Check whether a given point lies inside a triangle or not (original) (raw)

Last Updated : 23 Jul, 2025

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.

**Example:

**Input: A = (0, 0), B = (10, 30), C = (20, 0), P(10, 15)
**Output: Inside
**Explanation:
B(10,30)
/ \
/ \
/ \
/ P \ P'
/ \
A(0,0) ----------- C(20,0)
**Input: A = (0, 0), B = (10, 30), C = (20, 0), P(30, 15)
**Output: Outside
**Explanation:
B(10,30)
/ \
/ \
/ \
/ \ P
/ \
A(0,0) ----------- C(20,0)

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

Calculate the area of the given triangle, i.e., the area of the triangle ABC in the above diagram.
Area A = [ x1(y2 - y3) + x2(y3 - y1) + x3(y1-y2)]/2
Calculate the area of the triangle PAB. We can use the same formula for this. Let this area be A1.
Calculate the area of the triangle PBC. Let this area be A2.
Calculate the area of the triangle PAC. Let this area be A3.
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)) cout <<"Inside"; else cout <<"Not Inside";

return 0; }

// this code is contributed by shivanisinghss2110

C

#include <stdio.h> #include <math.h> #include <stdbool.h> #include <stdlib.h> /* 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); }

/* 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); }

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.

JavaScript

PHP

y1,y1, y1,x2, y2,y2, y2,x3, $y3) { return abs(($x1 * ($y2 - $y3) + x2∗(x2 * (x2∗(y3 - $y1) + x3∗(x3 * (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,y1, y1,x2, $y2, x3,x3, x3,y3, x,x, x,y) { /* Calculate area of triangle ABC */ A=area(A = area (A=area(x1, y1,y1, y1,x2, y2,y2, y2,x3, $y3); /* Calculate area of triangle PBC */ A1=area(A1 = area (A1=area(x, y,y, y,x2, y2,y2, y2,x3, $y3); /* Calculate area of triangle PAC */ A2=area(A2 = area (A2=area(x1, y1,y1, y1,x, y,y, y,x3, $y3); /* Calculate area of triangle PAB */ A3=area(A3 = area (A3=area(x1, y1,y1, y1,x2, y2,y2, y2,x, $y); /* Check if sum of A1, A2 and A3 is same as A */ return ($A == A1+A1 + 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. ?>

**Time Complexity: _O(1)
**Auxiliary Space: _O(1)

[embed]https://www.youtube.com/watch?v=H9qu9Xptf-w%5B/embed%5D

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.

**Another Approach - Using Barycentric Coordinate Method: Below is the algorithm to check if a point P lies inside a triangle ABC using the Barycentric Coordinate Method:

Define a function "isInsideTriangle" that takes four input parameters: A, B, C, and P.
Calculate the barycentric coordinates of point P with respect to the triangle ABC. To do this, we first need to calculate the area of the triangle ABC. We can use the cross product to find the area of the triangle ABC as:

Area(ABC) = 0.5 * ||AB x AC||, where ||AB x AC|| is the magnitude of the cross product of vectors AB and AC.

Then, we can calculate the barycentric coordinates of point P as:
1. a = 0.5 * ||PB x PC|| / Area(ABC)
2. b = 0.5 * ||PC x PA|| / Area(ABC)
3. c = 0.5 * ||PA x PB|| / Area(ABC), where PB, PC, and PA are vectors from point P to vertices B, C, and A, respectively.
If all three barycentric coordinates are non-negative, then the point P lies inside the triangle ABC. Return "Inside". Otherwise, point P lies outside the triangle ABC. Return "Outside".

Below is the implementation of the above approach:

C++ `

// c++ code addition

#include #include using namespace std;

// Function to check if the point is inside // the triangle or not string isInsideTriangle(vector A, vector B, vector C, vector P) { // Calculate the barycentric coordinates // of point P with respect to triangle ABC double denominator = ((B[1] - C[1]) * (A[0] - C[0]) + (C[0] - B[0]) * (A[1] - C[1])); double a = ((B[1] - C[1]) * (P[0] - C[0]) + (C[0] - B[0]) * (P[1] - C[1])) / denominator; double b = ((C[1] - A[1]) * (P[0] - C[0]) + (A[0] - C[0]) * (P[1] - C[1])) / denominator; double c = 1 - a - b;

// Check if all barycentric coordinates
// are non-negative
if (a >= 0 && b >= 0 && c >= 0) {
    return "Inside";
} else {
    return "Outside";
}

}

// Driver Code int main() { vector A = {0, 0}; vector B = {10, 30}; vector C = {20, 0}; vector P = {10, 15};

// Call the isInsideTriangle function with
// the given inputs
string result = isInsideTriangle(A, B, C, P);

// Print the result
cout << result << endl;
return 0;

}

// The code is contributed by Arushi Goel.

Java

import java.awt.Point;

class Main {

// Function to check if the point is inside // the triangle or not public static String isInsideTriangle(Point A, Point B, Point C, Point P) {

// Calculate the barycentric coordinates
// of point P with respect to triangle ABC
double denominator = ((B.y - C.y) * (A.x - C.x) +
                      (C.x - B.x) * (A.y - C.y));
double a = ((B.y - C.y) * (P.x - C.x) +
            (C.x - B.x) * (P.y - C.y)) / denominator;
double b = ((C.y - A.y) * (P.x - C.x) +
            (A.x - C.x) * (P.y - C.y)) / denominator;
double c = 1 - a - b;

// Check if all barycentric coordinates
// are non-negative
if (a >= 0 && b >= 0 && c >= 0) {
  return "Inside";
} else {
  return "Outside";
}

}

public static void main(String[] args) { Point A = new Point(0, 0); Point B = new Point(10, 30); Point C = new Point(20, 0); Point P = new Point(10, 15);

// Call the isInsideTriangle function with
// the given inputs
String result = isInsideTriangle(A, B, C, P);

// Print the result
System.out.println(result);

} }

Python3

Python program for the above approach

Function to check if the point is inside

the triangle or not

def isInsideTriangle(A, B, C, P):

# Calculate the barycentric coordinates
# of point P with respect to triangle ABC
denominator = ((B[1] - C[1]) * (A[0] - C[0]) +
               (C[0] - B[0]) * (A[1] - C[1]))
a = ((B[1] - C[1]) * (P[0] - C[0]) +
     (C[0] - B[0]) * (P[1] - C[1])) / denominator
b = ((C[1] - A[1]) * (P[0] - C[0]) +
     (A[0] - C[0]) * (P[1] - C[1])) / denominator
c = 1 - a - b

# Check if all barycentric coordinates
# are non-negative
if a >= 0 and b >= 0 and c >= 0:
    return "Inside"
else:
    return "Outside"

Driver Code

A = (0, 0) B = (10, 30) C = (20, 0) P = (10, 15)

Call the isInsideTriangle function with

the given inputs

result = isInsideTriangle(A, B, C, P)

Print the result

print(result)

C#

using System;

public class PointInsideTriangle { // Function to check if the point is inside // the triangle or not public static string IsInsideTriangle(int[] A, int[] B, int[] C, int[] P) { // Calculate the barycentric coordinates // of point P with respect to triangle ABC double denominator = ((B[1] - C[1]) * (A[0] - C[0]) + (C[0] - B[0]) * (A[1] - C[1])); double a = ((B[1] - C[1]) * (P[0] - C[0]) + (C[0] - B[0]) * (P[1] - C[1])) / denominator; double b = ((C[1] - A[1]) * (P[0] - C[0]) + (A[0] - C[0]) * (P[1] - C[1])) / denominator; double c = 1 - a - b;

    // Check if all barycentric coordinates
    // are non-negative
    if (a >= 0 && b >= 0 && c >= 0)
    {
        return "Inside";
    }
    else
    {
        return "Outside";
    }
}

// Driver Code
public static void Main(string[] args)
{
    int[] A = { 0, 0 };
    int[] B = { 10, 30 };
    int[] C = { 20, 0 };
    int[] P = { 10, 15 };

    // Call the IsInsideTriangle function with
    // the given inputs
    string result = IsInsideTriangle(A, B, C, P);

    // Print the result
    Console.WriteLine(result);
}

}

JavaScript

// JavaScript program for the above approach

// Function to check if the point is inside // the triangle or not function isInsideTriangle(A, B, C, P) { // Calculate the barycentric coordinates // of point P with respect to triangle ABC let denominator = ((B[1] - C[1]) * (A[0] - C[0]) + (C[0] - B[0]) * (A[1] - C[1])); let a = ((B[1] - C[1]) * (P[0] - C[0]) + (C[0] - B[0]) * (P[1] - C[1])) / denominator; let b = ((C[1] - A[1]) * (P[0] - C[0]) + (A[0] - C[0]) * (P[1] - C[1])) / denominator; let c = 1 - a - b;

// Check if all barycentric coordinates // are non-negative if (a >= 0 && b >= 0 && c >= 0) { return "Inside"; } else { return "Outside"; } }

// Driver Code let A = [0, 0]; let B = [10, 30]; let C = [20, 0]; let P = [10, 15];

// Call the isInsideTriangle function with // the given inputs let result = isInsideTriangle(A, B, C, P);

// Print the result console.log(result);

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