为什么我的代码比它应该产生的多一个？

Question

I am trying to solve the following problem: Your task is to construct a building which will be a pile of n cubes. The cube at the bottom will have a volume of n^3, the cube above will have volume of (n-1)^3 and so on until the top which will have a volume of 1^3.

You are given the total volume m of the building. Being given m can you find the number n of cubes you will have to build?

The parameter of the function findNb (find_nb, find-nb, findNb) will be an integer m and you have to return the integer n such as n^3 + (n-1)^3 +... + 1^3 = m if such an exists or -1 if there is no such n.

Examples:

findNb(1071225) --> 45
findNb(91716553919377) --> -1

Here is my code:

def find_Nb(m):
    n = 1
    while n <= m // 2:
        total_n = 0
        for i in range(0, n):
            total_n += (n - i) ** 3
        n += 1
    
        if total_n == m:
            return n
            break
    
        else: 
            return -1

For some reason, the output produces (n+1) in the case where total_n ==m. So for example, if we do m = 100, the output should be 4; but the above code produces 5; any idea why this is?

Or as per the example, when I run m=1071225 into the code I should get an output of 45, but my code produces 46.

Answer 1

this is the code that I wrote and it works but when I first tried it I had the same problem as you. I fixed it by putting the n = i before i+=1 so that n has the right value so I think you too have to make a copy of n before you say n += 1 and to return that value.

def findN(m):
    sum = 0
    i = 1
    while sum < m:
        sum += i ** 3
        n = i
        i += 1
    if sum == m:
        return n
    else:
        return -1
print(findN(1071225))