I have a big table in R, now I want to select the odd rows and paste a label before the first element of this row

A=matrix(0,4,2)

A[1,1]=2
A[1,2]=3
A[2,1]=2
A[2,2]=3
A[3,1]=2
A[3,2]=3
A[4,1]=2
A[4,2]=3

Now I want to pick up row 2,4 and return this is odd before the fi开发者_如何学运维rst element of the row.

But I don't know how to make a loop to pick up row 2,4

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If I understand your question correctly, you want to display some text and the first element of all odd rows. You can try this:

cat(paste("This is odd", A[c(2,4),1], "\n"))

No need for a loop there. Should you want to work with a larger matrix, and take all odd rows, you can use seq(2, nrow(A), by=2) instead of c(2,4).