Seb Seb - 6 months ago 15
Python Question

Return tuple with smallest y value from list of tuples

I am trying to return a tuple the smallest second index value (y value) from a list of tuples. If there are two tuples with the lowest y value, then select the tuple with the largest x value (i.e first index).

For example, suppose I have the tuple:

x = [(2, 3), (4, 3), (6, 9)]


The the value returned should be
(4, 3)
.
(2, 3)
is a candidate, as
x[0][1]
is
3
(same as
x[1][1]
), however,
x[0][0]
is smaller than
x[1][0]
.

So far I have tried:

start_point = min(x, key = lambda t: t[1])


However, this only checks the second index, and does not compare two tuples first index if their second index's are equivalent.

Answer

Include the x value in a tuple returned from the key; this second element in the key will be then used when there is a tie for the y value. To inverse the comparison (from smallest to largest), just negate that value:

min(x, key=lambda t: (t[1], -t[0]))

After all, -4 is smaller than -2.

Demo:

>>> x = [(2, 3), (4, 3), (6, 9)]
>>> min(x, key=lambda t: (t[1], -t[0]))
(4, 3)
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