Thread: more with my 1st structure program

Thanks, Troll King.

I think it is fabulous you understand the C language with only 2 1/2 years experience. You learn well, and remember well. I suspect you know other programming languages too. I don't. I am at a disadvantage by not having a frame of reference with which to compare things to. To me C is like learning Greek. It would take years to switch to a new speaking language, and be a pro at it. Heck, I ain't a pro at English, and I have been speaking it for 33 years!!!!

The program is to do the following:
1) find area of rectangle r (done)
2) find the center of rectangle r(having many problems with this part as far as using C syntax to explain it
3) move r by x units in the x direction and y units in the y direction, returning the modified version of r (x and y are additional arguments to the function) --- I think this means that rectangle r is the same size as before but moved to where the lower left corner is at the sight of the former upper right corner (these points were not referenced in the point structure.)
4) then find if a point (p) lies within r; returning TRUE or FALSE
(p is an additional argument of type struct point)....now is p in the old r or the new r .... I have no idea.

1) is done
2) is almost done
3) is confusing
4) is confusing++

>when to use struct and when not, when using a variable of a >structure

Use a struct for only the most generic purposes! I mean, if you had a "struct car" to hold useful data about a model of car, you would not, say have a variable in it that contains your favorite radio station, or a variable that contains the fact that you spilled your cappucino on the floorboard! That's a little extreme but the point is, some people frankly misuse the structure to hold unrelated facts and figures.

In other words, if there is some piece of data you might need frequently that is related to, say any given rectangle, then make it a data member!


>how to reference a global variable with structures

Since global variables can be seen anywhere in the program, you can literally refer to it/ access it in any function without even passing it in as a parameter! (But you won't be able to change it in these functions unless you DO pass it in, as a *pointer or a &reference).




>how to reference a local variable

Since a local variable cannot be "seen" by anything except for manipulations within it's own scope, to access it in a function, you must pass it in as a parameter.


The less parameters, the better! So don't write a function that passes more parameters than necessary!





>when to know to use a global vs a local

Generally, ALWAYS use a local variable, and only create global variables about as often as you take trips to Alaska!! So RARELY.

having problems with this part; the line is too long to fit across screen; how do I continue assignment line onto next line but have compiler know that it is taking two lines for one assignment???

NOTE: as it appears here, it looks fine, but on my screen where actual program is, it doesn't fit across so well

Code:

struct point center_of_r ( struct rectangle rect )
{
    struct point center;
    center.x = rect.upper_left.x + ( rect.lower_right.x - rect.upper_left.x ) / 2;
    center.y = rect.upper_left.x + ( rect.lower_right.y - rect.upper_left.y ) / 2;
    return center;
}

Except inside " " and ' ', wherever you have a space, you can also have a newline.

So just insert newlines and indentation as you need them to make the code look nicely formatted.

nevermind; got it to compile now


Now my problem is that the center point is not being calculated correctly. And there is one problem. In some instances the
center point could be in between whole integer numbers (eg. 4.5 or 6.5). Where do I typecast for a float so as to represent the values correctly??? In the local function, in the main??



if for instance my upper_left coords are 2,7
and my lower_right coordsd are 7,2
the area is 25
the center is 4.5, 4.5

but it is returning center is 1 1

Why? Where is problem??? And how do I fix...including typecasting??

There are 3 things you need to do
1. make a backup copy of working code

2. Change your point structure like so

Code:

struct point {
    float x, y;
};

3. Change your input to read floats instead of ints

Code:

scanf( "%f %f", &r.upper_left.x, &r.upper_left.y );

scanf( "%f %f", &r.lower_right.x, &r.lower_right.y );

Ok here's my revised code;
I don't think I am allowed to change the members of struct point.
Is there any other way to get floating-point variables to be returned in the
FUNCTON: struct point center_of_r(struct rectangle rect);
????

I didn't change anything here. I tried typecasting the assignments to center.x and center.y to float but that didn't work (and yes I changed the printf line to %5.1f to write a float )

What else can I do here??

Code:

#include <stdio.h>

struct point {
   int x, y;
};

struct rectangle {
   struct point upper_left, lower_right;
} ;


int area_of_r(struct rectangle rect);
struct point center_of_r(struct rectangle rect);

int main() {

  struct rectangle r;
  struct point center;
  int area;

  int coord_x, coord_y;

  printf( "input upper left corner of rectangle r "
           "as 0 0\n");
  scanf( "%d %d", &r.upper_left.x, &r.upper_left.y );
  printf( "input lower right corner of rectangle r  "
          "as 0 0\n");
  scanf( "%d %d", &r.lower_right.x, &r.lower_right.y );

  area = area_of_r(r);
  printf( "\nThe area of rectangle r is "
          "%d\n", area);

  center = center_of_r(r);
  printf("\nThe center point of rectangle r is "
         "%d %d\n\n",center.x, center.y);

  return 0;
}

int area_of_r ( struct rectangle rect )
{
  int area;
  area = (rect.lower_right.x - rect.upper_left.x) *
         (rect.upper_left.y - rect.lower_right.y);
  return area;
}

struct point center_of_r(struct rectangle rect)
{
    struct point center;

    center.x = ((rect.lower_right.x) +
                            (rect.upper_left.x))/2;
    center.y = ((rect.upper_left.y) +
                           (rect.lower_right.y))/2;

    return center;
}

Oh, the coordinates are correct now;
forgot to change my subject line of last post
sorry.

> I don't think I am allowed to change the members of struct point.
In which case, you're sunk on the rounding problem - there is no easy way out.

Before you try and solve it, you'd better make sure with your tutor that this is acceptable.

I don't think it matters for the purpose of this exercise, which seems to be aimed at how to handle structures.

can't typecast the varibles in function ??


struct point center_of_r(struct rectangle rect)
{
struct point center;

center.x = (float)((rect.lower_right.x) +
(rect.upper_left.x))/2;
center.y = (float)((rect.upper_left.y) +
(rect.lower_right.y))/2;

return center;
}