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# Program to check points are forming concave polygon or not in Python

Suppose we have outer points of a polygon in clockwise order. We have to check these points are forming a convex polygon or not. A polygon is said to be concave if any one of its interior angle is greater than 180°.

From this diagram it is clear that for each three consecutive points the interior angle is not more than 180° except CDE.

So, if the input is like points = [(3,4), (4,7),(7,8),(8,4),(12,3),(10,1),(5,2)], then the output will be True.

To solve this, we will follow these steps −

- n := size of points
- for i in range 0 to size of points, do
- p1 := points[i-2] when i > 1, otherwise points[n-2]
- p2 := points[i-2] when i > 0, otherwise points[n-1]
- p3 := points[i]
- if angle between points (p1, p2, p3) > 180, then
- return True

- return False

## Example

Let us see the following implementation to get better understanding −

import math def get_angle(a, b, c): angle = math.degrees(math.atan2(c[1]-b[1], c[0]-b[0]) - math.atan2(a[1]-b[1], a[0]-b[0])) return angle + 360 if angle < 0 else angle def solve(points): n = len(points) for i in range(len(points)): p1 = points[i-2] p2 = points[i-1] p3 = points[i] if get_angle(p1, p2, p3) > 180: return True return False points = [(3,4), (4,7),(7,8),(8,4),(12,3),(10,1),(5,2)] print(solve(points))

## Input

[(3,4), (4,7),(7,8),(8,4),(12,3),(10,1),(5,2)]

## Output

True

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