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pythonnumpyloopsnumpy-ndarray
Matt Winther
Matt Winther

Reputation: 33

numpy ndarray object has no attribute append

I am struggling with a program I am making on the part where I have to store values that I get from my loop in an array.

What I tried to do is to make an empty array called M. Then for every new value "a" calculated in a loop I use the command M.append(a) to add the value a to the array M.

The thing is, python says this error : 'numpy.ndarray' object has no attribute 'append'

and I don't know how to fix it.

Here is my code :

import numpy as np
from matplotlib import pyplot as plt
with open('expFcn0.txt') as f:
    M = np.array([])
    print(M)
    lines = f.readlines()
    x = [float(line.split()[0]) for line in lines]
    y = [float(line.split()[1]) for line in lines]
    for i in range(0,181):
        a=np.log(y[i])/x[i]
        print(a)
        i=i+1
        M.append(a)
    print(M)

    
    plt.plot(x, y, 'r--')
    plt.xlabel('Time')
    plt.ylabel('Biomass')
    plt.title('Exponential Function')
    plt.show()

Thank you very much!

Upvotes: 1

Views: 12061

Answers (3)

jkr
jkr

Reputation: 19260

Other answers explain that numpy arrays do not have an .append() method and point to numpy.append. Using numpy.append, however, is bad practice because it creates a new array each time. A better solution is to create one numpy and fill it during the for loop (see end of answer).

An even better solution would make use of numpy's broadcasting. That's a core feature of numpy, and it's what helps make numpy fast.

import numpy as np

with open('expFcn0.txt') as f:
    lines = f.readlines()
    x = np.array([float(line.split()[0]) for line in lines])
    y = np.array([float(line.split()[1]) for line in lines])

M = np.log(y) / x

You can also look into numpy.loadtxt to read the file into a numpy array directly.

How to fill a numpy array in a for loop:

import numpy as np

with open('expFcn0.txt') as f:
    lines = f.readlines()
    x = [float(line.split()[0]) for line in lines]
    y = [float(line.split()[1]) for line in lines]

M = np.zeros(181)
for i in range(181):
    a = np.log(y[i])/x[i]
    print(a)
    M[i] = a

Upvotes: 1

Shradha
Shradha

Reputation: 2442

Numpy arrays do not have an append method. Use the Numpy append function instead:

M = np.append(M, a)

Upvotes: 0

Yehuda
Yehuda

Reputation: 1893

Numpy arrays don't have a method append(). You need to use np.append(array, values) as per the documentation, or for your case, np.append(M, a).

Upvotes: 3

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