Python：我试图找到列表中两个元素之间的最大差异

Question

I need to find the maximum difference in a list between any two elements. In list [1,2,3,4,5] the maximum difference is 4 (between elements 1 and 5) using for loops.

This program needs to output the location of these two elements (0 and 4) and their values (1 and 5).

I can only figure out how to find the max difference between consecutive values, but this creates a problem if the maximum starts elsewhere, eg [4,1,6,3,10,8] where the largest difference is between 1 and 10 (positions 1 and 4). Can someone help me?

Answer 1

You can use the built-in functions max and min to find the maximum and minimum values respectively and then use the list method index to find their indices in the list.

numlist = [1, 2, 3, 4, 5]

max_val = max(numlist)
min_val = min(numlist)

max_pos = numlist.index(max_val)
min_pos = numlist.index(min_val)

Answer 2

In the naive approach, you simply have two nested loops in that makes sure that each elements visits every other list element. As you only need to check every pair once, it's enough to start the inner loop at the next index every time:

lst = [1, 2, 3, 4, 5]

max_i, max_j = None, None # stores the indexes
max_d = -1 # stores the maximum distance we have seen so far

# iterate through all indexes of the list
for i in range(len(lst)):
    # iterate through all indexes, but starting from the index `i+1`
    for j in range(i + 1, len(lst)):
        d = abs(lst[i] - lst[j])
        if d > max_d:
            # memorize everything if the distance is larger than what we know
            max_i, max_j, max_d = i, j, abs(d)

print(max_i, max_j, max_d) # 0 4 4

With two nested loops, this is of course not really efficient, but this is essentially the solution for when you really need to compare every list element with each other. In your case of finding the maximum distance, as others pointed out, you only need to look at the largest and the smallest list item which can both be determined in linear time.

---

As you said in a comment above, it seems that you are only allowed to use for loops, so we can still make it efficient, by doing the min/max finding ourself in linear time, by iterating only once:

# set the current maximum and minimum to the first index
max_i, min_i = 0, 0

# iterate the list from the second index
for i in range(1, len(lst)):
    # check if we’re larger than the current maximum
    if lst[i] > lst[max_i]:
        max_i = i

    # check if we’re smaller than the current minimum
    if lst[i] < lst[min_i]:
        min_i = i

distance = lst[max_i] - lst[min_i]
print(min_i, max_i, distance) # 0 0 4

This essentially does the same as the answer by mgilson does. We just do the job of built-in functions max and min ourselves and find the minimum and maximum manually.

Answer 3

You can sort the list first, then get the minimum and maximum values. Also, use index() to get the position of an element:

L = [1, 2, 3, 4, 5]

temp = sorted(L) # sorted list

min = temp[0]
max = temp[-1] # index -1 will give the last element

Test:

print "min", min, L.index(min)
print "max", max, L.index(max)
print "difference", max - min

Output:

min 1 0
max 5 4
difference 4

Answer 4

Just subtract the maximum value from the minimum value. This is trivial in Python. A cool way to do this is with itemgetter . You can find the min/max index and value at the same time if you enumerate the items in the list, but perform min/max on the original values of the list. Like so:

>>> import operator
>>> values = [1, 2, 3, 4, 5]
>>>
>>> min_index, min_value = min(enumerate(values), key=operator.itemgetter(1))
>>> min_index, min_value
0, 1
>>> max_index, max_value = max(enumerate(values), key=operator.itemgetter(1))
4, 5
>>> difference = max_value - min_value
>>> difference
4

Answer 5

This can be accomplished using max and min + enumerate :

biggest_idx, biggest_value = max(enumerate(lst), key=lambda x: x[1])
smallest_idx, smallest_value = min(enumerate(lst), key=lambda x: x[1])

eg:

>>> lst =  [1,2,3,4,5] 
>>> biggest_idx, biggest_value = max(enumerate(lst), key=lambda x: x[1])
>>> smallest_idx, smallest_value = min(enumerate(lst), key=lambda x: x[1])
>>> print biggest_idx, biggest_value
4 5
>>> print smallest_idx, smallest_value
0 1

Answer 6

min_i = 0
max_i = 0
for i in xrange(len(alist)):
    if alist[i] < alist[min_i]:
        min_i = i
    if alist[i] > alist[max_i]:
        max_i = i
print "min=%d, max=%d" % (alist[min_i], alist[max_i])
print "min index=%d, max index=%d", (min_i, max_i)
print "difference=%d" % (alist[min_i] - alist[max_i])

Answer 7

It can be as simple as ...

numlist=[4,1,6,3,10,8]
print('min value index : ' , numlist.index(min(numlist)))
print('max value index : ' , numlist.index(max(numlist)))
print('Max Difference : ',max(numlist)-min(numlist))