check_collinearity_of_three_points.c

// C program to check collinearity of three given points
#include <stdio.h>
#include <stdbool.h>

typedef struct Point
{
    int x;
    int y;
} Point;

/*
We can calculate the area formed by the three points, and if the area is
zero then they lie on a same line. 
*/
bool check_collinear(Point a, Point b, Point c)
{
    int area = 0;

    /*
    The Area of a Triangle formed by three points (x1, y1), (x2, y2), (x3, y3)
    is determined by the following formula
    
    Area = (1/2) * {x1 * (y2 - y3) + x2 * (y3 - y1) + x3 * (y1 - y2)}     
    */
    area = a.x * (b.y - c.y) + b.x * (c.y - a.y) + c.x * (a.y - b.y);

    if (area == 0)
        return true;
    else
        return false;
}

int main()
{
    int x, y;
    Point a, b, c;
    printf("\nEnter the first co-ordinates:  ");
    scanf("%d %d", &a.x, &a.y);
    printf("Enter the second co-ordinates: ");
    scanf("%d %d", &b.x, &b.y);
    printf("Enter the third co-ordinates:  ");
    scanf("%d %d", &c.x, &c.y);

    if (check_collinear(a, b, c))
    {
        printf("\nThe given points are collinear\n");
    }
    else
    {
        printf("\nThe given points are not collinear");
    }
    return 0;
}

/*
Time Complexity: O(1)
Space Complexity: O(1)

SAMPLE INPUT AND OUTPUT

Enter the first co-ordinates:  1 1
Enter the second co-ordinates: 2 2
Enter the third co-ordinates:  3 3
The given points are collinear
*/