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iOS

Posted by Dali Qin
Dali Qin
1,319 Points

about pointer in c

Here is a small function defined by a typedef "sphere":

sphere Make_eyeball (float *c, float r) { 

sphere eyeball;

eyeball.center[0] = c[0];
eyeball.center[1] = c[1];
eyeball.center[2] = c[2];

eyeball.radius = r;
return eyeball;

}

Q: float *c given as a float pointer, why can the program just use C as an array?

1 Answer

John W
John W
21,558 Points
John W
John W
21,558 Points

Your code copies the values/data within the memory locations of c to those of eyeball.center. Two benefits of copying are that now the data for the center values are contiguous within the struct eyeball next to radius and independent of c. No matter what happens later to c, e.g. being changed or released, the values within your eyeball struct will remain unaffected.

Dali Qin
Dali Qin
1,319 Points

Thanks for the reply. What you are saying is c works as pointer and the actual data aren't stored in c but eyeball center. My confusion is that isn't it necessary to declare c as an array first, or array of pointers?

John W
John W
21,558 Points

(sry, didn't subscribe to this thread so missed your question)

The function takes in basically 4 numbers (ignore the fact that the first 3 are referenced by pointers) and outputs a struct. So this function is simply there to encapsulate the process of changing 4 arbitrary pieces of data into something that has semantic meanings. The usage of this function could be something like this:

float some_random_point_in_space[3] = {1.0, -2.5, 3.0};
float some_random_number = 1.2

sphere eyeball = Make_eyeball(some_random_point_in_space, some_random_number);
// Now you can use eyeball instead of 4 arbitrary numbers, knowing well what they actually mean.