[GitHub] [incubator-mxnet] sxjscience commented on issue #18078: A possible bug when computing a gradient vector

2020-04-15 Thread GitBox 
sxjscience commented on issue #18078: A possible bug when computing a gradient 
vector
URL: 
https://github.com/apache/incubator-mxnet/issues/18078#issuecomment-614406630
 
 
   @zleyk22 Yes, it's a bug and also exists in numpy.
   ```python
   import mxnet as mx
   mx.npx.set_np()
   a = mx.np.array([1,0])
   a.attach_grad()
   with mx.autograd.record():
   b = mx.np.prod(a)
   b.backward()
   print(a.grad)
   ```
   Output:
   ```
   [ 0. nan]
   ```


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[GitHub] [incubator-mxnet] sxjscience commented on issue #18078: A possible bug when computing a gradient vector

2020-04-16 Thread GitBox 
sxjscience commented on issue #18078: A possible bug when computing a gradient 
vector
URL: 
https://github.com/apache/incubator-mxnet/issues/18078#issuecomment-614766460
 
 
   @zleyk22 I think currently no one is looking at the issue. Would you try to 
solve it? Thanks for pointing out the problem!


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[GitHub] [incubator-mxnet] sxjscience commented on issue #18078: A possible bug when computing a gradient vector

2020-04-16 Thread GitBox 
sxjscience commented on issue #18078: A possible bug when computing a gradient 
vector
URL: 
https://github.com/apache/incubator-mxnet/issues/18078#issuecomment-614921875
 
 
   @zleyk22 I can think of two possible ways to solve this problem:
   1) Use two cumsums 
  - `[\log a_0, \log a_0 + \log a_1, ..., \log a_0 + \log a_1 + ... \log 
a_{n-3}]`,
  - `[\log a_2 + \log a_3 ... + \log a_{n-1}, \log a_3 + ... + \log 
a_{n-1}, ... \log a_{n-1}]`
   Then, sum up these two cumsums. Here, I think we should take the log-sum 
approach to avoid the overflow/underflow problem of multiplying lots of 
numbers. (Also this is the algorithm used to solve 
https://leetcode.com/problems/product-of-array-except-self/)
   2) Detect 0s and give them special treatment. We may detect the positions of 
the zeros and update the gradient of these positions with the correct value.


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