Overwrites array in end of loop Python

Question

I have an issue with writing data to a 2d array in a for loop. If I'm printing/plotting within the loop the values seems normal. While if I'm printing outside the loop, all rows in the array are equal. See attached screenshot. Do anyone have a suggest on how to prevent overwriting the values in the array?

import matplotlib.pyplot as plt
import math as mt
import numpy as np

I0=1 #
L=5 #[m] distance from slit screen to the observation screen
w=[668, 613, 575, 540, 505, 470, 425] #[nm] wavelengths
b=14 #[um] slit width

n=10 #length of array

x = np.linspace(-2*mt.pi,2*mt.pi, n)

bet=[[0.0]*n]*len(w) #beta
Ip=[[0.0]*n]*len(w) #intensity

fig,ax=plt.subplots()
ax2=ax.twinx()

# for 7 different wavelenths
for j in range(len(w)):
    # write to array
    for i in range(n):
        bet[j][i]=mt.pi*b/w[j]*mt.sin(x[i])
        Ip[j][i]=I0*(mt.sin(bet[j][i])/bet[j][i])**2
    ax.plot(x,bet[j])
    ax2.plot(x,Ip[j])
    print(Ip[j]) 
plt.show()

#to compare with the print within the loop
for k in range(len(w)):
    print(Ip[k]) #all rows is equal

Print/plot:

quantummind · Accepted Answer · 2021-04-23 15:00:19Z

2

You have the problem with these lines:

bet=[[0.0]*n]*len(w) #beta
Ip=[[0.0]*n]*len(w) #intensity

It is because when you multiply a list of lists then the rows will point to exactly the same list.

You should change these two lines to:

bet=[[0.0]*n for _ in range(len(w))] #beta
Ip=[[0.0]*n for _ in range(len(w))] #intensity

answered Apr 23, 2021 at 15:00

quantummind

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