I want to sort lists of mathematical operators (stored as strings) in order of precedence (*,/,+,-) within a list. There are thousands of lists within my list.
E.g.
my_lists = [['*','+','-'],['-','*','*'],['+','/','-']]
should become:
new_list = [['*','+','-'],['*','*','-'],['/','+','-']]
Is there a way to sort the lists within my list in a user defined order?
You can define the priority with a dictionary, like this
>>> priority = {'*': 0, '/': 1, '+': 2, '-': 3}
and then sort the individual lists, with the value from the priority, like this
>>> [sorted(item, key=priority.get) for item in my_lists]
[['*', '+', '-'], ['*', '*', '-'], ['/', '+', '-']]
The idea is to create a new list of operators and store the operators in the desired precedence you want(you may change the precedence anytime), and then use the index of operators as a key to do the sorting.
You may simply do it by passing a key to the sorted function as:
my_lists = [['*','+','-'],['-','*','*'],['+','/','-']]
operators = ['*', '/', '+', '-']
new_list = [sorted(i, key = operators.index) for i in my_lists]
>>> [['*', '+', '-'], ['*', '*', '-'], ['/', '+', '-']]
however you mentioned a tuple in the question as (*,/,+,-), so I tried to solve the issue using list, but using a dictionary would be a cleaner way as @thefourtheye suggested.
key=operators.index will work. ;)
operators is a little lengthier, operators.index would eat up a lot of time.
You have to use the optional key argument of the .sort() method.
If you want to do this in-place:
for sublist in my_lists:
sublist.sort(key="*/+-".index)
Or if you want to create a new list:
new_list = [sorted(sublist, key="*/+-".index) for sublist in my_lists]
Note that using dictionnary may be more efficient if you have many lists:
d = {"*": 0, "/": 1, "+": 2, "-": 3}
key = d.get
