CODEWITHSUNDEEP
cqsort
schegu
schegu

Reputation: 31

qsort in C give wrong results

I am trying to sort an array of structs (A SJF Scheduler). I am using the qsort library function to sort the structs in increasing order according to the attribute bursttime. However, the output is not correct. I've checked some SO questions about the same but they served no use.

struct job
{
    int jobno;
    int bursttime;
};
typedef struct job job_t;

int mycompare(const void* first, const void* second)
{
    int fb = ((job_t*)first)->bursttime;
    int sb = ((job_t*)second)->bursttime;
    return (fb - sb);
}

int main()
{
    int n;
    printf("Enter number of jobs: ");
    scanf("%d", &n);
    job_t* arr = (job_t*)malloc(sizeof(job_t) * n);
    for(int i = 1; i <= n; ++i)
    {
        printf("Enter Burst time for Job#%d: ",i);  
        scanf("%d", &(arr[i].bursttime));
        arr[i].jobno = i;
    }

    printf("\n");
    printf("Order of the Jobs before sort:\n");
    for(int i = 1; i <= n; ++i)
    {
        printf("%d\t", arr[i].jobno);
    }

    qsort(arr, n, sizeof(job_t), mycompare);    

    printf("\n");
    printf("Order of the Jobs after sort:\n");
    for(int i = 1; i <= n; ++i)
    {
        printf("%d\t", arr[i].jobno);
    }
    printf("\n");
    printf("\n");
    return 0;
}

This is my inputfile:

4
7
2
9
4

The output I'm getting is:

Order of the Jobs before sort:
1   2   3   4   
Order of the Jobs after sort:
2   1   3   4

The expected order should be: 2,4,1,3. Am I missing anything?

Upvotes: 0

Views: 195

Answers (2)

Kevin Boone
Kevin Boone

Reputation: 4307

You could change the indexing scheme to start from zero, as others have suggested, and this would certainly be the idiomatic way to do it.

But if you want to use 1-based indexing, you'll need to allocate an extra place in the array (position 0 that will never be used):

job_t* arr = (job_t*)malloc(sizeof(job_t) * (n + 1));

Then you'll need to start your sort at position 1 in the array:

qsort(&arr[1], n, sizeof(job_t), mycompare);

And, of course, you'll have to write your code to index from 1 -- but you've already done that.

The problem is that so many standard functions in C use zero-based indexing that doing anything else is inexpressive. That's a bigger problem than wasting one array position. But, for better or worse, I've had to convert to C a load of code from Fortran, so I've gotten used to working both ways.

Upvotes: 0

chux
chux

Reputation: 154312

At least this problem

for(int i = 1; i <= n; ++i) // bad

Use zero base indexing.

for(int i = 0; i < n; ++i)

Upvotes: 3

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