Sorting array of structures in C error

Question

I am writing a phonebook application in C. I have an array (sort2) of structure Nodes (contacts). I am trying to sort the array by nodes->pnum (phone number); s is the number of nodes in array. For some reason, the loop creates an error after first iteration. Why?

for(i=0; i<s;i++)
{
    for(j=0;j<s;j++)
    {
       num1= sort2[i];
       num2= sort2[j];
       if(num1->pnum<num2->pnum)
       {
            temp=sort2[j];
            sort2[j]=sort2[j+1];
            sort2[j+1]=temp;
            printf("%s",sort2[j]->lname);
            printf("%s",sort2[j+1]->lname);
        }
    }
}

This is what qsort is for. Anyway, you'd better hope sort2 has more than s elements. — Qaz
– Qaz, Commented Jul 31, 2014 at 14:51
Even when bug-fixed, that is a brutally inefficient way of sorting. — Jonathan Leffler
– Jonathan Leffler, Commented Jul 31, 2014 at 15:01

R Sahu · Accepted Answer · 2014-07-31 14:56:19Z

1

You are accessing beyond the bounds of the array in the following lines when j is equal to s-1:

        sort2[j]=sort2[j+1];
        sort2[j+1]=temp;
        printf("%s",sort2[j+1]->lname);

I think you meant to use:

        sort2[j]=sort2[i];
        sort2[i]=temp;
        printf("%s",sort2[i]->lname);

answered Jul 31, 2014 at 14:56

R Sahu

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3 Comments

R Sahu Over a year ago

@JProgrammer, what do you mean by that? Is sorting not working? Are you getting memory access related errors?

JProgrammer Over a year ago

I get a run time error the loop goes through one time and seems to break at the second assignment of num1 and num2 which i know are being assigned the node structure there of type nodeptr as the array is that type

BLUEPIXY Over a year ago

for(i=0; i<s;i++) { for(j=0;j<s;j++) is bad.

OshoParth · Accepted Answer · 2014-07-31 16:05:52Z

In the above code when you refer to sort2[j+1] at the last iteration of the loop it would result into access to the garbage memory as array indexing starts from 0 so the last index is limit-1 i.e s-1 in this case but code will access sort[s]. Thus this would result into abnormal termination of the program in certain cases.

And starting the inner loop from the next value of outer loop counter would save some iterations of your code.

so try

for(i=0; i<s;i++)
{
   for(j=(i+1);j<(s-1);j++)
   {
     num1= sort2[i];
     num2= sort2[j];
     if(num1->pnum<num2->pnum)
     {
        temp=sort2[j];
        sort2[j]=sort2[j+1];
        sort2[j+1]=temp;
        printf("%s",sort2[j]->lname);
        printf("%s",sort2[j+1]->lname);
     }
   }
}

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