convert string(<0.005) to floating point number(0.005) - python 2.7

Question

I have a sample of a list that i am iterating through, and trying to find the maximum value and require only the numbers. Im ignoring the blank spaces but need to change the '<0.005' to just 0.005, so i can use it.

lst = [['<0.005'],
       ['<0.005'],
       ['2.1'],
       ['2.1'],
       ['1.7'],
       ['0.098'],
       [''],
       ['1.7'],
       ['0.91'],
       ['1.2']]

here is the relevant part of my code

for i in range(0,9):
    if lst[i][0] != '':
       if lst[i][0] == '<0.005'
          lst[i][0] = 0.005
       elif float(lst[i][0]) > maxP[i][0]:
            maxP[i][0] = lst[i][0]

but the if lst[i][0] == '<0.005' doesn't seem to work ie the code seems to just ignore it and i get the ValueError: could not convert string to float: error for the second last line because it is still trying to access the <0.005 bits.

is there a way to fix this easily? ideally i would like it to deal with strings such as <0.098 and turn that into a float of 0.098

Any help / advice is greatly appreciated, thx

Answer 1

Your problem is an empty string (string with just white chars), not "<0.005" (which you handle correctly), you cannot convert empty string to float.

All you need is to add a .strip() in second line

for i in range(0,9):
    if lst[i][0].strip() != '':
       if lst[i][0] == '<0.005':
          lst[i][0] = 0.005
       elif float(lst[i][0]) > maxP[i][0]:
            maxP[i][0] = lst[i][0]

Answer 2

You can use this regex to strip any characters that do not belong into a number:

value = re.sub(r'[^0-9.-]', '', value)

Of course it would still let something like ...5 or 5-5 through but if you just want to get rid of leading/trailing crap the regex does its job.

lst = [[float(re.sub(r'[^0-9.-]', '', x[0]))] for x in lst if x[0].strip()]

Answer 3

You could remove all characters that are not a number using a regex.

import re

def get_num_from_str(in_str):
    return re.sub("[^0-9]", "", in_str)

Now you can let all items in the list be converted to a number

Answer 4

You can use regular expression

import re

lst = [['<0.005'],
        ['<0.005'],
        ['2.1   '],
        ['2.1   '],
        ['1.7   '],
        ['0.098 '],
        ['      '],
        ['1.7   '],
        ['0.91  '],
        ['1.2   ']]



def findMax(ilist):
    result = 0.0
    floatPattern = re.compile('\d+.\d+')
    for el in ilist:
        myFloat = floatPattern.search(el[0])
        if myFloat != None:
            if result < float(myFloat.group()):
                result = float(myFloat.group())
    return result


print findMax(lst)