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From |
"Joseph Coveney" <[email protected]> |

To |
<[email protected]> |

Subject |
st: Re: Linear indexing of matrices in Mata |

Date |
Sun, 22 Jul 2012 12:13:14 +0900 |

Joerg Luedicke wrote (excerpted): . . . So my first question would be: is my impression correct that there is no linear indexing for matrices in Mata? If yes, the second question would be how to best circumvent this? Consider the following problem: suppose we have two matrices, m1 and m2 with the same dimensions. Now we want to check a certain condition in m1 and if true, record the position index. Then we want to select elements in m2 that are corresponding to the recorded positions. For example, if we type in R *--R code-- m1=matrix(c(1,2,3,4), nrow = 2) m2=matrix(c(5,6,7,8), nrow = 2) index=which(m1>2) m2[index] *------------- then m1 looks like [,1] [,2] [1,] 1 3 [2,] 2 4 and m2 looks like [,1] [,2] [1,] 5 7 [2,] 6 8 Then, with the which() function we create a vector of integers that denote the position in the matrix m1 - for which the element is greater than 2 - by a single number. In this example, these are matrix positions 3 and 4 (coinciding with the values in that matrix). Finally, we can use the indices to index elements, as -m2[index]- yields [1] 7 8 i.e., the elements of m2 with the linear indices 3 and 4. So, the basic problem is how to refer to elements of a matrix, based on a selection of elements in another matrix. The above example demonstrates how this could be done in R. Any ideas about solutions in Mata? -------------------------------------------------------------------------------- I'll try to address your three questions in turn. I'm not aware of linear indexing for matrixes in Mata. Others more knowledgeable than I could ever hope to be might be able to point to such a feature, though. You would circumvent this by using Mata's -select()- function. One solution in Mata would be: select(vec(m2)', vec(m1 :> 2)') There are undoubtedly other, more elegant, solutions. This one is illustrated below using your two matrices. It assumes that the corresponding R code returns only row vectors. I've incorporated the statement into a rudimentary function for convenience, but you'd want to add some conformability- and type-checking for production use. Joseph Coveney . version 11.2 . . clear * . set more off . . mata: ------------------------------------------------- mata (type end to exit) ------ : mata set matastrict on : : real rowvector index_which(real matrix Target, real matrix Selector) > return(select(vec(Target)', vec(Selector)')) : : M1 = (1, 3 \ 2, 4) : M2 = (5, 7 \ 6, 8) : : index_which(M2, (M1 :> 2)) 1 2 +---------+ 1 | 7 8 | +---------+ : : end -------------------------------------------------------------------------------- . . exit end of do-file * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Linear indexing of matrices in Mata***From:*Joerg Luedicke <[email protected]>

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