Question

Below is my code:

typedef struct
{
  unsigned page;
  unsigned slot; 
} RID;

//Below struct has the Key on which I want to apply the sorting
struct LeafDataEntry
{
    void *key;
    RID rid;
};

//This is the sorting function I am using
bool leadNode_Key_asc( const LeafDataEntry &a, const LeafDataEntry &b){

    return strcoll((char *)a.key, (char *)b.key) > 0;
            //(strcmp((char *)a.key, (char *)b.key) > 0);
}

int main(){

    vector<LeafDataEntry> lde;

    char a[4] = {'D', 'B', 'C', 'D'};
    RID aRID = {0,0};
    char b[4] = {'A', 'C', 'B', 'A'};
    RID bRID = {0,1};

    unsigned size = sizeof(unsigned);

    lde.resize(2);
    char *tempPtr = (char *)malloc(8 + sizeof(RID));

    memcpy(tempPtr, &size, 4);
    tempPtr += 4;
memcpy(tempPtr, a, 4);

    tempPtr -= 4;
    lde[0].key = malloc(8);
    memcpy(lde[0].key, tempPtr, 8);
    memcpy(&lde[0].rid, &aRID, sizeof(RID));

    memcpy(tempPtr, &size, 4);
    tempPtr += 4;
    memcpy(tempPtr, b, 4);

    tempPtr -= 4;
    lde[1].key = malloc(8);
    memcpy(lde[1].key, tempPtr, 8);
    memcpy(&lde[1].rid, &bRID, sizeof(RID));

    std::sort(lde.begin(), lde.end(), leadNode_Key_asc);

    cout << "Sorted Data :: " << endl;
    for(int j=0; j<2; j++){
        cout << "KEY :: " << (char *)(lde[j].key);
        cout << ", RID ::" << "{" << lde[j].rid.pageNum << ", " <<        
    lde[j].rid.slotNum << "}";
  }
return 0;
}

I want to sort the above lde vector based on the *key value. It’s not working with the way given above.

NOTE: I cannot change the data types of any of the structs listed above.

Answer 1

Well there are two problems in your code:

first, you’re saving string in key of LeafDataEntry as [4 bytes for size of the string] + [the string] but strcoll takes strings ended with ‘\0’ and without size leading them, as you said that you don’t want any change in your datatypes, this would solve your problem:

bool leadNode_Key_asc( const LeafDataEntry &a, const LeafDataEntry &b){
    size_t size1 = *(unsigned*)(a.key), size2 = *(unsigned*)(b.key);
    string a1 = string((char*)a.key + 4, size1), a2 = string((char*)b.key + 4, size2);
    return a1 < a2;
}

second, these two lines:

    cout << ", RID ::" << "{" << lde[j].rid.pageNum << ", " <<        
    lde[j].rid.slotNum << "}";

should be replaced with:

    cout << ", RID ::" << "{" << lde[j].rid.page << ", " <<        
    lde[j].rid.slot << "}";