在函数中初始化的结构异常

Question

In my project, i have created 3 different typedef struct types:

typedef struct Point 
{ 
    float x; 
    float y;
    float z;
} Point;

typedef struct Triangle 
{ 
    Point A; 
    Point B;
    Point C;
    unsigned char color[3];//RGB
} Triangle;

typedef struct Structure 
{ 
    Triangle* triangles; 
    unsigned int nt; //number of triangles in array
} Structure;

As you might have noticed, the type Structure has a dynamic sized array of Triangle s, so i'll also post here the memory allocation and freeing functions:

Structure newStructure(unsigned int nt)
{
    Structure S;
    Triangle* tri = malloc ((nt) * sizeof(Triangle));
    if (tri!=NULL) 
    {
        S.triangles = tri;
        S.nt = nt;
    }
    else S.nt = 0;
    return S;
}

void delStructure (Structure S) 
{
    if (S.triangles != NULL) free (S.triangles);
}

Then i wanted to make a function to add a Triangle to a current Structure using the following syntax: S = addTriangle(T,S) . This is what i have:

Structure addTriangle(Triangle T, Structure S)
{

    Structure R = newStructure(S.nt+1);
    int i=0;

    while(i++<S.nt) R[0].triangles[i] = S.triangles[i];
    R[0].triangles[S.nt] = T;

    delStructure(S); //Is this necessary?

    return R[0];
}

It shows no error messages when compiling, however when i use the function, the first Triangle on the array gets random values. To be more clear, if I have a Structure S with Triangles T1 and T2 in the array, and then i use S = addTriangle(T3,S) the result will be a Structure with the following array of Triangle s: {T?,T2,T3} , where T? has apparently random values.

Why is this happening?

Answer 1

while(i++<S.nt) R[0].triangles[i] = S.triangles[i];

should be something like

for (i=0; i<S.nt; i++) {
    R.triangles[i] = S.triangles[i];
}

Currently, you skip assigning the first element of R.triangles and assign its second last element by reading beyond the end of S.triangles . You can fix this by deferring incrementing i until the end of each loop. (You could continue to do this using a while loop; I found a for loop clearer.)

Also, all use of R[0] can (should) be replaced by R . R is a single structure, not an array.