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jfmejiat
Asked: 2020-03-10 21:48:43 +0800 CST 2020-03-10 21:48:43 +0800 CST

How do I calculate the area of ​​a polygon knowing its coordinates?

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I want to calculate the area of ​​any polygon by entering its coordinates, I made the function to calculate the area according to the formula that is in the image and when I run the program the value of the area is wrong.

Point 1:

n=int(input("Ingrese el número de lados del polígono:"))

x=[]

y=[]

vértices=[]

for i in range(n):
    coordenada_x=int(input("Ingrese el valor de la coordenada x"+str(i)+":"))
    coordenada_y=int(input("Ingrese el valor de la coordenada y"+str(i)+":"))
    x.append(coordenada_x)
    y.append(coordenada_y)
    vértices.append(coordenada_x,coordenada_y)

x.append(x[0])
y.append(y[0])

Point 2:

from entrada import n
from entrada import x,y

for i in range(n):
  Suma=(x[i]*(y[i+1]-y[i-1]))

Área_Polígono=(1/2)*abs(Suma)

print("Área del polígono=:",Área_Polígono,"U^2")

When running the program:

Ingrese el número de lados del polígono:3
Ingrese el valor de la coordenada x0:0
Ingrese el valor de la coordenada y0:0
Ingrese el valor de la coordenada x1:1
Ingrese el valor de la coordenada y1:0
Ingrese el valor de la coordenada x2:1
Ingrese el valor de la coordenada y2:1
Coordenadas_x=[0,1,1,0]
Coordenadas_y=[0,0,1,0]
Vértices=[(0,0),(1,0),(1,1)]

The error occurs with the area, since the following appears:

Área del polígono=0.0 U^2

When the area in this case should be 0.5 U^2

python

1 Answers

  1. Best Answer

    You have the problem in:

    x.append([coordenada_x,x[0]])
y.append([coordenada_y,y[0]])

    What this does is create a new list using a literal:

    x.append([coordenada_x, x[0]])
         ^                  ^
         ^                  ^

    that contains as first element the last coordinate added in the for:

    x.append([coordenada_x, x[0]])
          ^^^^^^^^^^^^
          ^^^^^^^^^^^^

    and as second the first coordinate (index 0 of the list):

    x.append([coordenada_x, x[0]])
                        ^^^^
                        ^^^^

    and that new list adds it to the end of the list you already had as a new item, in your case:

    coordenadas_x -> [0, 1, 2, [0, 2]]
coordenadas_y -> [0, 1, 2, [0, 2]]

    Actually you have complicated yourself a lot without need, you just need to do:

    x.append(x[0])
y.append(y[0])

    The list is assumed to have at least one element, it actually must have at least 3 by the polygon definition itself, which you can consider validating in case the user enters n < 3.

    n = int(input("Ingrese el número de vértices del polígono: "))

x = []
y = []

for i in range(n):
    coordenada_x = int(input(f"Ingrese el valor de la coordenada x{i}: "))
    coordenada_y = int(input(f"Ingrese el valor de la coordenada y{i}: "))
    x.append(coordenada_x)
    y.append(coordenada_y)

x.append(x[0])
y.append(y[0])

print("coordenadas_x=", x)
print("coordenadas_y=", y)

    To iterate by grouping the pairs x, yof each coordinate you can:

    • It is best to use the builtin zip:

      for cord_x, cord_y in zip(x, y):
    print(f"Coordenada X: {cord_x}; Coordenada Y: {cord_y}")

    • for in+ enumerate+ indexed:

      for ind, cord_x in enumerate(x):
    cord_y = y[ind]
    print(f"Coordenada X: {cord_x}; Coordenada Y: {cord_y}")

    • Indexing with fory range(the least efficient and pythonic):

      for ind in range(len(x)):
    cord_x = x[ind]
    cord_y = y[ind]
    print(f"Coordenada X: {cord_x}; Coordenada Y: {cord_y}")

    Or if you want a new list with the pairs of coordinates per vertex:

    vertices = list(zip(x, y))
print(vertices)

    [(0, 0), (1, 1), (2, 2), (0, 0)]

    or generate the list directly in the for:

    x = []
y = []
vertices = []

for i in range(n):
    coordenada_x = int(input(f"Ingrese el valor de la coordenada x{i}: "))
    coordenada_y = int(input(f"Ingrese el valor de la coordenada y{i}: "))
    x.append(coordenada_x)
    y.append(coordenada_y)
    vertices.append((x, y))

    In both cases it would iterate the same:

    for cord_x, cord_y in vertices:
    print(f"Coordenada X: {cord_x}; Coordenada Y: {cord_y}")
    • 2