I don't understand broadcasting. The documentation explains the rules of broadcasting but doesn't seem to define it in English. My guess is that broadcasting is when NumPy fills a smaller dimensional array with dummy data in order to perform an operation. But this doesn't work:
>>> x = np.array([1,3,5])
>>> y = np.array([2,4])
>>> x+y
*** ValueError: operands could not be broadcast together with shapes (3,) (2,)
The error message hints that I'm on the right track, though. Can someone define broadcasting and then provide some simple examples of when it works and when it doesn't?
The term broadcasting describes how numpy treats arrays with different shapes during arithmetic operations.
It's basically a way numpy can expand the domain of operations over arrays.
The only requirement for broadcasting is a way aligning array dimensions such that either:
Aligned dimensions are equal.
One of the aligned dimensions is 1.
So, for example if:
x = np.ndarray(shape=(4,1,3))
y = np.ndarray(shape=(3,3))
You could not align x and y like so:
4 x 1 x 3
3 x 3
But you could like so:
4 x 1 x 3
3 x 3
How would an operation like this result?
Suppose we have:
x = np.ndarray(shape=(1,3), buffer=np.array([1,2,3]),dtype='int')
array([[1, 2, 3]])
y = np.ndarray(shape=(3,3), buffer=np.array([1,1,1,1,1,1,1,1,1]),dtype='int')
array([[1, 1, 1],
[1, 1, 1],
[1, 1, 1]])
The operation x + y would result in:
array([[2, 3, 4],
[2, 3, 4],
[2, 3, 4]])
I hope you caught the drift. If you did not, you can always check the official documentation here.
Cheers!
1.What is Broadcasting?
Broadcasting is a Tensor operation. Helpful in Neural Network (ML, AI)
2.What is the use of Broadcasting?
Without Broadcasting addition of only identical Dimension(shape) Tensors is supported.
Broadcasting Provide us the Flexibility to add two Tensors of Different Dimension.
for Example: adding a 2D Tensor with a 1D Tensor is not possible without broadcasting see the image explaining Broadcasting pictorially
Run the Python example code understand the concept
x = np.array([1,3,5,6,7,8])
y = np.array([2,4,5])
X=x.reshape(2,3)
x is reshaped to get a 2D Tensor X of shape (2,3), and adding this 2D Tensor X with 1D Tensor y of shape(1,3) to get a 2D Tensor z of shape(2,3)
print("X =",X)
print("\n y =",y)
z=X+y
print("X + y =",z)
You are almost correct about smaller Tensor, no ambiguity, the smaller tensor will be broadcasted to match the shape of the larger tensor.(Small vector is repeated but not filled with Dummy Data or Zeros to Match the Shape of larger).
3. How broadcasting happens?
Broadcasting consists of two steps:
1 Broadcast axes are added to the smaller tensor to match the ndim of
the larger tensor.
2 The smaller tensor is repeated alongside these new axes to match the full shape
of the larger tensor.
4. Why Broadcasting not happening in your code?
your code is working but Broadcasting can not happen here because both Tensors are different in shape but Identical in Dimensional(1D).
Broadcasting occurs when dimensions are nonidentical.
what you need to do is change Dimension of one of the Tensor, you will experience Broadcasting.
5. Going in Depth.
Broadcasting(repetition of smaller Tensor) occurs along broadcast axes but since both the Tensors are 1 Dimensional there is no broadcast Axis.
Don't Confuse Tensor Dimension with the shape of tensor,
Tensor Dimensions are not same as Matrices Dimension.
Broadcasting is numpy trying to be smart when you tell it to perform an operation on arrays that aren't the same dimension. For example:
2 + np.array([1,3,5]) == np.array([3, 5, 7])
Here it decided you wanted to apply the operation using the lower dimensional array (0-D) on each item in the higher-dimensional array (1-D).
You can also add a 0-D array (scalar) or 1-D array to a 2-D array. In the first case, you just add the scalar to all items in the 2-D array, as before. In the second case, numpy will add row-wise:
In [34]: np.array([1,2]) + np.array([[3,4],[5,6]])
Out[34]:
array([[4, 6],
[6, 8]])
There are ways to tell numpy to apply the operation along a different axis as well. This can be taken even further with applying an operation between a 3-D array and a 1-D, 2-D, or 0-D array.
>>> x = np.array([1,3,5])
>>> y = np.array([2,4])
>>> x+y
*** ValueError: operands could not be broadcast together with shapes (3,) (2,)
Broadcasting is how numpy do math operations with array of different shapes. Shapes are the format the array has, for example the array you used, x , has 3 elements of 1 dimension; y has 2 elements and 1 dimension.
To perform broadcasting there are 2 rules:
1) Array have the same dimensions(shape) or
2)The dimension that doesn't match equals one.
for example x has shape(2,3) [or 2 lines and 3 columns];
y has shape(2,1) [or 2 lines and 1 column]
Can you add them? x + y?
Answer: Yes, because the mismatched dimension is equal to 1 (the column in y). If y had shape(2,4) broadcasting would not be possible, because the mismatched dimension is not 1.
In the case you posted:
operands could not be broadcast together with shapes (3,) (2,);
it is because 3 and 2 mismatched altough both have 1 line.
I would like to suggest to try the np.broadcast_arrays, run some demos may give intuitive ideas. Official Document is also helpful. From my current understanding, numpy will compare the dimension from tail to head. If one dim is 1, it will broadcast in the dimension, if one array has more axes, such (256*256*3) multiply (1,), you can view (1) as (1,1,1). And broadcast will make (256,256,3).
Related
I am trying to perform non-consectuitive slicing of a multidimensional array like this (Matlab peudo code)
A = B(:,:,[1,3],[2,4,6]) %A and B are two 4D matrices
But when I try to write this code in Python:
A = B[:,:,np.array([0,2]),np.array([1,3,5])] #A and B are two 4D arrays
it gives an error: IndexError: shape mismatch: indexing arrays could not be broadcast...
It should be noted that slicing for one dimension each time works fine!
In numpy, if you use more than one fancy index (i.e. array) to index different dimension of the same array at the same time, they must broadcast. This is designed such that indexing can be more powerful. For your situation, the simplest way to solve the problem is indexing twice:
B[:, :, [0,2]] [..., [1,3,5]]
where ... stands for as many : as possible.
Indexing twice this way would generate some extra data moving time. If you want to index only once, make sure they broadcast (i.e. put fancy indices on different dimension):
B[:, :, np.array([0,2])[:,None], [1,3,5]]
which will result in a X by Y by 2 by 3 array. On the other hand, you can also do
B[:, :, [0,2], np.array([1,3,5])[:,None]]
which will result in a X by Y by 3 by 2 array. The [1,3,5] axis is transposed before the [0,2] axis.
Yon don't have to use np.array([0,2]) if you don't need to do fancy operation with it. Simply [0,2] is fine.
np.array([0,2])[:,None] is equivalent to [[0],[2]], where the point of [:,None] is to create an extra dimension such that the shape becomes (2,1). Shape (2,) and (3,) cannot broadcast, while shape (2,1) and (3,) can, which becomes (2,3).
I sometimes work with 1D arrays:
A = np.array([1, 2, 3, 4])
or 2D arrays (mono or stereo signals read with scipy.io.wavfile):
A = np.array([[1, 2], [3, 4], [5, 6], [7,8]])
Without having to distinguish these 2 cases with an if A.ndim == 2:..., is there a simple one-line solution to multiply this array A by an 1D array B = np.linspace(0., 1., 4)?
If A is 1D, then it's just A * B, and if A is 2D, what I mean is multiply each line of A by each element of B.
Note: this question arises naturally when working with both mono and stereo sounds read with scipy.io.wavfile.
Approach #1
We can use einsum to cover generic ndarrays -
np.einsum('i...,i->i...',A,B)
The trick that works here is the ellipsis that broadcasts for the trailing dimensions after the first axis as they are in A and keeps them in the output, while keeping the first axes for the two inputs aligned, which is the intended multiplication here. With A as 1D array, there's no broadcasting and that essentially reduces to : np.einsum('i,i->i',A,B) under the hoods.
Schematically put :
A : i x ....
B : i
out : i x ....
Hence, covers for A with any number of dimensions.
More info on the use of ellipsis from the docs :
To enable and control broadcasting, use an ellipsis. Default
NumPy-style broadcasting is done by adding an ellipsis to the left of
each term, like np.einsum('...ii->...i', a). To take the trace along
the first and last axes, you can do np.einsum('i...i', a), or to do a
matrix-matrix product with the left-most indices instead of rightmost,
you can do np.einsum('ij...,jk...->ik...', a, b).
Approach #2
Using the fact that we are trying to align the first axis of A with the only axis of 1D array B, we can simply transpose A, multiply with B and finally transpose back -
(A.T*B).T
I don't understand broadcasting. The documentation explains the rules of broadcasting but doesn't seem to define it in English. My guess is that broadcasting is when NumPy fills a smaller dimensional array with dummy data in order to perform an operation. But this doesn't work:
>>> x = np.array([1,3,5])
>>> y = np.array([2,4])
>>> x+y
*** ValueError: operands could not be broadcast together with shapes (3,) (2,)
The error message hints that I'm on the right track, though. Can someone define broadcasting and then provide some simple examples of when it works and when it doesn't?
The term broadcasting describes how numpy treats arrays with different shapes during arithmetic operations.
It's basically a way numpy can expand the domain of operations over arrays.
The only requirement for broadcasting is a way aligning array dimensions such that either:
Aligned dimensions are equal.
One of the aligned dimensions is 1.
So, for example if:
x = np.ndarray(shape=(4,1,3))
y = np.ndarray(shape=(3,3))
You could not align x and y like so:
4 x 1 x 3
3 x 3
But you could like so:
4 x 1 x 3
3 x 3
How would an operation like this result?
Suppose we have:
x = np.ndarray(shape=(1,3), buffer=np.array([1,2,3]),dtype='int')
array([[1, 2, 3]])
y = np.ndarray(shape=(3,3), buffer=np.array([1,1,1,1,1,1,1,1,1]),dtype='int')
array([[1, 1, 1],
[1, 1, 1],
[1, 1, 1]])
The operation x + y would result in:
array([[2, 3, 4],
[2, 3, 4],
[2, 3, 4]])
I hope you caught the drift. If you did not, you can always check the official documentation here.
Cheers!
1.What is Broadcasting?
Broadcasting is a Tensor operation. Helpful in Neural Network (ML, AI)
2.What is the use of Broadcasting?
Without Broadcasting addition of only identical Dimension(shape) Tensors is supported.
Broadcasting Provide us the Flexibility to add two Tensors of Different Dimension.
for Example: adding a 2D Tensor with a 1D Tensor is not possible without broadcasting see the image explaining Broadcasting pictorially
Run the Python example code understand the concept
x = np.array([1,3,5,6,7,8])
y = np.array([2,4,5])
X=x.reshape(2,3)
x is reshaped to get a 2D Tensor X of shape (2,3), and adding this 2D Tensor X with 1D Tensor y of shape(1,3) to get a 2D Tensor z of shape(2,3)
print("X =",X)
print("\n y =",y)
z=X+y
print("X + y =",z)
You are almost correct about smaller Tensor, no ambiguity, the smaller tensor will be broadcasted to match the shape of the larger tensor.(Small vector is repeated but not filled with Dummy Data or Zeros to Match the Shape of larger).
3. How broadcasting happens?
Broadcasting consists of two steps:
1 Broadcast axes are added to the smaller tensor to match the ndim of
the larger tensor.
2 The smaller tensor is repeated alongside these new axes to match the full shape
of the larger tensor.
4. Why Broadcasting not happening in your code?
your code is working but Broadcasting can not happen here because both Tensors are different in shape but Identical in Dimensional(1D).
Broadcasting occurs when dimensions are nonidentical.
what you need to do is change Dimension of one of the Tensor, you will experience Broadcasting.
5. Going in Depth.
Broadcasting(repetition of smaller Tensor) occurs along broadcast axes but since both the Tensors are 1 Dimensional there is no broadcast Axis.
Don't Confuse Tensor Dimension with the shape of tensor,
Tensor Dimensions are not same as Matrices Dimension.
Broadcasting is numpy trying to be smart when you tell it to perform an operation on arrays that aren't the same dimension. For example:
2 + np.array([1,3,5]) == np.array([3, 5, 7])
Here it decided you wanted to apply the operation using the lower dimensional array (0-D) on each item in the higher-dimensional array (1-D).
You can also add a 0-D array (scalar) or 1-D array to a 2-D array. In the first case, you just add the scalar to all items in the 2-D array, as before. In the second case, numpy will add row-wise:
In [34]: np.array([1,2]) + np.array([[3,4],[5,6]])
Out[34]:
array([[4, 6],
[6, 8]])
There are ways to tell numpy to apply the operation along a different axis as well. This can be taken even further with applying an operation between a 3-D array and a 1-D, 2-D, or 0-D array.
>>> x = np.array([1,3,5])
>>> y = np.array([2,4])
>>> x+y
*** ValueError: operands could not be broadcast together with shapes (3,) (2,)
Broadcasting is how numpy do math operations with array of different shapes. Shapes are the format the array has, for example the array you used, x , has 3 elements of 1 dimension; y has 2 elements and 1 dimension.
To perform broadcasting there are 2 rules:
1) Array have the same dimensions(shape) or
2)The dimension that doesn't match equals one.
for example x has shape(2,3) [or 2 lines and 3 columns];
y has shape(2,1) [or 2 lines and 1 column]
Can you add them? x + y?
Answer: Yes, because the mismatched dimension is equal to 1 (the column in y). If y had shape(2,4) broadcasting would not be possible, because the mismatched dimension is not 1.
In the case you posted:
operands could not be broadcast together with shapes (3,) (2,);
it is because 3 and 2 mismatched altough both have 1 line.
I would like to suggest to try the np.broadcast_arrays, run some demos may give intuitive ideas. Official Document is also helpful. From my current understanding, numpy will compare the dimension from tail to head. If one dim is 1, it will broadcast in the dimension, if one array has more axes, such (256*256*3) multiply (1,), you can view (1) as (1,1,1). And broadcast will make (256,256,3).
I am trying to concatenate two arrays: a and b, where
a.shape
(1460,10)
b.shape
(1460,)
I tried using hstack and concatenate as:
np.hstack((a,b))
c=np.concatenate(a,b,0)
I am stuck with the error
ValueError: all the input arrays must have same number of dimensions
Please guide me for concatenation and generating array c with dimensions 1460 x 11.
Try
b = np.expand_dims( b,axis=1 )
then
np.hstack((a,b))
or
np.concatenate( (a,b) , axis=1)
will work properly.
np.c_[a, b] concatenates along the last axis.
Per the docs,
... arrays will be stacked along their last axis after
being upgraded to at least 2-D with 1's post-pended to the shape
Since b has shape (1460,) its shape gets upgraded to (1460, 1) before concatenation along the last axis.
In [26]: c = np.c_[a,b]
In [27]: c.shape
Out[27]: (1460, 11)
The most basic operation that works is:
np.concatenate((a,b[:,None]),axis=1)
The [:,None] bit turns b into a (1060,1) array. Now the 1st dimensions of both arrays match, and you can easily concatenate on the 2nd.
There a many ways of adding the 2nd dimension to b, such as reshape and expanddims. hstack uses atleast_1d which does not help in this case. atleast_2d adds the None on the wrong side. I strongly advocate learning the [:,None] syntax.
Once the arrays are both 2d and match on the correct dimensions, concatenation is easy.
In numpy, I have two "arrays", X is (m,n) and y is a vector (n,1)
using
X*y
I am getting the error
ValueError: operands could not be broadcast together with shapes (97,2) (2,1)
When (97,2)x(2,1) is clearly a legal matrix operation and should give me a (97,1) vector
EDIT:
I have corrected this using X.dot(y) but the original question still remains.
dot is matrix multiplication, but * does something else.
We have two arrays:
X, shape (97,2)
y, shape (2,1)
With Numpy arrays, the operation
X * y
is done element-wise, but one or both of the values can be expanded in one or more dimensions to make them compatible. This operation is called broadcasting. Dimensions, where size is 1 or which are missing, can be used in broadcasting.
In the example above the dimensions are incompatible, because:
97 2
2 1
Here there are conflicting numbers in the first dimension (97 and 2). That is what the ValueError above is complaining about. The second dimension would be ok, as number 1 does not conflict with anything.
For more information on broadcasting rules: http://docs.scipy.org/doc/numpy/user/basics.broadcasting.html
(Please note that if X and y are of type numpy.matrix, then asterisk can be used as matrix multiplication. My recommendation is to keep away from numpy.matrix, it tends to complicate more than simplifying things.)
Your arrays should be fine with numpy.dot; if you get an error on numpy.dot, you must have some other bug. If the shapes are wrong for numpy.dot, you get a different exception:
ValueError: matrices are not aligned
If you still get this error, please post a minimal example of the problem. An example multiplication with arrays shaped like yours succeeds:
In [1]: import numpy
In [2]: numpy.dot(numpy.ones([97, 2]), numpy.ones([2, 1])).shape
Out[2]: (97, 1)
Per numpy docs:
When operating on two arrays, NumPy compares their shapes element-wise. It starts with the trailing dimensions, and works its way forward. Two dimensions are compatible when:
they are equal, or
one of them is 1
In other words, if you are trying to multiply two matrices (in the linear algebra sense) then you want X.dot(y) but if you are trying to broadcast scalars from matrix y onto X then you need to perform X * y.T.
Example:
>>> import numpy as np
>>>
>>> X = np.arange(8).reshape(4, 2)
>>> y = np.arange(2).reshape(1, 2) # create a 1x2 matrix
>>> X * y
array([[0,1],
[0,3],
[0,5],
[0,7]])
You are looking for np.matmul(X, y). In Python 3.5+ you can use X # y.
It's possible that the error didn't occur in the dot product, but after.
For example try this
a = np.random.randn(12,1)
b = np.random.randn(1,5)
c = np.random.randn(5,12)
d = np.dot(a,b) * c
np.dot(a,b) will be fine; however np.dot(a, b) * c is clearly wrong (12x1 X 1x5 = 12x5 which cannot element-wise multiply 5x12) but numpy will give you
ValueError: operands could not be broadcast together with shapes (12,1) (1,5)
The error is misleading; however there is an issue on that line.
Use np.mat(x) * np.mat(y), that'll work.
We might confuse ourselves that a * b is a dot product.
But in fact, it is broadcast.
Dot Product :
a.dot(b)
Broadcast:
The term broadcasting refers to how numpy treats arrays with different
dimensions during arithmetic operations which lead to certain
constraints, the smaller array is broadcast across the larger array so
that they have compatible shapes.
(m,n) +-/* (1,n) → (m,n) : the operation will be applied to m rows
Convert the arrays to matrices, and then perform the multiplication.
X = np.matrix(X)
y = np.matrix(y)
X*y
we should consider two points about broadcasting.
first: what is possible.
second: how much of the possible things is done by numpy.
I know it might look a bit confusing, but I will make it clear by some example.
lets start from the zero level.
suppose we have two matrices. first matrix has three dimensions (named A) and the second has five (named B).
numpy tries to match last/trailing dimensions. so numpy does not care about the first two dimensions of B.
then numpy compares those trailing dimensions with each other. and if and only if they be equal or one of them be 1, numpy says "O.K. you two match". and if it these conditions don't satisfy, numpy would "sorry...its not my job!".
But I know that you may say comparison was better to be done in way that can handle when they are devisable(4 and 2 / 9 and 3). you might say it could be replicated/broadcasted by a whole number(2/3 in out example). and i am agree with you. and this is the reason I started my discussion with a distinction between what is possible and what is the capability of numpy.
This is because X and y are not the same types. for example X is a numpy matrix and y is a numpy array!
Error: operands could not be broadcast together with shapes (2,3) (2,3,3)
This kind of error occur when the two array does not have the same shape.
to correct this you need reshape one array to match the other.
see example below
a1 = array([1, 2, 3]), shape = (2,3)
a3 =array([[[1., 2., 3.],
[2., 3., 2.],
[2., 4., 5.]],
[[1., 0., 3.],
[2., 3., 7.],
[2., 4., 6.]]])
with shape = (2,3,3)
IF i try to run np.multiply(a2,a3) it will return the error below
Error: operands could not be broadcast together with shapes (2,3) (2,3,3)
to solve this check out the broadcating rules
which state hat Two dimensions are compatible when:
#1.they are equal, or
#2.one of them is 1`
Therefore lets reshape a2.
reshaped = a2.reshape(2,3,1)
Now try to run np.multiply(reshaped,a3)
the multiplication will run SUCCESSFUL!!
ValueError: operands could not be broadcast together with shapes (x ,y) (a ,b)
where x ,y are variables
Basically this error occurred when value of y (no. of columns) doesn't equal to the number of elements in another multidimensional array.
Now let's go through by ex=>
coding apart
import numpy as np
arr1= np.arange(12).reshape(3,
output of arr1
array([[ 0, 1, 2, 3],
[ 4, 5, 6, 7],
[ 8, 9, 10, 11]])
arr2= np.arange(4).reshape(1,4)
or (both are same 1 rows and 4 columns)
arr2= np.arange(4)
ouput of arr2=>
array([0, 1, 2, 3])
no of elements in arr2 is equal no of no. of the columns in arr1 it will be excute.
for x,y in np.nditer([a,b]):
print(x,y)
output =>
0 0
1 1
2 2
3 3
4 0
5 1
6 2
7 3
8 0
9 1
10 2
11 3