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There has been some talk recently of simplifying the MxAlgebra interface in two regards. It would be convenient to specify constant values or matrices in an MxAlgebra statement, without having to explicitly declare a separate MxMatrix that contains constant values. It would also be convenient to specify free parameters in an MxAlgebra statment, without having to explicitly declare a separate 1 x 1 MxMatrix that contains the free parameter. We have been referring to these proposed changes as constant substitution and free parameter substitution, respectively.

As of revision 769, MxAlgebra are checked for correct dimensions before the optimizer is called.

> A <- mxMatrix("Full", nrow = 1, ncol = 1, name = 'A')
> B <- mxMatrix("Full", nrow = 1, ncol = 2, name = 'B')
> C <- mxAlgebra(A + B, name = 'C')
> model <- mxModel('model', A, B, C)
> mxRun(model)
Running model 
Error: The algebra 'model.C' in model 'model' generated the error message:
non-conformable arrays

If 'compute' is FALSE (the default), then MxAlgebra expressions returns their current value as they have been computed by the optimization call (using mxRun). If the 'compute' argument is TRUE, then MxAlgebra expressions will be calculated in R. Any references to an objective function that has not yet been calculated will return a 1 x 1 matrix with a value of NA. Users will like this feature because it allows them to compare the result of algebraic expressions as computed by the back-end against those values computed by R.