[Tutor] The '45' bug in round()

[Dick Moores]

Yesterday I was shocked, SHOCKED, to discover that round() is 
occasionally rounding incorrectly. For example,

 >>> print round(0.19945,4)
0.1994

For rounding of random samples of numbers between 0 and 1 ending in 
'45', the error ratio is about 0.041. Here are a few more examples:

 >>> print round(0.145, 2)
0.14

 >>> print round(0.5045, 3)
0.504

 >>> print round(0.34145, 4)
0.3414

 >>> print round(0.5170845, 6)
0.517084

 >>> print round(0.083400685245, 11)
0.08340068524

It may not be correct to call this a bug, but this is what I'm going 
to use until round() is revised:

def round2(n, digits=0):
     s = str(n)
     if '.' in s and s[-2:] == '45':
         mantissa = s.split('.')[1]
         if len(mantissa) - digits == 1:
             s = s[:-2] + '5'
             return s
         else:
             return round(float(s), digits)
     else:
         return round(float(n), digits)

Comments, Tutors? Am I way out in left field with this?

Dick Moores







0.504
0.505



0.083400685245
0.08340068524
0.08340068525

0.34145
0.3415
0.3414

0.29692817045
0.2969281705
0.2969281704

0.74945
0.7495
0.7494