I am trying to implement the A* algorithm in Python as a learning exercise. I have a list of nodes, called open, that contains strings that represent the names of graph nodes. Example: open["node_a", "node_b", "node_c"]. I also have two defaultdict dictionaries, f_estim and g, that contain values like {"name_of_node" : integer_representing_a_certain_cost_associated_to_the_node}. I am required to sort the open list ascending by the value associated in f_estim, and if that value is equal for two nodes, descending according to the value in g. I am sorting ascending like so: open.sort(key=lambda node: f_estim[node]). How can i also sort descending when reaching two equal values? I could not find the answer in the documentation of the sort function. Thank you!
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M_S
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have you tried sorted funtion [example](https://stackoverflow.com/questions/19199984/sort-a-list-in-python) – Shubham Shaswat Mar 21 '20 at 11:00
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Please add full examples (input and expected output) – Dani Mesejo Mar 21 '20 at 11:02
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3Your lambda can return a tuple with primary criterion first and negative of secondary criterion second. – Michael Butscher Mar 21 '20 at 11:03
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Have you tried doing something like this ? open.sort(key=lambda node: (f_estim[node], -g[node)) – Osama A.Rehman Mar 21 '20 at 11:03
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Examples: open ["a", "b", "c", "d"] f_estim {"a" : 5, "b" : 5, "c" : 4, " d" : 4} g: {"a" : 1, "b" : 2, "c" : 3, " d" : 4} after open.sort(???), i want to obtain open["d", "c", "b", "a"] – M_S Mar 21 '20 at 11:09
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The tuple suggestion is the solution to my problem. Thank you very much everyone for your help! – M_S Mar 21 '20 at 11:13
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You could do something like this:
open.sort(key=lambda node: (f_estim[node], -g[node]))
Osama A.Rehman
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Note that this will only work for ordering criteria that are numeric, and not for other types of comparable criteria (e.g. strings) – newacct Sep 13 '20 at 03:51