Merge Sort Using Vectors (int) C++

Question

I can't seem to figure out what's wrong with my code. As the title says, I am trying to implement merge sort using integer vectors.

HEADER:

using Val = int;
using Container = std::vector<Val>;
using Iter = long;
void mergeSort(Container& arr, Iter start, Iter end);

.CPP (I've included the definition of merge in the file, just not pictured here)

void mergeSort(Container& arr, Iter start, Iter end) {

if (start >= end) return;

int mid = (start + end) / 2;

mergeSort(arr, start, mid);
mergeSort(arr, mid + 1, end);
merge(arr, start, end, mid);


}

void merge(Container& arr, Iter start, Iter end, Iter mid)
{

int len = end - start + 1;
int x = 0;
Container temp;

temp.resize(arr.size());

int i, j, k;
i = start;
k = start;
j = mid + 1;

while (i <= mid && j <= end)
{
    if (arr[i] < arr[j])
    {
        temp[k++] = arr[i++];
    }
    else
    {
        temp[k++] = arr[j++];
    }
}



while (i <= mid) temp[k++] = arr[i++];


while (j <= end) temp[k++] = arr[j++];

for (k = 0; k < len; k++) arr[k + start] = temp[k];

}

many thanks!

Answer 1

I think there might be four problems with your code.

1. You assume the sequence <start,mid> and <mid+1,end> is sorted. If this condition does not hold (eg merge(v,0,3,2) on {6,5,7,4} ) the algorithm will give incorrect results.
2. You use incorrect end value when using a function (eg merge(v,0,4,2) on the array {6,5,7,4} . You always have to iterate over <0,size-1> .
3. As already said k should be always initialized to 0. You want to insert the sorted sequence to the beginning of the sorted array.
4. You assume that the argument mid is the index , not position of the element in the array. For example merge(v,0,3,2) would yield incorrect result on {1,6,2,4} , because in the function you sort the sequence from index mid+1=2+1=3 to 3 , which contains only {4} . Therefore, your first part {1,6,2} is not sorted, which is required by your algorithm.

The solution is to:

1. Initialize k to 0.
2. Check if mid<end .
3. Sort <0,mid> and <mid+1,end> using another sorting algorithm.

Answer 2

Only looking at your code, one problem I think is the initial value of k in the merge function.

If you put the temp value in the Container temp starting from position 0 , then you should initialize k to 0

k = 0;

Or if you want the position starting from start , then you would need to change the last for loop to

for (k = start; k <= end; k++) arr[k] = temp[k];

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However, please post more information about the problem you encounter.. is it a Compilation Error? or a Runtime Error? or the result doesn't match your expectation? In addition, show what have you done to solve the problem:)