I have the following code:
# initialize
a = []
# create the table (name, age, job)
a.append(["Nick", 30, "Doctor"])
a.append(["John", 8, "Student"])
a.append(["Paul", 22, "Car Dealer"])
a.append(["Mark", 66, "Retired"])
# sort the table by age
import operator
a.sort(key=operator.itemgetter(1))
# print the table
print(a)
It creates a 4x3 table and then it sorts it by age. My question is, what exactly key=operator.itemgetter(1)
does? Does the operator.itemgetter
function return the item's value? Why can't I just type something like key=a[x][1]
there? Or can I? How could with operator print a certain value of the form like 3x2
which is 22
?
How does exactly Python sort the table? Can I reverse-sort it?
How can I sort it based on two columns like first age, and then if age is the same b name?
How could I do it without operator
?
Looks like you're a little bit confused about all that stuff.
operator
is a built-in module providing a set of convenient operators. In two words operator.itemgetter(n)
constructs a callable that assumes an iterable object (e.g. list, tuple, set) as input, and fetches the n-th element out of it.
So, you can't use key=a[x][1]
there, because python has no idea what x
is. Instead, you could use a lambda
function (elem
is just a variable name, no magic there):
a.sort(key=lambda elem: elem[1])
Or just an ordinary function:
def get_second_elem(iterable):
return iterable[1]
a.sort(key=get_second_elem)
So, here's an important note: in python functions are first-class citizens, so you can pass them to other functions as a parameter.
Other questions:
reverse=True
: a.sort(key=..., reverse=True)
itemgetter
with multiple indices: operator.itemgetter(1,2)
, or with lambda: lambda elem: (elem[1], elem[2])
. This way, iterables are constructed on the fly for each item in list, which are than compared against each other in lexicographic(?) order (first elements compared, if equal - second elements compared, etc)a[2,1]
(indices are zero-based). Using operator... It's possible, but not as clean as just indexing.Refer to the documentation for details: