问题
I have a very big sparse csc_matrix x. I want to do elementwise exp() on it. Basically what I want is to get the same result as I would have got with numpy.exp(x.toarray()). But I can't do that(my memory won't allow me to convert the sparse matrix into an array). Is there any way out? Thanks in advance!
回答1:
If you don't have the memory to hold x.toarray(), you don't have the memory to hold the output you're asking for. The output won't be sparse; in fact, unless your input has negative infinities in it, the output probably won't have a single 0.
It'd probably be better to compute exp(x)-1, which is as simple as
x.expm1()
回答2:
If you want to do something on nonzeros only: the data attribute is writable at least in some representations including csr and csc. Some representations allow for duplicate entries, so make sure you are acting on a "normalised" form.
回答3:
To change non-zero elements, maybe this would work for you:
x = some big sparse matrix
np.exp( x.data, out=x.data ) # ask np.exp() to store results in existing x.data
presumably slower:
# above seems more efficient (no new memory alloc).
x.data = np.exp( x.data )
I've been wrestling with how to get an element-wise log2() of each non-zero array element. I ended up doing smth like:
np.log2( x.data, out=x.data )
The following two techniques seem like exactly what I was looking for. My matrix is sparse but it still plenty of non-zero elements.
Credit to @DSM here for the idea of directly changing x.data, I think that is a superb insight about sparse matrices.
Credit to @Mike Müller for the idea of using "out" as itself. In the same thread, @kmario23 points out an important caveat about promoting .data to floats (inputs could be int or smth) so it is compatible with the .exp() or whatever function, I would want to do that if I was writing smth for general use.
note: I'm just starting to learn about sparse matrices, so would like to know if this is a bad idea for reason(s) I'm not seeing. Please do let me know if I'm on thin ice with this.
Normally I wouldn't mess with private attributes, but .data shows up pretty clearly in the attributes documentation for the various sparse matrices I've looked at.
来源：https://stackoverflow.com/questions/42408772/element-wise-exp-of-scipy-sparse-matrix