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I have a list inside of a list, and the inner list has strings of numbers (float) and words. What I need to sort the list by, is in position list[0]. So for example, 

list = [['8.34', 'a'],['3.55', 'c'],['5.92', 'b']]

I'm trying to sort the list numerically to look like

list = [['3.55', 'c'],['5.92', 'b'],['8.34', 'a']]

I've tried 

sorted(list, key = float)

but I get an error message: 'float() argument must be a string or a number' and I've tried using lambda as well. Neither works. Could someone help please?

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3
  • possible duplicate of How to sort a list of lists by a specific index of the inner list? Commented Apr 1, 2014 at 8:08
  • list is a python type, using it as a variable name will override current type. Commented Apr 1, 2014 at 8:13
  • Okay, I think I got it to work...apparently all that I needed to do was my_List.sort() ? Commented Apr 1, 2014 at 8:22

3 Answers 3

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3

You can try passing a lambda function.:

sorted(my_list, key = lambda x : float(x[0]))

x will be an element of the list (which is also a list, because my_list is a list of lists), and float(x[0]) will return the float representation of the first element of that list.

Demo:

>>> my_list = [['8.34', 'a'],['3.55', 'c'],['5.92', 'b']]
>>> print sorted(my_list, key = lambda x : float(x[0]))
[['3.55', 'c'], ['5.92', 'b'], ['8.34', 'a']]

Note:

  • Don't use list as the name of a variable, because you will hide its built-in implementation.
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2

Your list contains lists, so you cannot use float directly. You need to use a function that returns float value of the first item in each list.

>>> lis = [['8.34', 'a'],['3.55', 'c'],['5.92', 'b']]
>>> lis.sort(key=lambda x: float(x[0]))
>>> lis
[['3.55', 'c'], ['5.92', 'b'], ['8.34', 'a']]
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6 Comments

I get 'could not convert string to float:' error message
builtins.ValueError: could not convert string to float:
@jenncar That means one of the items in your list cannot be converted to float. What exactly does your list contain?
I just realized some of the inner lists have an empty string in position [0]
Sorry, I should have caught this earlier.
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-2

This earlier answer can be used in your case also: How to sort a list of lists by a specific index of the inner list?

from operator import itemgetter
list = [['8.34', 'a'],['3.55', 'c'],['5.92', 'b']]
print sorted(list, key=itemgetter(0))

gives the desired output:

[['3.55', 'c'], ['5.92', 'b'], ['8.34', 'a']]
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2 Comments

You are comparing strings not float.
list is a python type, using it as a variable name will override current type.

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