Replacing a data subset within a matrix by values coming from another matrix

Question

I got some questions about linking 2 matrix in R.

I got a matrix with depth associated to latitudes (rows, i) and longitudes (col, j) within a lagoon. Here is an example of the data :

      [,1] [,2] [,3]
[1,] 2.28 3.80 4.55
[2,] 1.35 5.70 2.88
[3,] 3.79 4.40 3.24

I got a list of couples of coordinates (i,j) indicating areas of interest within the lagoon. For example :

1,2, 3,3, 2,1

etc.

What I need to do is to replace the coordinates of the list by the corresponding depth values of the first matrix.

Please provide a [minimal reproducible example](http://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example/5963610#5963610). — Matthew Plourde, Nov 12 '13 at 15:10
I corrected my topic, is it clear enough ? Thank you for your answer — Doc Martin's, Nov 12 '13 at 16:52

IRTFM · Accepted Answer · 2013-11-12T18:45:57.650

1

 mat <- matrix( c(2.28, 3.80, 4.55,
                  1.35, 5.70, 2.88,
                  3.79 ,4.40 ,3.24),      3, byrow=TRUE)
 idxmat <- matrix( c(1,2, 3,3, 2,1 ), 3, byrow=TRUE)
 mat[idxmat]
#[1] 3.80 3.24 1.35

Using a 2 column-matrix to extract values from another matrix is a standard R maneuver. I'm sure it's been asked and answered (by me for one) before, but I suspect that searching for the strategy might be difficult.

edited Nov 12 '13 at 18:45

answered Nov 12 '13 at 16:56

IRTFM

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Thank you for your answer. However, I don't get it, since the answer should be : #[1] 2.28 3.24 1.35 – Doc Martin's Nov 12 '13 at 17:46
Sorry, I should have entered the second matrix as `byrow=TRUE`. The 2.28 value is (1,1) so I don't understand why you think it should be selected. – IRTFM Nov 12 '13 at 18:13
Should have entered both as `byrow=TRUE`. – IRTFM Nov 12 '13 at 18:46

Replacing a data subset within a matrix by values coming from another matrix

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