Convert string with engineer prefix to float

Question

I have a string like '102.3k' I would like to convert this string with an engineer prefix notation to a float number.

http://en.wikipedia.org/wiki/Engineering_notation

Allowed prefixes are

posPrefixes = ['k', 'M', 'G', 'T', 'P', 'E', 'Z', 'Y']
negPrefixes = ['m', 'µ', 'n', 'p', 'f', 'a', 'z', 'y']

k means 10^3

M means 10^6

m means 10^-3

µ means 10^-6

I think I should use regex to do this but I have very few experience with regex.

edit: ideally the solution should also be able to convert any string so '102.3' (without prefix) should also be converted to float

Answer 1

Try this out, no regex needed:

pos_postfixes = ['k', 'M', 'G', 'T', 'P', 'E', 'Z', 'Y']
neg_postfixes = ['m', 'µ', 'n', 'p', 'f', 'a', 'z', 'y']

num_postfix = n[-1]
if num_postfix in pos_postfixes:
    num = float(n[:-1])
    num*=10**((pos_postfixes.index(num_postfix)+1)*3)
elif num_postfix in neg_postfixes:
    num = float(n[:-1])
    num*=10**(-(neg_postfixes.index(num_postfix)+1)*3)
else:
    num = float(n)
print(num)

Another thing to note is that in python, it is more common to use underscore variable names than camelcasing, see the pep-8: http://www.python.org/dev/peps/pep-0008/

Answer 2

If you want to control the value, you could try this:

import decimal
posPrefixes = {'k':'10E3', 'M':'10E6', 'G':'10E9', 'T':'10E12', 'P':'10E15', 'E':'10E18', 'Z':'10E21', 'Y':'10E24'}
negPrefixes = {'m':'10E-3', '?':'10E-6', 'n':'10E-9', 'p':'10E-12', 'f':'10E-15', 'a':'10E-18', 'z':'10E-21', 'y':'10E-24'}
val='102.3k'
if val[-1] in posPrefixes.keys():
    v = decimal.Decimal(val[:-1])
    print v*decimal.Decimal(posPrefixes[val[-1]])

val ='102.3n'
if val[-1] in negPrefixes.keys():
    v = decimal.Decimal(val[:-1])
    print v*decimal.Decimal(negPrefixes[val[-1]])

Output:

1.0230E+6

1.023e-06