tensorregress (version 5.1)

fold: General Folding of Matrix

Description

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.

Usage

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

Value

Tensor object with modes given by modes

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

Details

This function uses aperm as the primary workhorse.

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

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)