Question

this is my mergesort function, I can not understand why am i getting always a “vector subscript out of range” error in VC++ express 2010.

vector<int> Ordinatore::doMerge(vector<int> &v1, vector<int> &v2) {
int dimV1 = v1.size();
int dimV2 = v2.size();

int dimTotale = dimV1 + dimV2;
vector<int> merged; 

int lh, rh;
lh=0;rh=0;


while(lh < dimV1)  {


    if (rh < dimV2 && v1[lh] <= v2[rh]) {

        merged.push_back(v1[lh]);

        lh++;
        /* the trouble should start here as far as I've seen with cout & debugging */
        while (rh < dimV2  && v2[rh] <= v1[lh]) {

            merged.push_back(v2[rh]);

            rh++;

        }

    }
    else {

        while (rh < dimV2  && v2[rh] <= v1[lh]) {
            merged.push_back(v2[rh]);
            rh++;
        }
        merged.push_back(v1[lh]);

        lh++;

    }
}

return merged;
}

Answer 1

The problem is in the top of your loop:

while(lh < dimV1)  {
    if (rh < dimV2 && v1[lh] <= v2[rh]) {
        merged.push_back(v1[lh]);
        lh++;                                       // |<-- Problem
        while (rh < dimV2  && v2[rh] <= v1[lh]) {   // |
           ...
        }
    }
    else {
        ...
        lh++;
    }
}

When that lh++ in the else is executed, lh is incremented and the while(lh < dimV1) restarts. Lets assume that it’s made it all the way to lh == dimV1 - 1…

• If the if condition evaluates to true, lh++ is executed right were you’ve marked “the trouble should start here“.
• The next line while (rh < dimV2 && v2[rh] <= v1[lh]) will cause a crash because lh now equals dimV1 but you’re using it to index v1.

You can read more about Merge Sorts in this Wikipedia Article.