# 1.2. Precision¶

## 1.2.1. Rationale¶

>>> 0.1
0.1

>>> 0.2
0.2

>>> 0.3
0.3

>>> 0.1 + 0.2 == 0.3
False

>>> 0.1 + 0.2
0.30000000000000004

>>> round(0.1+0.2, 16) == 0.3
True

>>> round(0.1+0.2, 17) == 0.3
False


## 1.2.2. IEEE 754 standard¶

>>> 1.234 == 1234 * 10e-4
True

>>> 1234 * 10e-4
1.234


## 1.2.3. Solutions¶

• Round values to 4 decimal places (generally acceptable)

• Store values as int, do operation and then divide. For example instead of 1.99 USD, store price as 199 US cents

• Use Decimal type

• Decimal type is much slower

Problem:

candy = 0.10      # price in dollars
cookie = 0.20     # price in dollars

print(result)
# 0.30000000000000004


Round values to 4 decimal places (generally acceptable):

candy = 0.10      # price in dollars
cookie = 0.20     # price in dollars

result = round(candy + cookie, 4)
print(result)
# 0.3


Store values as int, do operation and then divide:

candy = 10        # price in cents
cookie = 20       # price in cents

result = (candy + cookie) / 100   # divide by 100 (number of cents in dollar)
print(result)
# 0.30


Use Decimal type:

from decimal import Decimal

candy = Decimal('0.10')     # price in dollars
cookie = Decimal('0.20')    # price in dollars