Why doesn't this array of structure assignment compile?

Question

here is the code 

struct point_tag {
        int x;
        int y;
};

typedef struct point_tag Point;

int main(void) 
{
      Point pt[] = { {10,20}, {30, 40}, {50,60}};
      pt[0] = {100,200};
}

When i do pt[0] = {100, 200}, the compiler keeps complaining of

error:expected expression before '{' token

I don't really get that though. Isn't the expression before the { token assignment operator(=)?

I don't understand why the assignment would be an issue though. the value at address pt refers to an array of Point. I am just setting the 0th Point to be this new point and i know that assigning a struct in a format like {100,200} is legal where the elements inside that array are just fields.

Gyapti Jain · Accepted Answer · 2014-10-30 06:24:34Z

6

For assignment, typecast the value to type Point to make it a compound literal:

pt[0] = (Point){100,200};

Live code using gcc

This is equivalent to

{
  Point temp = {100,200};
  pt[0] = temp;
}

^{p.s. Compound literal is not available in old strict C89 compliant compiler. It is avilable in GCC for C89 as extension and in C99 compound literal is a core feature.}

edited Oct 30, 2014 at 6:24

answered Oct 30, 2014 at 6:14

Gyapti Jain

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5 Comments

committedandroider Over a year ago

Is there a reason you don't have to do this for {10,20}, {30,40}, and {50,60} ? That trips me up. Technically you're still assigning those values to the value at those indexes in the array

Gyapti Jain Over a year ago

Because the above construct is available only for initialization. In pt[0] = (Point){100,200};, you are creating a temp Point and assigning it to pt[0]. You must tell compiler whether {100, 200} is a Point or int [2] or something else.

committedandroider Over a year ago

but couldn't this be similar logic in that you're also creating a temp Point {10,20} and storing that as the value of the first index in the array?

Gyapti Jain Over a year ago

If my previous comment fails to answer your query, I would suggest you a read through C99 specs section 6.5.2.5 Compound literals.

Mohit Jain Over a year ago

@committedandroider You can also do something like [pt[0] = (Point){.x = 100, .y = 200};](ideone.com/Bkv5nu). Without telling the type, compiler would have no idea what x and y are.

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