How to get index in a loop in R

Question

I have a vector of character type which has all the list of names.

So I'm looping through each name and peforming some actions. This loop is not based on the index/length (and I want to keep it this way).

However, if I want to access the index value in the loop how do I get it.

Ex:

names <- c("name1", "name2") for(name in names){ #do something print(name) #here how would I get the index number? like 1, 2 etc? }

In the tidyverse world, check out imap. No time now for a full answer, if someone wants to write one up or link to an answer using imap, please go ahead. — Aaron - mostly inactive
– Aaron - mostly inactive, Commented Jun 8, 2023 at 21:24

Elin · Accepted Answer · 2018-01-11 11:15:12Z

You can do something like this, which is literally getting the i value.

names <- c("name1", "name2") i<-0 for(name in names){ i<-i+1 print(i) }

Or change the loop to use a numeric index

names <- c("name1", "name2") for(i in 1:length(names)){ print(i) }

Or use the which function.

names <- c("name1", "name2") for(name in names){ print(which(name == names)) }

Would the which way still be suitable if there are non unique entries in the list?
Did you try it? I think you will find it works, if I understand what you are asking.

steevee · Accepted Answer · 2019-04-03 14:09:20Z

For variety:

names <- c("name1", "name2") for(i in seq_along(names)){ print(i) }

seq_along is a fast primitive, and IMO slightly sweeter syntactic sugar.

2 Answers 2