C++ merge sort passed by value - c++

Question

I am trying to recursively implement the merge sort algorithm by only passing in a vector value to the function (no left or right index). The while loop in the following code works when the list to be sorted is passed as a pointer void merge_sort_array(int* v, int l, int r) or reference void merge_sort_ref(vector<int>& v, int l, int r) but I cannot for the life of me understand why the following code will not properly sort my list. I have a feeling it is something to do with either the starting values of i, j, k or the bounds within my while loop but I've tried anything that makes sense to me and can't figure it out.
#include <iostream>
#include <ctime>
#include <cstdlib>
#include <vector>
#include <algorithm>
using namespace std;
vector<int> merge_sort_value(vector<int> v) {
int n = v.size();
if(n == 1){
return v;
}
else{
int m = n/2;
vector<int> v1(v.begin(), v.begin()+m);
vector<int> v2(v.begin()+m, v.begin()+n);
merge_sort_value(v1);
merge_sort_value(v2);
vector<int> tmp(v.begin(), v.begin()+m);
int i = 0;
int j = m;
int k = 0;
while((i < m) or (j < n)){
if(i == m){
v[k] = v[j];
j +=1;
}
else if((j == n) or (tmp[i] < v[j])){
v[k] = tmp[i];
i+=1;
}
else{
v[k] = v[j];
j+=1;
}
k+=1;
# print output for debugging
for(auto x = v.begin(); x != v.end(); ++x)
cout << *x << " ";
cout << "" << endl;
cout << i << "\t"<< j << "\t" << k << endl;
}
return v;
}
}
int main(int argc, char** argv) {
vector<int> v(10);
for(int i=0; i < 10; ++i)
v[i] = rand() % 100;
v = merge_sort_value(v);
return 0;
}
I have included a sample output for reference below:
28 28
0 2 1
28 80
1 2 2
21 21
0 2 1
21 92
1 2 2
14 92 21
1 1 1
14 92 21
1 2 2
14 92 21
1 3 3
14 28 14 92 21
0 3 1
14 80 14 92 21
1 3 2
14 80 28 92 21
2 3 3
14 80 28 92 21
2 4 4
14 80 28 92 21
2 5 5
21 57
1 1 1
21 57
1 2 2
78 83
1 1 1
78 83
1 2 2
78 78 83
0 2 1
78 83 83
0 3 2
78 83 96
1 3 3
21 57 96 78 83
1 2 1
21 57 96 78 83
2 2 2
21 57 96 78 83
2 3 3
21 57 96 78 83
2 4 4
21 57 96 78 83
2 5 5
21 28 14 92 21 21 57 96 78 83
0 6 1
21 57 14 92 21 21 57 96 78 83
0 7 2
21 57 80 92 21 21 57 96 78 83
1 7 3
21 57 80 28 21 21 57 96 78 83
2 7 4
21 57 80 28 14 21 57 96 78 83
3 7 5
21 57 80 28 14 92 57 96 78 83
4 7 6
21 57 80 28 14 92 21 96 78 83
5 7 7
21 57 80 28 14 92 21 96 78 83
5 8 8
21 57 80 28 14 92 21 96 78 83
5 9 9
21 57 80 28 14 92 21 96 78 83
5 10 10
Thank you, any help is greatly appreciated!

Answer 1

after reviewing you code it seems you're making mistakes in the algorithm it self and in C++ as language so i've edited your algorithm to be more neat and more readable algorithm i will explain some part of the code
Code
#include <iostream>
#include <ctime>
#include <cstdlib>
#include <vector>
#include <algorithm>
using namespace std;
vector<int> merge_sort_value(vector<int> v) {
int n = v.size();
if(n == 1){
return v;
}
else{
int m = n/2;
vector<int> v1(v.begin(), v.begin()+m);
vector<int> v2(v.begin()+m, v.begin()+n);
v1 = merge_sort_value(v1); /* passing by value will left v1 with no sorting so you need to copy from the returned
object */
v2 = merge_sort_value(v2);
int i = 0;
int j = 0;
int k = 0;
const size_t left_vecS = v1.size();
const size_t right_vecS = v2.size();
while (i<left_vecS&&j<right_vecS) { // we must keep i (AND) j valid
if (v1[i] < v2[j])
v[k++] = v1[i++];
else
v[k++] = v2[j++];
}
while(i<left_vecS) // if we sorted v2 then what insert the rest of v1 in v as what kept from v1 will be sorted
v[k++] = v1[i++];
while(j<right_vecS)
v[k++] = v2[j++];
}
return v;
}
int main(int argc, char** argv) {
vector<int> v(10);
std::vector<int> x;
for(int i=0; i < 10; ++i)
v[i] = rand() % 100;
v = merge_sort_value(v);
for(auto&i:v)
std::cout << i << std::endl;
return 0;
}
1- I get rid of the printing inside the sorting function so we keep the code clean
2-
the first error you've did at the language level is you didn't copy the returned sorted vector object from merge_sort_value to the vectors.(i've mentioned that in the code in a comment) so that's the first thing to keep in mind
3- the logic part of the algorithm wasn't clear to me because i didn't see how you're sorting specially that part else if ((j == n) or (tmp[i] < v[j])) {
v[k] = tmp[i];
i += 1;
}
like you're comparing unsorted sub vector to another unsorted vector and you're giving it unsorted value again (you must compare v1 against v2)
the whole logic is missed up i think you need to review it
anyway i hope that helped