Python中list [index，：]的含义是什么

Question

What's the difference between list[index, :] and list[index:]??

for line in arraylines:
   line  = line.strip()
   listFromLine = line.split('\t')
   returnMat[index, :] = listFromLine[0:3]

At the beginning, returnMat is a zero matrix.

Answer 1

returnMat[index, :] is an example of numpy's array slicing syntax. It would retrieve rows (different rows correspond to the first index) of the matrix ( : means "get all of this index"). Example:

import numpy as np

mat = np.zeros((3, 5))

print(mat)  # 3 rows and 5 columns of zeros
mat[1, :] = 1
print(mat)  # all of second row is now ones

---

To go a bit further, it looks like you're constructing a matrix row-by-row from a file in your code snippet, which means you probably need to be doing something with index as well. I would suggest looking into enumerate .

Answer 2

list[index,:]: starts from index number till the end
list[index:]: it has ranged between numbers which are the output.