In R, what does a negative index do?

r syntax indexing matrix-indexing
Brad picture Brad · Sep 7, 2011 · Viewed 17.6k times · Source

I am porting part of a program (not enough to compile and run) from R to C++. I am not familiar with R. I have done okay using the references online, but was stumped by the following line:

cnt2.2<-cnt2[,-1]

I am guessing:

  1. cnt2 is a 2 dimensional matrix
  2. cnt2.2 is a new variable being declared with a period '.' used the same way an alphabetic character would be.
  3. <- is an assignment.
  4. [,-1] accesses part of the array. I thought [,5] meant all rows, 5th column only. If this is correct, I have no idea what -1 refers to.

Answer

Chase picture Chase · Sep 7, 2011

This is covered in section 2.7 of the manual: http://cran.r-project.org/doc/manuals/R-intro.html#Index-vectors

It is a negative index into the cnt2 object specifying all rows and all columns except the first column.

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