Errors in the following C program

[erald23]

Struggling with these errors it seems that most of them are in the same area can't figure out why?

#include "area.h"#include<stdio.h>
#include<math.h>




/* this function calculates the area of a convex polygon */


double area(node *list)
{




 node * a;
 node * b;
 node * c = list->next->next;
 double area = (0.5*( a(b-c) + b(c - a)) + c(a - b)); /* formula used to calculate triangle areas*/
 double total=0;


 while(c!=a){
  area =(0.5*( a(b-c) + b(c - a)) + c(a - b));
  total+=area;
  b=c;
  c=c->next;


          }


   return area;


 }

error

icc -ansi -Wall -c -o area.o area.c
area.c(27): error: expression preceding parentheses of apparent call must have (pointer-to-) function type
   double area = (0.5*( a(b-c) + b(c - a)) + c(a - b)); /* formula used to calculate triangle areas, */
                        ^


area.c(27): error: expression preceding parentheses of apparent call must have (pointer-to-) function type
   double area = (0.5*( a(b-c) + b(c - a)) + c(a - b)); /* formula used to calculate triangle areas, /
                                 ^


area.c(27): error: expression preceding parentheses of apparent call must have (pointer-to-) function type
   double area = (0.5*( a(b-c) + b(c - a)) + c(a - b)); /* formula used to calculate triangle areas, */
                                             ^


area.c(31): error: expression preceding parentheses of apparent call must have (pointer-to-) function type
    area =(0.5*( a(b-c) + b(c - a)) + c(a - b));
                 ^


area.c(31): error: expression preceding parentheses of apparent call must have (pointer-to-) function type
    area =(0.5*( a(b-c) + b(c - a)) + c(a - b));
                          ^


area.c(31): error: expression preceding parentheses of apparent call must have (pointer-to-) function type
    area =(0.5*( a(b-c) + b(c - a)) + c(a - b));
                                      ^


compilation aborted for area.c (code 2)

[stahta01]

In math "b(" has a meaning that differs from C in C use "b*(" instead to have the same meaning as in math of times.

Tim S.

[john.c]

That's not the only problem. node is presumably a struct. a and b are uninitialized pointers to a node, and you are using them directly in an arithmetic calculation, which is certainly not what you want.

Firstly, they need to point to an actual node, presumably from the list parameter. This is a total guess, but maybe something like

node *a = list;
node *b = list->next;
node *c = list->next->next;
 
double total = 0.0;
 
while (c != list) {
    double area = 0.5*(a->value * (b->value - c->value) + b->value * (c->value - a->value)) + c->value * (a->value - b->value);
    total += area;
    a = b;
    b = c;
    c = c->next;
}
 
return total;

Don't just say "it doesn't work" since, as I said, this is a total guess just to give you the idea.
(For instance, I have no idea if the value member is called "value".)

[ProgrammingJedi]

while(c!=a) : you are comparing two pointers which store different addresses.
area =(0.5*( a(b-c) + b(c - a)) + c(a - b)); : if you want to use the values, you have to use the asterisk-operator: *a, *b, *b
c = c->next; : I would check if c is NULL after this line of code. you can do that like so directly in the loop-condition: while(c) etc.

[john.c]

@ProgrammingJedi (not)

while(c!=a) : you are comparing two pointers which store different addresses.

That's an assumption. How do you know they always contain different addresses? Have you heard of a circular linked list?

if you want to use the values, you have to use the asterisk-operator: *a, *b, *b

Obviously wrong in this case. They are clearly pointing to a struct, so simply dereferencing them isn't going to give a meaningful value for arithmetic operations. He needs to use the -> operator and access whatever the data field(s) is/are.

c = c->next; : I would check if c is NULL after this line of code.

Why? Clearly the code is written for a circular list, where the last node points back to the first. Actually, the code is so badly written, it's impossible to tell what the real situation is, so you are assuming yet again.


@erald23, Stop entitling your posts "error in program". Well, duh, obviously, that's why you're here. Put a little more effort into your questions. What are you trying to do? What does your node struct look like? Where does this "formula" come from? (post a link)

[Kernelpanic]

/* this function calculates the area of a convex polygon */

What for a polygon: with 3, 5, 6, or more vertices? Regular or irregular?