On Thu, 11 Feb 2010 12:32:52 am Owain Clarke wrote: > My son was doing a statistics project in which he had to sort some > data by either one of two sets of numbers, representing armspan and > height of a group of children - a boring and painstaking job. I came > across this piece of code:- > > li=[[2,6],[1,3],[5,4]] # would also work with li=[(2,6),(1,3),(5,4)] > li.sort(key=lambda x:x[1] ) > print li > > It occurred to me that I could adapt this so that he could input his > data at the command line and then sort by x:x[0] or x:x[1]. And I > have not discovered a way of inputting a list, only strings or > various number types.
Which command line do you mean? If you are talking about the Python interactive interpreter, then you can input either lists or strings: >>> li = [1, 2, 3] # a list >>> li = "[1, 2, 3]" # a string If you mean the external shell (say, "bash" under Linux or the DOS command line in Windows, or similar) then you can only input strings -- everything is a string in such shells. There are two ways to convert such strings to lists: the easy, unsafe way; or the slightly more difficult but safer way. Think of it like The Force, where the Dark Side is simpler and more attractive but ultimately more dangerous :) First, the easy way. If you absolutely trust the source of the data, then you can use the eval function: >>> s = "[1, 2, 3]" # quote marks make it a string >>> li = eval(s) The interpreter will read the string s as a Python expression, and do whatever it says. In this example, it says "make a list with the numbers 1, 2 and 3 in it". But it could say *anything*, like "erase the hard disk", and Python would dutifully do what it is told. This is why eval is easy and fast but dangerous, and you must absolutely trust the source of the string. Alternatively, you could do this: >>> s = "1 2 3 4" # numbers separated by spaces >>> li = s.split() # li now contains the strings "1", "2" etc. >>> li = map(int, li) # convert the strings to actual ints >>> print li [1, 2, 3, 4] Naturally the string s would come from the shell, otherwise there is no point! If you are typing the data directly into the Python interpreter, you would enter it directly as a list and not a string. Hope this helps, -- Steven D'Aprano _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
