The thread is quite old right now, but I would like to add a comment. The prevalent argument repeated over and over in this thread is that the transpose of CSR matrix can be constructed as a CSC matrix and then wrapped in transpose.
The problem with this argument is that it burdens the user with unwanted mental overhead. You see, the easiest way to construct the transpose is to use COO representation and then transform to CSC, because we can simply swap the two indices in COO representation. However, when you create the CSC matrix from COO matrix, you double the memory required to store the matrices. If COO already takes more than half of the RAM (I am talking based off my experience, it can happen), you simply won’t be able to create a CSC matrix, so the only option is to create it directly, i.e. by filling the colptr, rowval and nzval.
Filling directly colptr, rowval and nzval brings by itself some level of mental overhead. If simultaneously you have to construct it not for the matrix, but for its transpose, the mental overhead is doubled. More mental overhead means more opportunity to make a mistake.