将浮点数转换为字符串而不对其进行四舍五入

我正在编写一个程序，由于不需要解释的原因，它需要将一个 float 转换成一个字符串，然后使用 len ()进行计数。但是，str (float (x))在转换为字符串时会导致 x 被舍入，从而使整个过程失败。有人知道怎么修吗？ 下面是正在使用的代码，如果你想知道:

len(str(float(x)/3))
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len(repr(float(x)/3))

However I must say that this isn't as reliable as you think.

Floats are entered/displayed as decimal numbers, but your computer (in fact, your standard C library) stores them as binary. You get some side effects from this transition:

>>> print len(repr(0.1))
19
>>> print repr(0.1)
0.10000000000000001

The explanation on why this happens is in this chapter of the python tutorial.

A solution would be to use a type that specifically tracks decimal numbers, like python's decimal.Decimal:

>>> print len(str(decimal.Decimal('0.1')))
3
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最佳答案

Some form of rounding is often unavoidable when dealing with floating point numbers. This is because numbers that you can express exactly in base 10 cannot always be expressed exactly in base 2 (which your computer uses).

For example:

>>> .1
0.10000000000000001

In this case, you're seeing .1 converted to a string using repr:

>>> repr(.1)
'0.10000000000000001'

I believe python chops off the last few digits when you use str() in order to work around this problem, but it's a partial workaround that doesn't substitute for understanding what's going on.

>>> str(.1)
'0.1'

I'm not sure exactly what problems "rounding" is causing you. Perhaps you would do better with string formatting as a way to more precisely control your output?

e.g.

>>> '%.5f' % .1
'0.10000'
>>> '%.5f' % .12345678
'0.12346'

Documentation here.

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Other answers already pointed out that the representation of floating numbers is a thorny issue, to say the least.

Since you don't give enough context in your question, I cannot know if the decimal module can be useful for your needs:

http://docs.python.org/library/decimal.html

Among other things you can explicitly specify the precision that you wish to obtain (from the docs):

>>> getcontext().prec = 6
>>> Decimal('3.0')
Decimal('3.0')
>>> Decimal('3.1415926535')
Decimal('3.1415926535')
>>> Decimal('3.1415926535') + Decimal('2.7182818285')
Decimal('5.85987')
>>> getcontext().rounding = ROUND_UP
>>> Decimal('3.1415926535') + Decimal('2.7182818285')
Decimal('5.85988')

A simple example from my prompt (python 2.6):

>>> import decimal
>>> a = decimal.Decimal('10.000000001')
>>> a
Decimal('10.000000001')
>>> print a
10.000000001
>>> b = decimal.Decimal('10.00000000000000000000000000900000002')
>>> print b
10.00000000000000000000000000900000002
>>> print str(b)
10.00000000000000000000000000900000002
>>> len(str(b/decimal.Decimal('3.0')))
29

Maybe this can help? decimal is in python stdlib since 2.4, with additions in python 2.6.

Hope this helps, Francesco

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I know this is too late but for those who are coming here for the first time, I'd like to post a solution. I have a float value index and a string imgfile and I had the same problem as you. This is how I fixed the issue

index = 1.0
imgfile = 'data/2.jpg'
out = '%.1f,%s' % (index,imgfile)
print out

The output is

1.0,data/2.jpg

You may modify this formatting example as per your convenience.


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