Limiting floats to two decimal points

python

decimal-arithmetic

float-limitation

precision-matters

byAnton Shumikhin·Sep 15, 2024

Need to trim a float to two decimal places? Python gives you several ways to do it: use Python's round() function for mathematical operations or formatted strings for a nice print-out display.

round():

# Oh, I see you're a person of culture as well.
rounded_value = round(your_float, 2)

Formatted string:

# Just floating around here...
formatted_value = f"{your_float:.2f}"

Diving into decimal limitations

Our computing systems store floating points in binary format. So, occasionally, decimals don't exactly translate, leading to tiny inaccuracies. This is governed by the IEEE 754 standard for floating-point arithmetic.

So, when precision matters, like when you're working with currency or scientific computations, Python's Decimal type from the decimal module provides accurate fixed-point arithmetic and can bypass these errors by ensuring absolute precision.

Practical scenarios and solutions

Python offers us multiple tools to handle decimal points in various contexts. When precision is a game-changer in scenarios like financial calculations or scientific computations, these tools come in handy.

Formatting with Decimal: If absolute precision matters, employ the decimal.Decimal type.

from decimal import Decimal
# Precise as a Swiss watch. 
decimal_value = Decimal('10.123').quantize(Decimal('0.01'))

String Formatting: Delve deeper into Python's mini-language for string formatting for custom formats.
Quirks in round(): Python's round() uses banker's rounding. Peek into this for predictions in calculations.

Fun with floats and common gotchas

Round and Convert: First, round your float and then, convert it to a string. This strategy is the ultimate warrior against string to float inconsistencies.
```
# Round and round we go!
precise_float = round(your_float, 2)
# Smooth as ~Butter~ formatted strings!
display_string = f"{precise_float:.2f}"
```
Precision and Print: An f-string within a print statement gives a beautiful display of a float with two decimal places, thus leaving the original float value unaltered for further operations.

# Just floating by!
print(f"Value: {your_float:.2f}")

Deceptive Comparisons: Be vigilant while comparing floats! The trim in precision might affect comparisons. Stick to comparing the original values or make sure both numbers have been rounded similarly.

Alternatives in town

If round() and string formatting don't seem to fit your use case, Python offers other ways to limit floats to two decimal points:

Multiply and Divide: Multiply the float by 100, truncate, and then bring it back down by dividing by 100. It's perfect for those working with monetary calculations.
```
# Doing the math!
truncated_value = int(your_float * 100) / 100.0
```
Cents Representation: In the depiction of monetary values, store amounts in integer cents, and convert to dollars for display, keeping the precision spot-on!
```
# It's not just cents, it makes sense too!
cents = int(your_float * 100)
dollars = cents / 100.0
```

Precision in applications

The technique to limit decimals varies based on what you're building. Here's what you can do in some common scenarios:

Data Visualisation: If your focus is presenting data, formatted strings give you just the right output.
Scientific Computing: For pivotal calculations like in physics simulations, consider using improved precision libraries like decimal.Decimal.
Financial Applications: For applications involving monetary amounts, store the value as an integer and leverage the Decimal computations to get rid of rounding errors.