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# A question about Merge Sort

Hey,

I was trying to write a merge sort in C++, and this is what I got:

index size(number_sequence S) { return S.length; }

void merge_sort(number_sequence S) {
index n = size(S);
if (n < 2) return;
index mid = n/2;

// Sort the two halves separately.
merge_sort(subseq(S, 1, mid));
merge_sort(subseq(S, mid+1, n));

// Merge the two sorted halves.
merge(S, mid);
}

``````void merge(number_sequence S, index mid) { // Solve the Merge problem.

index n =size(S);
number* p = new number[n];
number_sequence A, B;
A.init(p, mid);
B.init(p+mid, n-mid);
copy(A, subseq(S, 1, mid));
copy(B, subseq(S, mid+1, n));

index a=1, b=1;
for (index k = 1; k <= n; ++k) {
if (a <= mid and b <= n-mid)
if (A[a] < B[b]) S[k] = A[a++];
else             S[k] = B[b++];
else if (a <= mid)
S[k] = A[a++];
else
break;
}
delete p; // Deallocate work space.
}
``````

But, my professor says it copies things more than it should... but I don't understand why that would be... I built my code from what I found here http://en.wikipedia.org/wiki/Merge_sort .

Could anyone give me a hint as to where I messed up?

0
errang
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2 Solutions

Commented:
subseq ??
Could you post your whole buildable program including test driver.
0

Commented:
You're copying S into A and B. Couldn't you operate on S directly, and then push the selected element to a single new sequence N?

for (index k = 1; k <= n; ++k) {
if (a <= mid and b <= n-mid)
if (S[a] < S\[b\]) N[k] = S[a++];
else             N[k] = S[b++];

0

Author Commented:
No... He was kinda specific about not touching anything else...

But... I was thinking,

``````for (index k = 1; k <= n; ++k) {
if (a <= mid and b <= n-mid)
if (A[a] < B[b]) S[k] = A[a++];
else             S[k] = B[b++];
else if (a <= mid)
S[k] = A[a++];
else
break;
}
``````

this loop is going from k = 1 to k<=n... but all a merge sort needs is to split the given sequence in half and sort the halves... right?

Would it be as simple as changing the boundaries of the loop? It seems to be copying things more than it should.
0

Commented:
the first unneeded copy is cause you pass the S by value and not by reference.

the second is the p array where i can't see any practical benefit (and must be delete []p; at end)

then A and B were used only for read purposes. it was some time ago when i implemented merge sort myself. but i am pretty sure you don't need the A and B but only a swap function, cause you can do the read operations on the input sequence.

what i don't understand is that all assignments were made to S which is a local copy. how does the caller get the sorted values back? is the copy constructor of number_sequence using same internal pointer????

Sara
0

Author Commented:
>>You're copying S into A and B. Couldn't you operate on S directly, and then push the selected element to a single new sequence N?

Sorry... I'm not sure I follow that.
0

Author Commented:
>>what i don't understand is that all assignments were made to S which is a local copy. how does the caller get the sorted values back? is the copy constructor of number_sequence using same internal pointer????

Yea, number_sequence is a struct.
0

Commented:
is it C or C++?

in case of c++ a struct is same as a class and if you don't provide an own copy constructor the compiler creates one for you. all those copies would use same pointer what means you can't delete it in destructor. that is really bad code technique even if it is c. In c you would have one instance of structure and pass a pointer to that structure. in c++ you pass the struct/class by reference.

Sara
0

Author Commented:
It is C++
0

Commented:
>>>You're copying S into A and B. Couldn't you operate on S directly, and then push the selected element to a single new sequence N?

>Sorry... I'm not sure I follow that.

What I was proposing is that you use S directly in the central loop; rather than first copying to A and B; see the code snippet I provided.

Sarabande point about passing by value, rather than by reference, is also relevant.
0

Author Commented:
>>What I was proposing is that you use S directly in the central loop; rather than first copying to A and B; see the code snippet I provided

Ah... I see it now, will have to change the starting index of b for that to work... I'll try that out.
0

Commented:
also change interface of functions to

void merge_sort(number_sequence & S);

void merge(number_sequence & S, index mid);

index size(number_sequence & S);

void copy(number_sequence & S1, number_sequence & S2);

number_sequence & subseq(number_sequence & S1, int start, int end);

Sara

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