How to get row index number in R?

Question

Suppose I have a list or data frame in R, and I would like to get the row index, how do I do that?

Answer 1

It not quite clear what exactly you are trying to do.

To reference a row in a data frame use df[row,]

To get the first position in a vector of something use match(item,vector), where the vector could be one of the columns of your data frame, eg df$cname if the column name is cname.

Edit:

To combine these you would write:

df[match(item,df$cname),]

Note that the match gives you the first item in the list, so if you are not looking for a unique reference number, you may want to consider something else.

Answer 2

Have you read An Introduction to R?

Answer 3

I'm interpreting your question to be about getting row numbers.

• You can try as.numeric(rownames(df)) if you haven't set the rownames. Otherwise use a sequence of 1:nrow(df).
• The which() function converts a true/false row index into row numbers.

Answer 4

If i understand your question, you just want to be able to access items in a data frame (or list) by row:

x = matrix( ceiling(9*runif(20)), nrow=5  )   
colnames(x) = c("col1", "col2", "col3", "col4")
df = data.frame(x)      # create a small data frame

df[1,]                  # get the first row
df[3,]                  # get the third row
df[nrow(df),]           # get the last row

lf = as.list(df)        

lf[[1]]                 # get first row
lf[[3]]                 # get third row

etc.