How to access data from a structure pointer within a structure

Question

I am trying to print the x value for newBall, but don't understand how to access it. Can someone help me out? I have a pointer to a struct Ball within the struct AllBalls. I get a compile error saying x is not a member of a structure.

    #include <stdlib.h>
    #include <stdio.h>


    struct Ball {
        char id;
        double x;
        double y;
        double Vx;
        double Vy;
    };

    struct AllBalls {
        int count; 
        struct Ball *ballPtr; 
    };

int main(void) 
{   
    int index = 1; 
    struct AllBalls list = {0, NULL};
    struct Ball newBall;
    double x, y, Vx, Vy;
    int input;

    printf("Enter input: ");
    input = scanf("%lf %lf %lf %lf", &x, &y, &Vx, &Vy);
    list.count++;
    list.ballPtr = &newBall;
    newBall.id = 64 + list.count;
    newBall.x = x;
    newBall.y = y;
    newBall.Vx = Vx;
    newBall.Vy = Vy;
    printf("%lf", *(list.ballPtr).x);
}

dbush · Accepted Answer · 2019-02-07 02:17:16Z

3

The problem is in this line near the end:

 printf("%lf", *(list.ballPtr).x);

The member selection operator . has higher precedence than the dereference operator *. So the compiler thinks you're trying to access a pointer as a struct. You can fix this by moving the parenthesis:

 printf("%lf", (*list.ballPtr).x);

Or by using the pointer to member operator -> instead:

 printf("%lf", list.ballPtr->x);

answered Feb 7, 2019 at 2:17

dbush

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