CODEWITHSUNDEEP
rdataframeindexing
Najeha Mohamed
Najeha Mohamed

Reputation: 21

What does index do in r?

I have a code I'm working with which has the following line,

data2 <- apply(data1[,-c(1:(index-1))],2,log)

I understand that this creates a new data frame, from the data1, taking column-wise values log-transformed and some columns are eliminated, but I don't understand how the columns are removed. what does 1:(index-1) do exactly?

Upvotes: 1

Views: 93

Answers (2)

IRTFM
IRTFM

Reputation: 263382

The ":" operator creates an integer sequence. Because (1:(index-1) ) is numeric and being used in the second position for the extraction operator"[" applied to a dataframe, it is is referring to column numbers. The person writing the code didn't need the c-function. It could have been more economically written:

data1[,-(1:(index-1))]  
# but the outer "("...")"'s are needed so it starts at 1 rather than -1

So it removes the first index-1 columns from the object passed to apply. (As MrFlick points out, index must have been defined before this gets passed to R. There's not default value or interpretation for index in R.

Upvotes: 2

akrun
akrun

Reputation: 887251

Suppose the index is 5, then index -1 returns 4 so the sequence will be from 1 to 4 i.e. and then we use - implies loop over the columns other than the first 4 columns as MARGIN = 2

Upvotes: 0

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