Question

我有这个问题：

int nCab = 11;
int nCabCombo;
std::vector<int>counter(nCab);
for(int i = nCab; i > 0; i--)
{
    counter[i] = i-1;
    std::cout<<counter[i]<<std::endl;
}
nCabCombo = std::accumulate(counter.begin(),counter.end(),0);
std::cout<<nCabCombo<<std::endl;

nCabCombo的输出是45，当它应该是55时，由于某种原因，累加功能没有添加10在我的计数器向量的索引11处。有人能告诉我，我在做错了吗？谢谢！

Answer 1

You do not have an index 11. You have 11 elements in you vector which means the valid indexes are [0, 10]. counter[i] = i-1; is undefined behavior since you are accessing one past the end of the vector.

Ignoring the UB your vector actually contains {0,0,1,2,3,4,5,6,7,8,9} which is 45

Note: The reason it does not contain a -1 is the for loop runs for i > 0 so i will never be 0 and we will never set the first element of the vector so it remains 0 from the construction of the vector.

Answer 2

As others mentioned, you are accessing the element out of bounds.

However, instead of writing a loop and getting into this sort of trouble, you could use std::iota:

#include <vector>
#include <algorithm>

int main()
{
    int nCab = 11;
    std::vector<int>counter(nCab);
    std::iota(counter.begin(), counter.end(), 0);
    //...
}

Live Example

If for some reason you really wanted to loop and populate the array in reverse (high index to low index), you could use std::generate and use the reverse iterators:

#include <vector>
#include <algorithm>

int main()
{
    int nCab = 11;
    std::vector<int>counter(nCab);
    int i = 11;
    std::generate(counter.rbegin(), counter.rend(), [&i] {return --i;});
    //...
}

Live Example (std::generate)

Answer 3

counter[i] = i-1; //when i == 11, this is an off by one error

An array, and by extension a vector, has range from [0, size). If it has nCab elements, the maximum index is only nCab - 1. You're storing the last element outside the bounds of the vector. You should change it to:

 counter[i - 1] = i-1;

The missing element would have been 10. When accumulate runs, that is why your sum is off by 10.

累积不给出我的矢量的正确总和

3 个答案: