Question

Here’s what I’m trying to do.
There are 3 arrays, cost[] node1[] and node2[].
These entires correspond to edges of a graph with node1[i],node2[i] and cost[i] specifying that there is an edge going from vertex node1[i] to node2[i] with an edge weight of cost[i].

I’m trying to sort these edges with respect to their weights, i.e sort the cost[] array using merge-sort. However whenever I’m, changing an entry in the cost[] array I also want to change the corresponding entries in the node1 and node2 array since even the nodes of the graph have to be modified. Ie if node1[]=1,2,3 and node2[]=2,3,1 cost[]={7 4 8} then after sorting the cost array the node1 and node2 should look like node1[]=2,1,3 node2[]=3,2,1. and cost[]=4,7,8

Here’s my code.

        #include<stdio.h>
        #include<stdlib.h>
        int merge_sort(int arr[],int low,int high,int node1[],int node2[])

        {

        int mid;

        if(low<high) {

        mid=(low+high)/2;

        // Divide and Conquer

        merge_sort(arr,low,mid,node1,node2);

        merge_sort(arr,mid+1,high,node1,node2);

        // Combine

        merge(arr,low,mid,high,node1,node2);

        }



        return 0;

        }



        int merge(int arr[],int l,int m,int h,int node1[],int node2[])

        {

        int arr1[80000],arr2[80000]; // Two temporary arrays to
        int arr3[70000],arr4[70000];
        int arr5[70000],arr6[70000];
        int n1,n2,i,j,k;

        n1=m-l+1;

        n2=h-m;



        for(i=0; i<n1; i++)
        {

        arr1[i]=arr[l+i];
        arr3[i]=node1[l+i];
        arr5[i]=node2[l+i];

        }
        for(j=0; j<n2; j++)
        {
        arr2[j]=arr[m+j+1];
        arr4[i]=node1[m+j+1];
        arr6[i]=node2[m+j+1];
        }


        arr1[i]=99999; // To mark the end of each temporary array
        arr2[j]=99999;
        arr3[i]=99999;
        arr4[j]=99999;
        arr5[i]=99999;
        arr6[j]=99999;


        i=0;

        j=0;

        for(k=l; k<=h; k++) { //process of combining two sorted arrays

        if(arr1[i]<=arr2[j])
        {

        arr[k]=arr1[i++];
        //node1[k]=arr3[i++]; COMMENTED LINES!!!!!!!!!!!
        //node2[k]=arr5[i++];

        }
        else
        {
        arr[k]=arr2[j++];
        //node1[k]=arr4[j++]; COMMENTED LINES!!!!!!!!~!
        //node2[k]=arr6[j++];
        }
        }
        return(0);
        }

        int main(void)
        {
            int i,j,n,vert1,vert2,weight;
            scanf("%d",&n);
            int adjmat[n+1][n+1],cluster[n+1][n+1];
            int *cost,*node1,*node2;
            node1=malloc(sizeof(int)*1000000);
            node2=malloc(sizeof(int)*1000000);
            cost=malloc(sizeof(int)*1000000);
            for(i=0;i<n+1;i++)
                for(j=0;j<n+1;j++)
                {
                    adjmat[i][j]=0;
                    cluster[i][j]=0;
                }
            for(i=1;i<n+1;i++)
                cluster[i][0]=i;
            for(i=1;i<(n+1)*(n+1);i++)
            {
                scanf("%d %d %d",&vert1,&vert2,&weight);

                node1[i]=vert1;
                node2[i]=vert2;
                cost[i]=weight;
                if(node1[i]==node1[i-1] && node2[i]==node2[i-1] && cost[i]==cost[i-1])
                    break;
                //  printf("%d %d %d\n",node1[i],node2[i],cost[i]);
                adjmat[vert1][vert2]=weight;
                adjmat[vert2][vert1]=weight;
            }
            printf("\n%d\n",i);
            merge_sort(cost,1,124751,node1,node2);
            for(j=1;j<i;j++)
                printf("%d %d %d\n",node1[j],node2[j],cost[j]);
            return(0);
        }

Whenever I comment the lines in the merge function the code manages to sort the cost array. However whenever I un comment these lines somehow everything gets equated to 0. i.e all entires of the node1 node2 and cost arrays are 0. Could anyone tell me why this is happening? Thanks!

Answer 1

You probably have forgotten to take care of the side effect of the i++ operation. There is no need at all at that place to work with side effects, don’t do that.