查找列表的最大绝对值的Pythonic方法

Question

The following list is given:

lst = [3, 7, -10]

I want to find the maximum value of absolute value. For the above list it will be 10 (abs(-10) = 10).

I can do it as follows:

max_abs_value = lst[0]
for num in lst:
    if abs(num) > max_abs_value:
        max_abs_value = abs(num)

What are better ways of solving this problem?

Answer 1

The built-in max takes a key function, you can pass that as abs :

>>> max([3, 7, -10], key=abs)
-10

You can call abs again on the result to normalise the result:

>>> abs(max([3, 7, -10], key=abs))
10

Answer 2

Use map , and just pass abs as your function, then call max on that:

>>> max(map(abs, [3, 7, -10]))
10

Answer 3

max(max(a),-min(a))

It's the fastest for now, since no intermediate list is created (for 100 000 values):

In [200]: %timeit max(max(a),-min(a))
100 loops, best of 3: 8.82 ms per loop

In [201]: %timeit abs(max(a,key=abs))
100 loops, best of 3: 13.8 ms per loop

In [202]: %timeit max(map(abs,a))
100 loops, best of 3: 13.2 ms per loop

In [203]: %timeit max(abs(n) for n in a)
10 loops, best of 3: 19.9 ms per loop

In [204]: %timeit np.abs(a).max()
100 loops, best of 3: 11.4 ms per loop

Answer 4

You can use max() with a generator comprehension:

>>> max(abs(n) for n in [3, 7, -10])
10
>>> 

ProTip: Try to avoid naming your variables after builtin names such as list . Rename to something else like lst or L .