使用递归：Python在列表中添加最小和最大数字

Question

I wrote a recursive function to find the summation of a list, here is my code:

def rsum (eleList):
    if len(eleList) == 1:
        return eleList[0]
    else:
        return eleList[0] + rsum(eleList[1:])

However, right now I want to write a recursive function to find the summation of the max and min of a list, and I have no clue where I should start. Could anyone give me some hint?

Answer 1

If you want to write a recursive function, you have to figure out how to solve this problem based on having solved a smaller problem. You know that the sum of this list is the first element plus the sum of the rest of the list. If you have a list:

[1,2,3,4,5]

and you know that the max of the last four elements is 5 and the first element is 1 , and you want to find the total maximum, how do you do that in a constant number of operations?

Answer 2

I would search for maximum and minimum value in each iteration. And if that is the case, I will return the sum.

Something like:

def rsum(eleList, ans, index):
    if index == len(eleList) - 1:
        if max(eleList) == eleList[index] or min(eleList) == eleList[index]:
            return ans+eleList[index]
        else:
            return ans
    else:
        if max(eleList) == eleList[index] or min(eleList) == eleList[index]:
            return ans + eleList[index] + rsum(eleList, ans, index + 1)
        else:
            return rsum(eleList, ans, index + 1)

print rsum([9, 2, 3, 4], 0, 0)

Output:

11

Might not be the smartest one or most pythonic but it gets things done.

Answer 3

For finding the sum of all the elements of the list use sum function:

temp = [1,2,3,4]
sum(temp)
#output = 10

If you want to find sum of min and max elements, get the list sorted and add first and last element:

temp = [1,2,3,4]
sorted_list =     sorted(temp)
total = sorted_list[0] + sorted_list[-1]

Avoid making your own functions whenever there is a possibility of built-in function being present.