fold function

General Folding of Matrix

General folding of a matrix into a Tensor. This is designed to be the inverse function to unfold-methods, with the same ordering of the indices. This amounts to following: if we were to unfold a Tensor using a set of row_idx and col_idx, then we can fold the resulting matrix back into the original Tensor using the same row_idx and col_idx.

fold(mat, row_idx = NULL, col_idx = NULL, modes = NULL)

Arguments

• mat: matrix to be folded into a Tensor
• row_idx: the indices of the modes that are mapped onto the row space
• col_idx: the indices of the modes that are mapped onto the column space
• modes: the modes of the output Tensor

Returns

Tensor object with modes given by modes

Details

This function uses aperm as the primary workhorse.

Examples

tnsr <- new('Tensor',3L,c(3L,4L,5L),data=runif(60))
matT3<-unfold(tnsr,row_idx=2,col_idx=c(3,1))
identical(fold(matT3,row_idx=2,col_idx=c(3,1),modes=c(3,4,5)),tnsr)

References

T. Kolda, B. Bader, "Tensor decomposition and applications". SIAM Applied Mathematics and Applications 2009, Vol. 51, No. 3 (September 2009), pp. 455-500. URL: https://www.jstor.org/stable/25662308.

See Also

unfold-methods