trouble trying to use a struct variable - c

Question

I’m learning C programming and I’m trying to make a simple program to store points in the XY plane.
At first I did it like this:
#include<stdio.h>
#include<stdlib.h>
typedef struct point { float x, y; } PNT;
typedef struct plane
{
int n_points;
PNT pt[50];
} plane;
void points_input(plane planexy);
void points_distance(plane planexy);
int main()
{
plane planexy;
printf("How many points do you want to enter? : ");
scanf("%d", &planexy.n_points);
points_input(planexy);
points_distance(planexy);
}
void points_input(plane planexy)
{
int i;
for (i = 0; i < planexy.n_points; i++)
{
printf("\nEnter a coordinate for x%d ", i);
scanf("%f", &planexy.pt[i].x);
printf("\nEnter a coordinate for y%d ", i);
scanf("%f", &planexy.pt[i].y);
system("cls");
}
}
void points_distance(plane planexy)
{
int i;
printf("Select first point :\n");
for (i = 0; i < planexy.n_points; i++)
{
printf("\n %d.(%.1f ,%.1f)", i, planexy.pt[i].x, planexy.pt[i].y);
}
}
And it doesn’t work, when I try to access one of the variables of a point it always returns a 0.
After messing with the variable name “planexy” and using it as a vector of size 1, and then using all mentions of it as planexy[0], it suddenly started to work :
#include<stdio.h>
#include<stdlib.h>
typedef struct point { float x, y; } PNT;
typedef struct plane
{
int n_points;
PNT pt[50];
} plane;
void points_input(plane planexy[]);
void points_distance(plane planexy[]);
int main()
{
plane planexy[1];
printf("How many points do you want to enter? : ");
scanf("%d", &planexy[0].n_points);
points_input(planexy);
points_distance(planexy);
}
void points_input(plane planexy[])
{
int i;
for (i = 0; i < planexy[0].n_points; i++)
{
printf("\nEnter a coordinate for x%d ", i);
scanf("%f", &planexy[0].pt[i].x);
printf("\nEnter a coordinate for y%d ", i);
scanf("%f", &planexy[0].pt[i].y);
system("cls");
}
}
void points_distance(plane planexy[])
{
int i;
printf("Select first point :\n");
for (i = 0; i < planexy[0].n_points; i++)
{
printf("\n %d.(%.1f ,%.1f)", i, planexy[0].pt[i].x, planexy[0].pt[i].y);
}
}
Now, this code works, but I don’t know why I had to put it this way for it to function, and it doesn't seem like it's good code.
I get that if I want to use multiple planes "XY" I should store the variable as a vector, but I just want to use one, I don't understand why it works when I use a vector of size 1 but it doesn't when I just want to use one variable to store a single plane.
What's wrong with the first approach?
Sorry for bad English.

Answer 1

I've edited your question to English so other people can understand it in SO.
I hope you accept it as I'll answer in English.
You're passing a copy of the object planexy in the function points_input() so the actual planexy that's defined in main() doesn't change. Instead, you should pass a pointer that points to it then modify the object pointed by that pointer.
The modified code:
Forward declartion
void points_input(plane* planexy); // ingreso_puntos function
Code
void points_input(plane* planexy)
{
int i;
for (i = 0; i < planexy->n_points; i++)
{
printf("\nEnter a coordinate for x%d ", i);
scanf("%f", &planexy->pt[i].x);
printf("\nEnter a coordinate for y%d ", i);
scanf("%f", &planexy->pt[i].y);
system("cls");
}
}
When using pointers, you use -> instead of the dot . which is only for objects.
Why did it work with an array as your second code?
Simply, arrays/vectors are actual pointers by themselves.
Proof:
scanf("%f", &planexy[0].pt[i].x); is the same as scanf("%f", &planexy->pt[i].x);.
In main(). You should pass the address of the planexy which creates the pointer.
points_input(&planexy);
Tip: When you want to modify an object through a function, you pass a pointer to it exactly as you do with scanf. Otherwise, when you want to just read an object, you pass it normally.

Answer 2

The errors you get from the compiler when you compile this tell you what is wrong:
program.c: In function ‘main’:
program.c:23:20: error: incompatible type for argument 1 of ‘ingreso_puntos’
ingreso_puntos(planexy);
^
program.c:15:6: note: expected ‘plane * {aka struct plane *}’ but argument is of type ‘plane {aka struct plane}’
void ingreso_puntos(plane planexy[]);
^
You've declared your functions as taking a pointer to a plane (plane *), but you're calling them with a plane. Use the & operator to get the address of planexy to pass to the function.
program.c: In function ‘ingreso_puntos’:
program.c:31:28: error: request for member ‘n_puntos’ in something not a structure or union
for (i = 0; i < planexy.n_puntos; i++)
^
Here in the function, planexy is a pointer to a plane, so you need -> instead of . to access fields from it.