Sorting a list of based on the 2nd element of a tuple

Question

I have a dictionary and want to convert it to a list. Then I would like to sort the resulting list consisting of {Key, Value} pairs from min to max depending on the 2nd element(Value).

Is there a built in sort method for Lists to handle this or how does one do this?

Thanks

Answer 1

The easiest way to sort by the second element would be to define your own sorting function that could work as follows:

fun({KeyA,ValA}, {KeyB,ValB}) -> {ValA,KeyA} =< {ValB,KeyB} end.

And call it in lists:sort/2:

1> lists:sort(fun({KeyA,ValA}, {KeyB,ValB}) -> {ValA,KeyA} =< {ValB,KeyB} end., [{a,b},{b,a},{b,b}]).
[{b,a},{a,b},{b,b}]

This is because Erlang will always automatically compare tuples from first to last element. This function swaps the first and the second element so the second one acts as the first point of comparison. The Key in your dict will then be used to order entries where values are the same.

Answer 2

The function lists:keysort/2 fits like glove for this.

1> lists:keysort(2, [{a,b},{b,a},{b,b}]).

[{b,a},{a,b},{b,b}]

2> lists:keysort(2, [{1,14},{3,10},{2,13}]).

[{3,10},{2,13},{1,14}]