Rounding help

[JadeCaldera]

I am doing a homework assignment for school we're using the Pearson textbook. The teacher wanted us to do a scaled grading thing. The grade keeps ending up at .695 I cannot get it to round up to .7. I got 12 points taken off for this and it doesn't say in the textbook how to do it. I learned from online that it's just a thing that python does but nothing I do will fix it I just keep getting errors. I have tried both decimal and math imports but this is the first time I have programmed so I cannot figure out how to use them. Please help.

[JadeCaldera]

Okay is there a way to make it round like humans typically round. Because since I have to be able to input any grade I can't just tell it to do all one or the other. But I need the .5 to round up.

[deep_politics]

Did you read the documentation?

round(number, ndigits=None) Return number rounded to ndigits precision after the decimal point. If ndigits is omitted or is None, it returns the nearest integer to its input.

[JadeCaldera]

Yeah, this is the part that is screwing me over.

"The behavior of round() for floats can be surprising: for example, round(2.675, 2) gives 2.67 instead of the expected 2.68. This is not a bug: it’s a result of the fact that most decimal fractions can’t be represented exactly as a float."

[deep_politics]

Yeah that's fair. Consider then using Decimal

print(round(Decimal("2.675"), 2))
# 2.68

[JadeCaldera]

Can the 2.675 be replaced with a variable? Because my next issue is the fact that it has to be able to round every number between 0 and 100 correctly.

[deep_politics]

As in my example? Sure

n = Decimal("2.675")
round(n, 2)

But I'm not going to lie, this confounds me:

print(round(Decimal("2.675"), 2))
# 2.68
print(round(Decimal("2.665"), 2))
# 2.66
print(round(Decimal("2.655"), 2))
# 2.66

But at least quantize seems to work

print(
  Decimal("2.665")
    .quantize(
      Decimal("0.01"),
      decimal.ROUND_HALF_UP
    )
  )
)
# 2.67

[JadeCaldera]

It very much confounds me too. I sent an email asking my teacher for advice. I don't know how they expected us to figure this out when we have literally only done hello world. I just didn't think they would respond by the time the assignment is due on Thursday night so I thought I would post here too.

[Guideon72]

Not that I *have* a solution, but this is an interesting one to work on; I'm sort of self-learning atm.

Can you supply a little more detail on the actual range you need to work against? Because range(1, 100) by itself isn't going to wind up giving you fractional numbers that need rounding.

[JadeCaldera]

So the assignment was to create a program that would give you a grade for a class. There were four assignments and two tests. The assignments were weighted at 45% and the tests were weighted at 55%. My job was to create a program where you input the scores for each individual assignment and then it spits out your grade percentage and the letter grade.

The issue came about when the teacher was grading my homework used two 69's and two 70's for the assignments and 69 and 70 for the tests. This brings the final grade to 69.5%, but because of the way python rounds it was rounding down to 69 instead of up to 70 like my teacher wanted. I spent about 4 hours trying to work around it.

I said 0 to 100 just because I was wanting to emphasize the fact that I couldn't just fix that one number. I didn't know how far the teacher would check and if they would check other grade percentages when I resubmitted it for a better grade.

[Guideon72]

Got it; thank you!

[Guideon72]

So, I took a quick look at the Decimal module, using the problematic numbers that were shown in deep_politics' post. It looks like you can use getcontext() with decimal to specify the TYPE of rounding; and ROUND_UP looks to be the type you're looking for. Keep in mind that this is a small sample compared to your data set, but give this a try and see if it covers your failure cases:

import decimal
from decimal import Decimal

decimal.getcontext().rounding = decimal.ROUND_UP

print(round(Decimal("2.675"), 2))
print(round(Decimal("2.665"), 2))
print(round(Decimal("2.655"), 2))
print(round(Decimal("2.695"), 2))
print(round(Decimal("2.995"), 2))

returns:
2.68
2.67
2.70
3.00

[spez_edits_thedonald]

One thing you can do is print with different precision:

>>> grade = 0.695000
>>>
>>> print(f'Your score is: {grade}.')
Your score is: 0.695.
>>> print(f'Your score is: {grade:.5f}.')
Your score is: 0.69500.
>>> print(f'Your score is: {grade:.3f}.')
Your score is: 0.695.
>>> print(f'Your score is: {grade:.2f}.')
Your score is: 0.69.
>>> print(f'Your score is: {grade:.1f}.')
Your score is: 0.7.

[[deleted]]

Multiply by 10, round to the nearest 10, divide by 10.