rksh - 1 year ago 213
Python Question

# Find maximum value and index in a python list?

I have a python list that is like this,

``````[[12587961, 0.7777777777777778], [12587970, 0.5172413793103449], [12587979, 0.3968253968253968], [12587982, 0.88], [12587984, 0.8484848484848485], [12587992, 0.7777777777777778], [12587995, 0.8070175438596491], [12588015, 0.4358974358974359], [12588023, 0.8985507246376812], [12588037, 0.5555555555555555], [12588042, 0.9473684210526315]]
``````

This list can be up to thousand elements in length, how can I get the maximum value in the list according to the second item in the sub-array, and get the index of the maximum value which is the fist element in the sub-array in python?

Use the `max` function and its `key` parameter, to use only the second element to compare elements of the list.

For example,

``````>>> data = [[12587961, 0.7777777777777778], [12587970, 0.5172413793103449], [12587979, 0.3968253968253968].... [12588042, 0.9473684210
526315]]
>>> max(data, key=lambda item: item[1])
[12588042, 0.9473684210526315]
``````

Now, if you want just the first element, then you can simply get the first element alone, or just unpack the result, like this

``````>>> index, value = max(data, key=lambda item: item[1])
>>> index
12588042
>>> value
0.9473684210526315
``````

Edit: If you want to find the maximum index (first value) out of all elements with the maximum value (second value), then you can do it like this

``````>>> _, max_value = max(data, key=lambda item: item[1])
>>> max(index for index, value in data if value == max_value)
``````

You can do the same in a single iteration, like this

``````max_index = float("-inf")
max_value = float("-inf")

for index, value in data:
if value > max_value:
max_value = value
max_index = index
elif value == max_value:
max_index = max(max_index, index)
``````
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