Given three corner points of a triangle, and one more point P. Write a function to check whether P lies within the triangle or not.

For example, consider the following program, the function should return true for P(10, 15) and false for P'(30, 15)

B(10,30) / \ / \ / \ / P \ P' / \ A(0,0) ----------- C(20,0)

Source: Microsoft Interview Question

**Solution:**

Let the coordinates of three corners be (x1, y1), (x2, y2) and (x3, y3). And coordinates of the given point P be (x, y)

1) Calculate area of the given triangle, i.e., area of the triangle ABC in the above diagram. Area A = [ x1(y2 – y3) + x2(y3 – y1) + x3(y1-y2)]/2

2) Calculate area of the triangle PAB. We can use the same formula for this. Let this area be A1.

3) Calculate area of the triangle PBC. Let this area be A2.

4) Calculate area of the triangle PAC. Let this area be A3.

5) If P lies inside the triangle, then A1 + A2 + A3 must be equal to A.

## C++

#include <bits/stdc++.h> using namespace std; /* A utility function to calculate area of triangle formed by (x1, y1), (x2, y2) and (x3, y3) */ float area(int x1, int y1, int x2, int y2, int x3, int y3) { return abs((x1*(y2-y3) + x2*(y3-y1)+ x3*(y1-y2))/2.0); } /* A function to check whether point P(x, y) lies inside the triangle formed by A(x1, y1), B(x2, y2) and C(x3, y3) */ bool isInside(int x1, int y1, int x2, int y2, int x3, int y3, int x, int y) { /* Calculate area of triangle ABC */ float A = area (x1, y1, x2, y2, x3, y3); /* Calculate area of triangle PBC */ float A1 = area (x, y, x2, y2, x3, y3); /* Calculate area of triangle PAC */ float A2 = area (x1, y1, x, y, x3, y3); /* Calculate area of triangle PAB */ float A3 = area (x1, y1, x2, y2, x, y); /* Check if sum of A1, A2 and A3 is same as A */ return (A == A1 + A2 + A3); } /* Driver program to test above function */ int main() { /* Let us check whether the point P(10, 15) lies inside the triangle formed by A(0, 0), B(20, 0) and C(10, 30) */ if (isInside(0, 0, 20, 0, 10, 30, 10, 15)) printf ("Inside"); else printf ("Not Inside"); return 0; }

## Java

// JAVA Code for Check whether a given point // lies inside a triangle or not import java.util.*; class GFG { /* A utility function to calculate area of triangle formed by (x1, y1) (x2, y2) and (x3, y3) */ static double area(int x1, int y1, int x2, int y2, int x3, int y3) { return Math.abs((x1*(y2-y3) + x2*(y3-y1)+ x3*(y1-y2))/2.0); } /* A function to check whether point P(x, y) lies inside the triangle formed by A(x1, y1), B(x2, y2) and C(x3, y3) */ static boolean isInside(int x1, int y1, int x2, int y2, int x3, int y3, int x, int y) { /* Calculate area of triangle ABC */ double A = area (x1, y1, x2, y2, x3, y3); /* Calculate area of triangle PBC */ double A1 = area (x, y, x2, y2, x3, y3); /* Calculate area of triangle PAC */ double A2 = area (x1, y1, x, y, x3, y3); /* Calculate area of triangle PAB */ double A3 = area (x1, y1, x2, y2, x, y); /* Check if sum of A1, A2 and A3 is same as A */ return (A == A1 + A2 + A3); } /* Driver program to test above function */ public static void main(String[] args) { /* Let us check whether the point P(10, 15) lies inside the triangle formed by A(0, 0), B(20, 0) and C(10, 30) */ if (isInside(0, 0, 20, 0, 10, 30, 10, 15)) System.out.println("Inside"); else System.out.println("Not Inside"); } } // This code is contributed by Arnav Kr. Mandal.

## Python

# A utility function to calculate area # of triangle formed by (x1, y1), # (x2, y2) and (x3, y3) def area(x1, y1, x2, y2, x3, y3): return abs((x1 * (y2 - y3) + x2 * (y3 - y1) + x3 * (y1 - y2)) / 2.0) # A function to check whether point P(x, y) # lies inside the triangle formed by # A(x1, y1), B(x2, y2) and C(x3, y3) def isInside(x1, y1, x2, y2, x3, y3, x, y): # Calculate area of triangle ABC A = area (x1, y1, x2, y2, x3, y3) # Calculate area of triangle PBC A1 = area (x, y, x2, y2, x3, y3) # Calculate area of triangle PAC A2 = area (x1, y1, x, y, x3, y3) # Calculate area of triangle PAB A3 = area (x1, y1, x2, y2, x, y) # Check if sum of A1, A2 and A3 # is same as A if(A == A1 + A2 + A3): return True else: return False # Driver program to test above function # Let us check whether the point P(10, 15) # lies inside the triangle formed by # A(0, 0), B(20, 0) and C(10, 30) if (isInside(0, 0, 20, 0, 10, 30, 10, 15)): print('Inside') else: print('Not Inside') # This code is contributed by Danish Raza

## C#

// C# Code to Check whether a given point // lies inside a triangle or not using System; class GFG { /* A utility function to calculate area of triangle formed by (x1, y1) (x2, y2) and (x3, y3) */ static double area(int x1, int y1, int x2, int y2, int x3, int y3) { return Math.Abs((x1 * (y2 - y3) + x2 * (y3 - y1) + x3 * (y1 - y2)) / 2.0); } /* A function to check whether point P(x, y) lies inside the triangle formed by A(x1, y1), B(x2, y2) and C(x3, y3) */ static bool isInside(int x1, int y1, int x2, int y2, int x3, int y3, int x, int y) { /* Calculate area of triangle ABC */ double A = area(x1, y1, x2, y2, x3, y3); /* Calculate area of triangle PBC */ double A1 = area(x, y, x2, y2, x3, y3); /* Calculate area of triangle PAC */ double A2 = area(x1, y1, x, y, x3, y3); /* Calculate area of triangle PAB */ double A3 = area(x1, y1, x2, y2, x, y); /* Check if sum of A1, A2 and A3 is same as A */ return (A == A1 + A2 + A3); } /* Driver program to test above function */ public static void Main() { /* Let us check whether the point P(10, 15) lies inside the triangle formed by A(0, 0), B(20, 0) and C(10, 30) */ if (isInside(0, 0, 20, 0, 10, 30, 10, 15)) Console.WriteLine("Inside"); else Console.WriteLine("Not Inside"); } } // This code is contributed by vt_m.

## PHP

<?php /* A utility function to calculate area of triangle formed by (x1, y1), (x2, y2) and (x3, y3) */ function area($x1, $y1, $x2, $y2, $x3, $y3) { return abs(($x1 * ($y2 - $y3) + $x2 * ($y3 - $y1) + $x3 * ($y1 - $y2)) / 2.0); } /* A function to check whether P(x, y) lies inside the triangle formed by A(x1, y1), B(x2, y2) and C(x3, y3) */ function isInside($x1, $y1, $x2, $y2, $x3, $y3, $x, $y) { /* Calculate area of triangle ABC */ $A = area ($x1, $y1, $x2, $y2, $x3, $y3); /* Calculate area of triangle PBC */ $A1 = area ($x, $y, $x2, $y2, $x3, $y3); /* Calculate area of triangle PAC */ $A2 = area ($x1, $y1, $x, $y, $x3, $y3); /* Calculate area of triangle PAB */ $A3 = area ($x1, $y1, $x2, $y2, $x, $y); /* Check if sum of A1, A2 and A3 is same as A */ return ($A == $A1 + $A2 + $A3); } // Driver Code /* Let us check whether the P(10, 15) lies inside the triangle formed by A(0, 0), B(20, 0) and C(10, 30) */ if (isInside(0, 0, 20, 0, 10, 30, 10, 15)) echo "Inside"; else echo "Not Inside"; // This code is contributed by anuj_67. ?>

Ouptut:

Inside

**Exercise:** Given coordinates of four corners of a rectangle, and a point P. Write a function to check whether P lies inside the given rectangle or not.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.