Python 四舍五入不正确的结果

Question

我想在 python 中对数字进行四舍五入，但它总是给我不准确的结果例如我想将 99.999999946 舍入为 99.99，或将 56.3633333 舍入为 56.36这是我尝试过的:

int(99.999999946*100)/100   #result = 99
int(99.999999946*100)/100.0 #result = 99.989999999999995
round(99.999999946, 2)      #result = 100.0

先谢谢了

Answer 1

在您熟悉二进制浮点的限制之前，您可能会对 decimal 模块感到更满意，该模块实现了丰富的十进制浮点模型。除此之外，它还允许对舍入模式进行精细控制:

>>> import decimal
>>> d = decimal.Decimal("99.999999946")
>>> print d
99.999999946
>>> chopped = d.quantize(decimal.Decimal(".01"), decimal.ROUND_DOWN)
>>> print chopped
99.99

概括

这是一个函数，它将截断到您喜欢的任何数字位置，并返回一个浮点值(通常，这是不精确的!):

def chop_to_n_decimals(x, n):
    # This "rounds towards 0".  The decimal module
    # offers many other rounding modes - see the docs.
    import decimal
    d = decimal.Decimal(repr(x))
    targetdigit = decimal.Decimal("1e%d" % -n)
    chopped = d.quantize(targetdigit, decimal.ROUND_DOWN)
    return float(chopped)

例如，

for x in 5555.5555, -5555.5555:
    for n in range(-3, 4):
        print x, n, "->", chop_to_n_decimals(x, n)

显示:

5555.5555 -3 -> 5000.0
5555.5555 -2 -> 5500.0
5555.5555 -1 -> 5550.0
5555.5555 0 -> 5555.0
5555.5555 1 -> 5555.5
5555.5555 2 -> 5555.55
5555.5555 3 -> 5555.555
-5555.5555 -3 -> -5000.0
-5555.5555 -2 -> -5500.0
-5555.5555 -1 -> -5550.0
-5555.5555 0 -> -5555.0
-5555.5555 1 -> -5555.5
-5555.5555 2 -> -5555.55
-5555.5555 3 -> -5555.555