There are two ways to index the matrices, using indexes of the row and 
columns or using names of rows and columns.
In your example, it seems that you need to index your matrix by row names.

X[rownames(X) %in% 17:19, ]

could give the result you want.



Nils Trebing wrote:

>Hi,
>
>I am trying to index the rows of a model matrix where, due to missing
>values in the dataset used for the regression, the row numbers sometimes
>"jump" like this:
>
>  
>
>>X[17:19,]
>>    
>>
>   (Intercept)  PCINIFM
>17           1 36.76471
>19           1 13.23529
>20           1 32.35294
>
>So, the problem is if I select for example X[20, ], I actually get the
>21th observation. I've tried to work around this problem by using
>X[as.character(20) , ]. Thus I actually get what I want. However, I need
>to access sequences of rows, but if there is some missing row, this
>doesn't work:
>
>  
>
>>X[as.character(17:19),]
>>    
>>
>Error: subscript out of bounds
>
>Is there a way to make R handle this more gracefully, so that, in this
>case, it would just return the rows 17 and 19 without complaining?
>
>I apologize in advance for this dumb question of the beginner who I am
>and for which I didn't find a solution in the documentation!
>
>Thanks in advance,
>Nils
>
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>  
>



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