qsort结构中的结构数组

Question

I'm attempting to sort an array of structs in a struct based on an int value. I've successfully sorted an array of structs but I'm guessing I'm passing a wrong value somewhere for the nested structs.

I just need the structs in the array sorted the value of a.

The structs are set up as:

struct s2{
    int a;
    int b;
};

struct s1{
    int c;
    struct s2 arr[10];
}

I have a compare function:

int comp(const void *a, const void *b){

    struct s1 *q1 = (struct s1 *)a;
    struct s1 *q2 = (struct s1 *)b;

    return(q1->arr->a - q2->arr->a); 
}

And I call qsort:

struct s1 myStruct; 
size_t theLen = sizeof(myStruct.arr) / sizeof(struct s2);
qsort(myStruct.arr, 10, theLen, comp);

For the input:

10, 5, 7, 20, 17, 9, 3, 11, 15, 1

I get output:

2147451181, 589824, 327680, 65536, 131072, 4, 5, 11, 15, 8

I'm guessing it may be something to do with how I declare the length?

Thanks!

The file line is:

10 5 7 20 17 9 3 11 15 1

myStruct.arr[i].a is filled from file input using fgets and sscanf:

fgets(t, sizeof(t), fp);
sscanf(t, "%d,...,%d", &myStruct.arr[0].a,...,&myStruct.arr[9].a); 

myStruct.arr[i].b is filled with a for loop:

for(int i = 0; i < 10; i++){
    myStruct.arr[i].b = i+1;
}

Answer 1

qsort(myStruct.arr, 10, theLen, comp);

You are sorting myStruct.arr here where each element is of type struct s2 .So your compare should be

int comp(const void *a, const void *b){

struct s2 *q1 = (struct s2 *)a;
struct s2 *q2 = (struct s2 *)b;

return(q1->a - q2->a); 
}

EDIT: the third parameter to qsort is the size of each element of array to be sorted.So it should be

qsort(myStruct.arr, theLen, sizeof(struct s2), comp);

Answer 2

There are two mistakes with your code

1. You are using q1->arr->a to compare where you should use q1->a (where q1 is of type const struct s2 ). This has also been explained by @GauravSehgal in his answer

2. If you look at the third argument to qsort , it is acutally the size of each element to compare in bytes. But you have passed the number of elements. Change your call to -

   qsort(myStruct.arr, 10, sizeof(struct s2), comp);

and you should get the desired result.

There are some other points you need to take care of (pointed out by @Stargateur) -

1. Declare q1 and q2 to be of type const struct s2* , because you do not want to discard the const qualifier.

2. Do not cast a and b explicitly while assigning to q1 and q2 because they are of type const void* which auto-promotes to pointer of any const type.