Question

I made a list from a CSV file. It looks like this:

['18', '29', '0'...'0', '-14', '9']

It had a text element in the beginning, which I popped. Now, Python doesn’t work with the list elements as with proper numbers. Whenever I try to parse the elements into floats, for example, I get the error message:

ValueError: could not convert string to float: -

My code looks like this:

list2=[[float(column) for column in row] for row in list1]

Or this:

list2 = [map(float, x) for x in list1]

Both versions produce the same result. If I try a different data type, I get messages like:

ValueError: invalid literal for int() with base 10: '-'

Can anyone tell me how do I get Python to parse the dash into a number correctly?

Answer 1

It looks like the list you’re creating is 1D but when you try and parse it you’re treating it as a 2D list which will try converting each character in each element to a float (hence it throws an error while trying to do float('-')). Try replacing the line with the following to fix the problem:

list2 = map(float, list1)

You should also try adding print list1 before that line to confirm that the list has the structure you think it has.