Exercise 1.7: You should get an answer of 966279.5999999957 not 966,279.6

[Jock2018]

# mortgage.py

principal = 500000.0
rate = 0.05
payment = 2684.11
total_paid = 0.0

while principal > 0:
    principal = principal * (1+rate/12) - payment
    total_paid = total_paid + payment

print('Total paid', total_paid)

Enter this program and run it. I get an answer of 966279.5999999957. Not 966,279.6.

I can get an anwer of 966,279.6 by following:

# Exercise 1.7


principal = 500000.0
rate = 0.05
payment = 2684.11
total_paid = 0.0

while principal > 0:
    principal = principal * (1+rate/12) - payment
    total_paid = total_paid + payment

print('Total paid', round(total_paid, 1))

[dabeaz]

The question is worded as intended. The fact that floating point numbers don't give exact results is mentioned a few places. However, the fact that you get a slightly "off" answer here would be a discussion point in a live-taught course. One could also point people at the decimal built-in module.

[atsalolikhin-spokeo]

Hello! I am learning Python. I appreciate that you shared your course materials, thank you!

What does the decimal built-in module have to do with this mortgage exercise, please? Is that covered later in the course? Is there a way to get the .6 answer using the decimal module?

[dabeaz]

Floating point point numbers are not exact. If this matters to you, consider the use of the decimal module (you'd need to look at the library documentation for more information).