Python-Numpy数组索引作为元组

Question

If we create an array with Numpy , we can use many functionalities given by numpy library.

For example if c is a matrix

print(c[:,1])

will print every value in the column 1.

Now, when i index the c matrix in this way, am i indexing using a tuple ? If yes, how is possible to have a tuple with ':' inside ?

Answer 1

The colon syntax is syntactical sugar for a slice(..) object. Your expression is equvalent to:

#        v slice object
print(c[(slice(None), 1)])
#       ^   tuple      ^

So you have passed a tuple containing a slice(None) object as first element, and 1 as second element.

The mapping of slice syntax to slice(..) objects is as follows:

1. the colon : is equivalent to slice(None) ;
2. if it is :b , then it is equivalent to slice(b) ;
3. a: is equivalent to slice(a, None) ;
4. a:b is equivalent to slice(a, b) ;
5. ::c is equivalent to slice(None, None, c) ;
6. :b:c to slice(None, b, c) ;
7. a::c is equivalent to slice(a, None, c) ; and
8. a:b:c to slice(a, b, c) .

Note that slice syntax is only supported in the context of an itemgetter (so x[..] ).