CODEWITHSUNDEEP
pythonnumpy
David
David

Reputation: 103

About Numpy，a=np.array([1,2,3,4]),print a.shape[0]. why it will output 4?

import numpy as np
a = np.array([1,2,3,4])
print a.shape[0]

Why it will output 4?

The array [1,2,3,4], it's rows should be 1, I think , so who can explain the reason for me?

Upvotes: 1

Views: 5988

Answers (6)

Wildan Aulia
Wildan Aulia

Reputation: 1

import numpy as np
X = np.array([[1,2,3], [4,5,6], [7,8,9]])
y = np.array([[9,7,8], [3,2,1], [6,5,4]])
Z = x[:2,1:] + y[1:3,:2]
print(z)

Upvotes: -1

alexisrozhkov
alexisrozhkov

Reputation: 1632

If you'll print a.ndim you'll get 1. That means that a is a one-dimensional array (has rank 1 in numpy terminology), with axis length = 4. It's different from 2D matrix with a single row or column (rank 2).

More on ranks

Related questions:

Upvotes: 1

hiro protagonist
hiro protagonist

Reputation: 46849

because

print(a.shape)  # -> (4,)

what you think (or want?) to have is

a = np.array([[1],[2],[3],[4]])
print(a.shape)  # -> (4, 1)

or rather (?)

a = np.array([[1, 2 , 3 , 4]])
print(a.shape)  # -> (1, 4)

Upvotes: 2

Peiqin
Peiqin

Reputation: 388

a = np.array([1, 2, 3, 4])

by doing this, you get a a as a ndarray, and it is a one-dimension array. Here, the shape (4,) means the array is indexed by a single index which runs from 0 to 3. You can access the elements by the index 0~3. It is different from multi-dimensional arrays.

You can refer to more help from this link Difference between numpy.array shape (R, 1) and (R,).

Upvotes: 0

Chandan Purohit
Chandan Purohit

Reputation: 2403

numpy arrays returns the dimensions of the array. So, when you create an array using,

a = np.array([1,2,3,4])

you get an array with 4 dimensions. You can check it by printing the shape,

print(a.shape) #(4,)

So, what you get is NOT a 1x4 matrix. If you want that do,

a = numpy.array([1,2,3,4]).reshape((1,4))
print(a.shape)

Or even better,

a = numpy.array([[1,2,3,4]])

Upvotes: 0

gauri
gauri

Reputation: 31

The shape attribute for numpy arrays returns the dimensions of the array. If a has n rows and m columns, then a.shape is (n,m). So a.shape[0] is n and a.shape[1] is m.

Upvotes: 0

Related Questions