How do I get the value of a tensor in PyTorch?
Printing the tensor gives:
>>> x = torch.tensor([3])
>>> print(x)
tensor([3])
Likewise indexing its .data
gives:
>>> x.data[0]
tensor(3)
How do I get just the value 3
?
You can use x.item()
to get a Python number from a tensor that has one element.
Convert tensor to numpy:
x.numpy()[0]
To get a value from single element tensor x.item()
works always:
Example : Single element tensor on CPU
x = torch.tensor([3])
x.item()
Output:
3
Example : Single element tensor on CPU with AD
x = torch.tensor([3.], requires_grad=True)
x.item()
Output:
3.0
NOTE: We needed to use floating point arithmetic for AD
Example : Single element tensor on CUDA
x = torch.tensor([3], device='cuda')
x.item()
Output:
3
Example : Single element tensor on CUDA with AD
x = torch.tensor([3.], device='cuda', requires_grad=True)
x.item()
Output:
3.0
Example : Single element tensor on CUDA with AD again
x = torch.ones((1,1), device='cuda', requires_grad=True)
x.item()
Output:
1.0
To get a value from non single element tensor we have to be careful:
The next example will show that PyTorch tensor residing on CPU shares the same storage as numpy array na
Example: Shared storage
import torch
a = torch.ones((1,2))
print(a)
na = a.numpy()
na[0][0]=10
print(na)
print(a)
Output:
tensor([[1., 1.]])
[[10. 1.]]
tensor([[10., 1.]])
Example: Eliminate effect of shared storage, copy numpy array first
To avoid the effect of shared storage we need to copy()
the numpy array na
to a new numpy array nac
. Numpy copy()
method creates the new separate storage.
import torch
a = torch.ones((1,2))
print(a)
na = a.numpy()
nac = na.copy()
nac[0][0]=10
print(nac)
print(na)
print(a)
Output:
tensor([[1., 1.]])
[[10. 1.]]
[[1. 1.]]
tensor([[1., 1.]])
Now, just the nac
numpy array will be altered with the line nac[0][0]=10
, na
and a
will remain as is.
Example: CPU tensor requires_grad=True
import torch
a = torch.ones((1,2), requires_grad=True)
print(a)
na = a.detach().numpy()
na[0][0]=10
print(na)
print(a)
Output:
tensor([[1., 1.]], requires_grad=True)
[[10. 1.]]
tensor([[10., 1.]], requires_grad=True)
In here we call:
na = a.numpy()
This would cause: RuntimeError: Can't call numpy() on Tensor that requires grad. Use tensor.detach().numpy() instead.
, because tensors that require_grad=True
are recorded by PyTorch AD.
This is why we need to detach()
them first before converting using numpy()
.
Example: CUDA tensor requires_grad=False
a = torch.ones((1,2), device='cuda')
print(a)
na = a.to('cpu').numpy()
na[0][0]=10
print(na)
print(a)
Output:
tensor([[1., 1.]], device='cuda:0')
[[10. 1.]]
tensor([[1., 1.]], device='cuda:0')
In here we just don't convert the CUDA tensor to CPU. There is no effect of shared storage here.
Example: CUDA tensor requires_grad=True
a = torch.ones((1,2), device='cuda', requires_grad=True)
print(a)
na = a.detach().to('cpu').numpy()
na[0][0]=10
print(na)
print(a)
Output:
tensor([[1., 1.]], device='cuda:0', requires_grad=True)
[[10. 1.]]
tensor([[1., 1.]], device='cuda:0', requires_grad=True)
Without detach()
method the error RuntimeError: Can't call numpy() on Tensor that requires grad. Use tensor.detach().numpy() instead.
will be set.
Without .to('cpu')
method TypeError: can't convert cuda:0 device type tensor to numpy. Use Tensor.cpu() to copy the tensor to host memory first.
will be set.
You can use x.cpu().detach().numpy()
to get a Python array from a tensor that has one element and then you can get a number from this array