Slicing columns in Python

Question

I am new in Python. I want to slice columns from index 1 to end of a marix and perform some operations on the those sliced out columns. Following is the code:

import numpy as np
import pandas as pd

train_df = pd.read_csv('train_475_60_W1.csv',header = None) 
train = train_df.as_matrix()
y = train[:,0]
X = train[:,1:-1]

The problem is if I execeute "train.shape", it gives me (89512, 61). But when I execute "X.shape", it give me (89512, 59). I was expecting to get 60 as I want to execute operations on all the colunms except the first one. Can anyone please help me in solving this?

don't you want X = train[:,1:]? with slicing here the end slice isn't included which is not what you want — EdChum
– EdChum, Commented Jul 7, 2017 at 7:42
"From index 1 to end of a matrix" is train[:,1:], not train[:,1:-1] — Daniel F
– Daniel F, Commented Jul 7, 2017 at 7:43

perigon · Accepted Answer · 2017-07-07 07:42:31Z

3

In the line

X = train[:,1:-1]

you cut off the last column. -1 refers to the last column, and Python includes the beginning but not the end of a slice - so lst[2:6] would give you entries 2,3,4, and 5. Correct it to

X = train[:,1:]

BTW, you can make your code format properly by including four spaces before each line (you can just highlight it and hit Ctrl+K).

answered Jul 7, 2017 at 7:42

perigon

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Sumaiya Iqbal Over a year ago

Thanks...for both the suggestions! It's working now!

Rayhane Mama · Accepted Answer · 2017-07-07 07:46:43Z

3

The thing you should know with slicing for single dimension even in normal lists is that it looks like this:

[start : end]

with start included and end excluded.

you can also use these:

[:x] # from the start to x
[x:] # from x to the end

you can then generalize than to 2D or more, so in your case it would be:

X = train[:,1:] # the first : to get all rows, and 1: to get all columns except the first

you can learn more about these in here if you want, it's a good way to practice

answered Jul 7, 2017 at 7:46

Rayhane Mama

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2 Comments

Sumaiya Iqbal Over a year ago

Thanks...I didn't know that Python doesn't include the last column in slicing! It's working Great! Thanks for the tutorial link!!

Rayhane Mama Over a year ago

No problem at all, don't forget to accept the answer that suited you most as a sign for future users who might see this question ;) @SumaiyaIqbal

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