Ballpark orders

Question

Hey! Can you please help me with the advice - what’s wrong with my code in the challenge Ballpark orders?
It passes 1 and 2 tests but not the 3,4,5.
Thank you in advance!

zakaz = input ()
zak = zakaz.split()

cost = 0
for i in zak:
if i == "Pizza":
cost += 6.00
elif i == "Nachos":
cost += 6.00
elif i == "Cheeseburger":
cost += 10.00
elif i == "Water":
cost += 4.00
elif i == "Coke":
cost += 5.00
else:
cost += 5.00

total = cost * 1.07
print (total)

Answer 1

Hi,

Please check the difference in the output of these two commands to find out what is wrong:

x = 10
print(x * 1.07)
print(x + x * 0.07)

Answer 2

Bad news after some experimenting: it's a rounding error. Python doesn't make that easy to fix by just using a native long double or decimal type instead of regular double, unfortunately. One ugly solution is to do tax = cost*7, total = cost + tax/100 ; otherwise you'll need to import something with better precision.

Answer 3

It's different because binary representations of decimals are a little bit weird. The classic example is 1.01+2.02; try running print(1.01+2.02) and you'll see what I mean.

As for the underlying reason it's weird, that's because of the way binary floating point numbers are stored: in two parts, let's say a and b, where the number being stored is equal to a * 2^b (plus an extra bit for the sign). This can cause small rounding errors sometimes.

Answer 4

Thanks a lot! Not it works however i do not undrtstand the reason it shows different reaults in calculating using different approaches:)

Answer 5

Okay, thanks a lot for the clarification!