CODEWITHSUNDEEP
pythonarraysloopscounter
blablabla
blablabla

Reputation: 314

How to calculate how many coordinates lie between 2 circles of radius 35 and 65?

I know that the answer should be around 60% of my coordinates lie within this but im only getting about 20%

import random
import pylab
import numpy
pylab.close("all")                                              #all import statements
x = [(random.randint(0,100)) for i in range(3000)]      #creating list of x coordinates
y = [(random.randint(0,100)) for j in range(3000)]      #creating list of y coordinates
array=zip(x,y)                                                  #creating an array by combining the x and y coordinates
counter = 0
for i, j in array:
        if 35**2 <= (i**2+j**2) <= 65**2:
                counter+= 1
print counter

Can anyone tell me what im doing wrong?

Upvotes: 0

Views: 197

Answers (4)

HYRY
HYRY

Reputation: 97291

You can visualize this by pylab.imshow():

import numpy as np
import pylab as pl
y, x = np.mgrid[:100, :100]
d = x**2 + y**2
mask = (35**2 <= d) & (d <= 65**2)
pl.imshow(mask, origin="lower")

output:

enter image description here

Upvotes: 1

Andr&#233;
Andr&#233;

Reputation: 1024

Your area is 10000, in the first quadrant. You're looking at a sample with an area of 

pi(65**2 - 35**2)/4

which is 2356 ... or 23.56% of the total area. So, you are getting the right answer.

BTW, your code does not need those pylab and numpy imports.

Upvotes: 2

abarnert
abarnert

Reputation: 365747

The problem is this line:

if 35**2 <= (i**2+j**2) <= 65**2:

This is checking whether the coordinates are between a circle of radius 35 centered at the origin, and a circle of radius 65 centered at the origin.

But since the coordinates you're testing are always in the positive-positive quadrant, only 1/4th as many will fall within the circles as you're expecting.

The simplest fix is to change the randint calls to use (-50, 50) instead of (0, 100).

Upvotes: 3

lollercoaster
lollercoaster

Reputation: 16503

Nope, you should be getting less than 30%. You are only filling the first quadrant!

Here's a plot:

enter image description here

Upvotes: 0

Related Questions