Python 中的无穷整数

Python 3有 float('inf')和 Decimal('Infinity')，但没有 int('inf')。那么，为什么在语言中缺少一个表示无限整数集的数字呢？int('inf')不合理吗？

138373 次浏览
小开

Taken from here: https://www.gnu.org/software/libc/manual/html_node/Infinity-and-NaN.html

IEEE 754 floating point numbers can represent positive or negative infinity, and NaN (not a number)

That is, the representation of float and Decimal can store these special values. However, there is nothing within the basic type int that can store the same. As you exceed the limit of 2^32 in an unsigned 32-bit int, you simply roll over to 0 again.

If you want, you could create a class containing an integer which could feature the possibility of infinite values.

小开

For python 2. It is sometimes the case that you need a very large integer. For example, I may want to produce a subarray with x[:n] and, I may wish to sometimes set n to a value such that the whole array will be produced. Since you can't use a float for n (python wants an integer), you need a "large integer". A good way of doing this is to use the largest integer available: sys.maxint. Here is an example:

# MAX_SOURCES = sys.maxint # normal setting
MAX_SOURCES = 5 # while testing


# code to use an array ...
... sources[:MAX_SOURCES]

So, while testing, I could use a smaller sources array but use the full array in production.

小开

You are right that an integer infinity is possible, and that none has been added to the Python standard. This is probably because math.inf supplants it in almost all cases (as Martijn stated in his comment).

In the meantime, I added an implementation of extended integers on PyPI:

In [0]: from numbers import Integral, Real


In [0]: from extended_int import int_inf, ExtendedIntegral, Infinite


In [0]: i = int_inf


In [4]: float(i)
Out[4]: inf


In [5]: print(i)
inf


In [6]: i ** i


Out[6]: inf


In [7]: i
Out[7]: inf


In [9]: isinstance(i, Real)


Out[9]: True


In [10]: isinstance(i, Integral)


Out[10]: False


In [11]: isinstance(i, Infinite)


Out[11]: True


In [12]: isinstance(i, ExtendedIntegral)


Out[12]: True


In [13]: isinstance(2, ExtendedIntegral)


Out[13]: True


In [14]: isinstance(2, Infinite)


Out[14]: False

提交答案