Assign an integer for each group in a column

Question

I have a data set that is organized like this

Date:
4/2/2018
5/6/2018
4/2/2018
7/1/2018
7/1/2018
5/6/2018

I would like it to be organized like this:

Date:                ID:
4/2/2018             1 
5/6/2018             2
4/2/2018             1
7/1/2018             3
7/1/2018             3
5/6/2018             2

Does anyone know how to do this? My data is organized in a data table with date as one of the columns and I need to create a new column called id.

Welcome to SO. What code have you attempted so far? are you open to using other packages or is a data.table solution preferred? — Nate, Aug 13 '18 at 17:20
I have attempted to use the dplyr package. Today is my first day in R, so I am unfamiliar with how to do things. A data table solution is preferred. — Savannah Morrissey Martin, Aug 13 '18 at 17:25

acylam · Answer 1 · 2018-08-13T17:43:49.993

0

We can use rleid from data.table:

library(data.table)
dt[order(Date),ID:=rleid(Date)]

Result:

       Date ID
1: 4/2/2018  1
2: 5/6/2018  2
3: 4/2/2018  1
4: 7/1/2018  3
5: 7/1/2018  3
6: 5/6/2018  2

Data:

dt <- read.table(text = "Date
4/2/2018
                 5/6/2018
                 4/2/2018
                 7/1/2018
                 7/1/2018
                 5/6/2018", header = TRUE, stringsAsFactors = FALSE)

setDT(dt)

edited Aug 13 '18 at 17:43

answered Aug 13 '18 at 17:28

acylam

18,231
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having problems with this as the date field is a string, any thoughts? – Savannah Morrissey Martin Aug 13 '18 at 17:34
@SavannahMorrisseyMartin Are you saying it's a string in your data or in my answer? What is the desired format for `Date`? – acylam Aug 13 '18 at 17:36
the date variable in my table is a string. I appreciate your help! – Savannah Morrissey Martin Aug 13 '18 at 17:42
@SavannahMorrisseyMartin Format doesn't matter for this answer. It should work whether your `Date` is a string or factor or Date format. Nonetheless, I've changed the format of the sample `Date` from a factor to a string. – acylam Aug 13 '18 at 17:45
Awesome works thanks! – Savannah Morrissey Martin Aug 13 '18 at 17:55

Assign an integer for each group in a column

1 Answers1