Add new column to dataframe, based on values at specific rows within that dataframe [duplicate] - r

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Matching up two vectors in R
(2 answers)
Closed 4 years ago.
Suppose I have a dataframe such as the below
people.dat <- data.frame("ID" = c(2001, 1001, 2005, 2001 5000), "Data"
= c(100, 300, 500, 900, 200))
Which looks something like this
+------+------+
| ID | Data |
+------+------+
| 2001 | 100 |
| 1001 | 300 |
| 2005 | 500 |
| 2001 | 900 |
| 5000 | 200 |
+------+------+
Suppose the first thing I do is work out how many unique ID values are in the dataframe (this is necessary, due to the size of the real dataset in question)
unique_ids <- sort(c(unique(people.dat$ID)))
Which gives
[1] 1001 2001 2005 5000
Where I get stuck is that I would like to add a new column, say "new_id", which looks at the "ID" value in the dataframe and evaluates its position in unique_ids, and assigns positional value (so the column "new_id" consists of values at each row which range from 1:length(unique_ids)
An example of the output would be as follows
+------+------+--------+
| ID | Data | new_id |
+------+------+--------+
| 2001 | 100 | 2 |
| 1001 | 300 | 1 |
| 2005 | 500 | 3 |
| 2001 | 900 | 1 |
| 5000 | 200 | 4 |
+------+------+--------+
I thought about using a for loop with if statements, but my first attempts didn't quite hit the mark. Although, if I just wanted to replace "ID" with a sequential value, the following code would work (but where I get stuck is that I want to keep ID, but add another "new_id" column)
for (i in 1:48){
people.dat$ID[people.dat$ID == unique_ids[i]] <- i
}
Thank you for any help. Hope I have made the question as clear as possible (although I struggled to phrase some of it, so please let me know if there is anything specific that needs clarifying)

Answer 1

This is more like a 'rank' problem
people$rank=as.numeric(factor(people$ID))
people
ID Data rank
1 2001 100 2
2 1001 300 1
3 2005 500 3
4 2001 900 2
5 5000 200 4