为什么我的基数排序会出现 C6385 错误

Question

I am trying to write my own version of radix sort in order to better understand the algorithm. The problem is that in the first inner for loop("j") i get the C6385 error in VS. I have no idea of how to rephrase that line in order to make it work. What am i doing wrong here?

The warning is:

warning C6385: Reading invalid data from 'countQueues': the readable size is '400' bytes, but '4000' bytes may be read.

void radixSort(int arr[], int arraySize)
{
    int countDigits = GetMax(arr, arraySize);   //max number of digits
    queue<int> countQueues[10];
    int modulo = 1;

    for (int i = 0; i < countDigits; i++)
    {
        modulo *= 10;
        for (int j = 0; j < arraySize; j++)     //store the values in the queues
        {
            countQueues[arr[j] % modulo].push(arr[j]);  //error here
        }


        for (int k = 0; k < 10; k++)        // move them back in the array
        {
            while (!countQueues[k].empty())
            {
                arr[i] = countQueues[k].front();
                countQueues[k].pop();
            }
        }
    }
}

Answer 1

Consider the second iteration of the first loop when modulo = 100. Then this line of code:

            countQueues[arr[j] % modulo].push(arr[j]);  //error here

could be trying to access countQueues[up to 99], but countQueues has a size of 10.

This is an issue with trying to implement radix sort most significant digit first. The 1st iteration uses 10 queues, 2nd iteration 100 queues, 3rd iteration 1000 queues, ... , which could be simplified using recursion, but would consume a lot of space, 10 raised to the countDigits power.

Implementing a radix sort least significant digit first avoids this problem. A [countDigits][10] matrix could be used. The initial pass generates counts and stores them into the matrix, where matrix[i][] is an array of 10 counts for the number of occurrences of each digit, 0 through 9. The counts are converted into the starting indexes for each logical bin of matrix[i]. The radix sort is then done in countDigits passes, least significant digit/bin first.