Getting elements of a 1 dimensional array Numpy Python

Question

How would I be able to print the last value of element_val numpy arrays. It is the second index of [ 2. 2. 30.] which is 30. So the values will go like 30,40,50...670,680. How would I be able to do that since the numpy array is 1 dimensional?

input.csv file:

element,LNPT,SNPT,NLP,NSP,TNT,TPnL,MxPnL,MnPnL,MxU,MxD
[ 2.  2. 30.],0,0,4,4,8,-0.1,-0.0,-0.1,17127,-3
[ 2.  2. 40.],0,0,2,2,4,0.0,-0.0,-0.0,17141,-3
[ 2.  2. 50.],0,0,2,2,4,0.0,-0.0,-0.0,17139,-3
[ 2.  2. 60.],2,0,6,6,12,0.5,2.3,-1.9,17015,-3
[ 2.  2. 70.],1,0,4,4,8,0.3,0.3,-0.0,17011,-3

Code:

import numpy as np
import pandas as pd

my_data = pd.read_csv('input.csv').to_numpy()
element_vals = my_data[:,0]
print(element_vals)

Output:

['[ 2.  2. 30.]' '[ 2.  2. 40.]' '[ 2.  2. 50.]' ... '[  6.    7.5 660. ]'
 '[  6.    7.5 670. ]' '[  6.    7.5 680. ]']

That output looks like you printed an array to a file, rather than actually writing CSV, then tried to read the file as CSV anyway. — user2357112
– user2357112, Commented Apr 15, 2021 at 21:30
print should never be used as a way of saving NumPy array data. It discards data by default, and it's awkward to parse even when it doesn't discard data. Use numpy.save and numpy.load, or numpy.savetxt and numpy.loadtxt. — user2357112
– user2357112, Commented Apr 15, 2021 at 21:33
(Actually, considering details like the spacing and lack of extra brackets, it looks like you probably printed a number of arrays individually, possibly printing each row of a 2D array. Either way, it's a bad idea, and pandas.read_csv is the wrong tool to read this.) — user2357112
– user2357112, Commented Apr 15, 2021 at 21:41
Plus, element_vals has probably lost the first data row, since read_csv probably interpreted it as a column header. — user2357112
– user2357112, Commented Apr 15, 2021 at 21:42
If I read it directly from numpy it would give me an error and the [ 2. 2. 30.] values get outputted as nan when I use genfromtxt hence why I used pandas. I have updated to show what the issue to show what input.csv looks like. — tony selcuk
– tony selcuk, Commented Apr 15, 2021 at 22:24

tdy · Accepted Answer · 2021-04-16 00:21:29Z

1

Use str.strip(), str.split(), and to_numpy() to manually convert the df.element column into a real numpy array:

df = pd.read_csv('input.csv')
element_vals = df.element.str.strip('[]').str.split(expand=True).astype(float).to_numpy()

# array([[ 2.,  2., 30.],
#        [ 2.,  2., 40.],
#        [ 2.,  2., 50.],
#        [ 2.,  2., 60.],
#        [ 2.,  2., 70.]])

Then you can index as expected:

element_vals[:, 2]

# array([30., 40., 50., 60., 70.])

edited Apr 16, 2021 at 0:21

answered Apr 15, 2021 at 23:25

tdy

42k42 gold badges124 silver badges125 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Getting elements of a 1 dimensional array Numpy Python

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related