The variable declared in for-loop of c++ is not initialised

#include <iostream>
#include <Windows.h>

void selection_sort(int*,int);
void output_array(int*, int);

int main() {
    using namespace std;
    int numbers[] = { 4, 6, 8, 2, 7, 5, 0,1, 3, 9 };
    int length = 10;

    selection_sort(numbers, length);
    output_array(numbers, length);

    Sleep(10000);
    return 0;
}

void output_array(int* start, int length) {
    for (int i = 0; i < length; i++) {
        std::cout << *(start + i) << " ";
    }
    std::cout << std::endl;
}

void selection_sort(int* start, int length) {
    for (int i = 0; i < length; i++) {
        int max = *start;
        int max_i = 0;
        for (int j = 1; j < (length - i); j++) {
            //find out the maximum
            if (max < *(start + j)) {
                max = *(start + j);
                max_i = j;
            }
        }
        //put it at the end
        for (int k = max_i; k < (length - i -1); k++) { //The problem is HERE
            *(start + k) = *(start + k + 1);
        }
        *(start + length - i) = max;
    }
}

Perhaps it is a problem simple enough, but why when it comes to the last for loop, k is undefined? Is it because max_i is not a compile-time constant?
Variables
Program at this step
This doesn’t make sense to me.

When it comes to the second largest number, k behaves as expected.

>Solution :

void selection_sort(int* start, int length) {
    for (int i = 0; i < length; i++) {
        int max = *start;
        int max_i = 0;
        for (int j = 1; j < (length - i); j++) {
            //find out the maximum
            if (max < *(start + j)) {
                max = *(start + j);
                max_i = j;
            }
        }
        //put it at the end
        for (int k = max_i; k < (length - i -1); k++) {
            *(start + k) = *(start + k + 1);
        }
        // The problem was here, you wanted to access memory that does not belong to you.
        *(start + length - i - 1) = max;
    }