Why do I get two different results? (issue with booleans) - c

Question

I am a Python developer for the most part, but recently I have needed to solve a few problems using C and, honestly, it makes me suffer.
func.c
#define _CRT_SECURE_NO_WARNINGS
#define M_PI 3.1415926535897932384626433832795028841971693993751058209
#include <stdio.h>
#include <stdbool.h>
#include <stdlib.h>
#include <math.h>
#include "Point.h"
bool point_on_line(struct Point p1, struct Point p2, struct Point point) {
double min_x = min(p1.x, p2.x);
double max_x = max(p1.x, p2.x);
double min_y = min(p1.y, p2.y);
double max_y = max(p1.y, p2.y);
if (p2.y - p1.y == 0) {
return point.y == p2.y && point.x >= min_x && point.x <= max_x && point.y >= min_y && point.y <= max_y;
}
if (p2.x - p1.x == 0) {
return point.x == p2.x && point.x <= max_x && point.x >= min_x && point.y >= min_y && point.y <= max_y;
}
double k = (p2.y - p1.y) / (p2.x - p1.x);
double b = (p2.x * p1.y - p1.x * p2.y) / (p2.x - p1.x);
return point.y == (k * point.x + b) && point.x >= min_x && point.x <= max_x && point.y >= min_y && point.y <= max_y;
}
double calculate_angle(struct Point p1, struct Point p2) {
double dot_product = p1.x * p2.x + p1.y * p2.y;
double lenp1 = sqrt(p1.x * p1.x + p1.y * p1.y);
double lenp2 = sqrt(p2.x * p2.x + p2.y * p2.y);
double orientation = p1.x * p2.y - p1.y * p2.x;
int sign;
if (orientation > 0) {
sign = 1;
}
else {
sign = -1;
}
return sign * acos(dot_product / (lenp1 * lenp2));
}
bool check_border(struct Point p, struct Point points[], int size) {
for (int i = 0; i < size - 1; i++) {
if (point_on_line(points[i], points[i + 1], p)) {
return true;
}
}
return false;
}
bool calc_angle_sum(struct Point p1, struct Point points[], int size) {
struct Point* vectors = malloc(size * sizeof(struct Point));
for (int i = 0; i < size; i++) {
struct Point temp = { points[i].x - p1.x,points[i].y - p1.y };
vectors[i] = temp;
}
double total_sum = 0;
for (int i = 0; i < size - 1; i++) {
total_sum += calculate_angle(vectors[i], vectors[i + 1]);
}
bool res = (fabs(total_sum - 2 * M_PI)) < 0.00005;
printf("TOTAL SUM %.100f\n", total_sum);
printf("DIFFERENCE SMALL %d\n", fabs(total_sum - 2 * M_PI) < 0.00005);
return fabs(total_sum - 2 * M_PI) < 0.00005;
//return res;
}
Source.c
#define _CRT_SECURE_NO_WARNINGS
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include "Point.h"
int main() {
int length = 5;
struct Point p1 = { 1,2 };
struct Point p2 = { 5,0 };
struct Point p3 = { 7,4 };
struct Point p4 = { 5,6 };
struct Point p5 = { 2,5 };
struct Point p6 = { 1,2 };
struct Point points_test[6] = {p1,p2,p3,p4,p5,p6};
struct Point point_test = {7,3};
//bool result = calc_angle_sum(point, points, length + 1);
//printf("%s", result ? "true\n" : "false\n");
if (check_border(point_test, points_test, length + 1)) {
printf("BORDER");
return 0;
}
else if (calc_angle_sum(point_test, points_test, length + 1)) {
printf("INSIDE");
return 0;
}
else {
printf("OUTSIDE");
return 0;
}
}
Point.h
#pragma once
struct Point {
double x, y;
};
coordinates.txt
1 2 5 0 7 4 5 6 2 5
lap.txt (the first number is the number of rectangle vertices, the second and third - the coordinates of the point.
5 7 3
This algorithm determines whether a point is inside/outside/on an edge of a given polygon using the winding number method.
The point (7,3) (the second and third numbers inside lap.txt) lies outside the polygon so the correct answer is "OUTSIDE/FALSE". Nevertheless, the output differs depending on Debug/Release and the way I return from calc_angle_sum function.
When I return this way:
return fabs(total_sum - 2 * M_PI) < 0.00005;
I get inconsistent results depending on debug/release mode.
The following way, however, seems to work fine:
bool res = (fabs(total_sum - 2 * M_PI)) < 0.00005
return res
Below is the part inside Source.c that calls the method:
else if (calc_angle_sum(point, points, length + 1)) {
printf("INSIDE");
return 0;
}
There is something I cannot understand about how expressions are evaluated in C.
In Python, I am used to returning like return <some_expression> and expect it to get converted to True/False. In C, however, this doesn't seem to be true, or else there is some other error.

Answer 1

One obvious problem is that of implicit function declarations.
Neither
bool calc_angle_sum(struct Point p1, struct Point points[], int size)
and
bool check_border(struct Point p, struct Point points[], int size)
are not declared in the translation unit source.c. Therefore the compiler assumes that the functions are of type
int calc_angle_sum()
int check_border()
Were the return type of the functions int, they could be called compatibly in this manner - with these arguments. But because the actual return type of the functions is bool, the behaviour of the function calls is undefined.
C99 removed implicit function definitions. A compliant C99, C11, C17 compiler must complain about these function calls in your source.c.But the substandard MSVC compiler barely meets the long obsolete C89 specification...
Thus, try declaring
bool calc_angle_sum(struct Point p1, struct Point points[], int size);
bool check_border(struct Point p, struct Point points[], int size);
in Point.h.