Kruskal's algorithm implementation

Question

I'm tying to implement Kriskal's algorithm in C++ but...

Unhandled exception at 0x0127160d in DAA.exe: 0xC0000005: Access violation reading location 0xdd2021d4.

It stop on this line in getRoot function:

while(cities[root].prev != NO_PARENT)

I think that the problem is with data in cities array. When I prinf all data in array it's not what I want to be. The names of cities are like this "════════════════¤¤¤¤ллллллллю■ю■" and numbers (int) - like this (-842150451). Below is full code.

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>

#define NO_PARENT -1

struct city {
    char name[11];
    int prev;
};

struct path {
    unsigned i, j, price;
};

bool comparsion(path p1, path p2) {
    return p1.price > p2.price;
}

int getRoot(city *cities, int cityNumber) {
    int root = cityNumber, tmp;

    while(cities[root].prev != NO_PARENT)
        root = cities[root].prev;

    while(cityNumber != root) {
        tmp = cityNumber;
        cityNumber = cities[cityNumber].prev;
        cities[tmp].prev = root;
    }

    return root;
}

bool isListed(city *cities, int n, char cityName[]) {
    for(int i = 0; i < n; i++)
        if(strcmp(cities[i].name, cityName))
            return true;
    return false;
}

int getCityNumber(city *cities, int n, char cityName[]) {
    for(int i = 0; i < n; i++)
        if(strcmp(cities[i].name, cityName))
            return i;
    return NO_PARENT;
}

int minPrice(city *cities, path *paths, int cityCount, int pathCount) {
    unsigned minPrice = 0;
    // sort paths by price
    std::sort(paths, &paths[pathCount-1], comparsion);

    for(int k = 0; k < pathCount; k++) {
        printf("path: %d - %d\n", paths[k].i, paths[k].j);
        int c1 = getRoot(cities, paths[k].i), c2 = getRoot(cities, paths[k].j);
        if(c1 != c2) {
            minPrice += paths[k].price;
            cities[c2].prev = c1;
        }
    }

    return minPrice;
}

    int main() {
    int n, m, k;
    do {
        scanf("%d %d %d", &n, &m, &k);
    } while(n < 2 || n > 10001 || m < -1 || m > 100001 || k < -1 || k > 100001);

    city* cities = (city*)malloc(n*sizeof(city));
    path* paths = (path*)malloc((m + k)*sizeof(path));
    int addCities = 0;
    char city1[11], city2[11];
    for(int i = 0; i < (m + k); i++) {
        scanf("%s %s", city1, city2);

        if(addCities < n && !isListed(cities, n, city1)) { // if city1 is not into cities
            // add it
            strcpy(cities[addCities].name, city1);
            cities[addCities].prev = NO_PARENT;
            addCities++;
        }
        paths[i].i = getCityNumber(cities, n, city1); // number of city1

        if(addCities < n && !isListed(cities, n, city2)) { // if city2 is not into cities
            // add it
            strcpy(cities[addCities].name, city2);
            cities[addCities].prev = NO_PARENT;
            addCities++;
        }
        paths[i].j = getCityNumber(cities, n, city1); // number of city2

        if(i >= m)
            scanf("%d", &paths[i].price);
    }

    for(int i = 0; i < (m + k); i++)
        printf("%s: %d\n", cities[i].name, cities[i].prev);

    // Calculate min price
    printf("%d ", minPrice(cities, paths, n, k + m));

    system("pause");
    return 0;
}

Answer 1

you have to initialize the "cities". There are (m+k) paths between n cities but this doesn't necessarily means that all n cities are included in these paths, since you have set the prev member of a city to NO_PARENT whenever it's listed as city1 or city2, when a city is never listed as those its prev member would be undefined and when you use it as an index in getRoot function while(cities[root].prev != NO_PARENT) root = cities[root].prev; this would cause the problem.

Answer 2

In isListed() and getCityNumber() you use strcmp() to check string equality. There's two problems with the way you're doing it:

1. strcmp returns 0 when two strings are equal, therefore you need to check if( strcmp(...) == 0 ). It's one of these weird things in C.

2. After malloc'ing you need to set cities[i].name to something e.g. "unnamed" or just "\0". Otherwise, strcmp will get called on uninitialised strings - and if they don't contain a null character within 11 chars, it will fail. Add this code after the malloc lines:

   for( int i = 0 ; i < n ; ++ i ) {
       cities[ i ].name[ 0 ] = '\0';
       cities[ i ].parent    = NO_PARENT;
   }