Summing up floats elements of array in python is adding garbage value to the end of the answer?

Question

My code is:

import array as arr

print("\t\t\tEnter Marks of the following subjects out of 100 :\n")

marks = arr.array('f') 


for x in range(5):
    marks.append(float(input("Enter marks of {0} subject : ".format(x+1))))

else:
    print("Calculating...")

sum = float(0.0)
for x in marks:
    sum += x

print("Total Marks (Out Of 500) = ",sum)

Code and Output

Also: you are using single precision floating points (type code `'f'`). Standard python `float`'s are double precision, and you should use that for basically everything (use type code `'d'` in `arr.array`) — Rob, Mar 19 '20 at 08:07

Filip · Answer 1 · 2020-03-19T08:02:50.713

I don't know if you are required to used the array library, but I don't see a use for it here. Do you want to use it to be more efficient? If that's the case then also see 2 In python you can set a variable's values as an empty list, so you can use it as one, further in your code. For example:

l = []
for i in range(3):
    l.append(i)

print(l)  # prints [0, 1, 2]

So in your case you can do this:

1)

print("\t\t\tEnter Marks of the following subjects out of 100 :\n")

marks = []
for x in range(5):
    marks.append(float(input("Enter marks of {0} subject : ".format(x+1))))

print("Calculating...")

sum = 0
for mark in marks:
    sum += x  # if x is a float in any of the loops then the sum will be a float.

print("Total Marks (Out Of 500) = ", sum)

Or if you are required to use the array package:

2)

import array

marks = array.array("f")
for i in range(5):
    marks.append(float(input("Enter marks of {0} subject : ".format(i+1))))

sum = 0
for mark in marks:
    sum += mark

print("Total Marks (Out Of 500) =", float(sum))  # prints the float value of the sum

This doesn't seem like it needs to be that efficient and you probably wont be getting the program any faster by using the array library. Use the timeit module to see which program runs faster.

import timeit

timeit.timeit(stmt="""import array

in_marks = [80, 80.2, 80, 80, 80]
marks = array.array("f")
for i in range(5):
    marks.append(float(in_marks[i]))

sum = 0
for mark in marks:
    sum += mark

# print("Total Marks (Out Of 500) =", float(sum))  # prints the float value of the sum""", number=1000000)

# This took 1.7964587999999821 seconds to run 1.000.000 times
# And 0.19057900000007066 to run 100.000 times

And this is without the library:

import timeit

timeit.timeit(stmt="""
in_marks = [80, 80.2, 80, 80, 80]
marks = []
for i in range(5):
    marks.append(float(in_marks[i]))

sum = 0
for mark in marks:
    sum += mark

# print("Total Marks (Out Of 500) =", float(sum))  # prints the float value of the sum""", number=100000)

# This took 1.271052900000086 seconds to run 1.000.000 times
# And 0.12818579999998292 seconds to run 100.000 times

Python seems to be faster without it, because the program doesn't do any heavy calculations, so by importing the library you lose some time and don't gain any in the long run.

To remove the ending numbers in you code you can round() the float to 2 decimal points like @Laurent Bristiel suggested.

score 0 · Accepted Answer · answered Mar 19 '20 at 07:39

That's because floats are not exact values. So when you add them you often ends up with ,00001 or ,..999. One way to solve this is to round your final result. For example do :

print("Total Marks (Out Of 500) = ", round(sum,2))

You can check this detailed article in Real Python on the matter, it should help.

Summing up floats elements of array in python is adding garbage value to the end of the answer?

2 Answers2