding both positive and negative numbers.