Charlie Zhu - 2 months ago 14

R Question

Assuming my dataframe has one column, I wish to add another column to indicate if my ith element is unique within the first i elements. The results I want is:

`c1 c2`

1 1

2 1

3 1

2 0

1 0

For example, 1 is unique in

`{1}`

`{1,2}`

`{1,2,3}`

`{1,2,3,2}`

`{1,2,3,2,1}`

Here is my code, but is runs extremely slow given I have nearly 1 million rows.

`for(i in 1:nrow(df)){`

k <- sum(df$C1[1:i]==df$C1[i]))

if(k>1){df[i,"C2"]=0}

else{df[i,"C2"]=1}

}

Is there a quicker way of achieving this?

Answer

The following works:

```
x$c2 = as.numeric(! duplicated(x$c1))
```

Or, if you prefer more explicit code (I do, but itâ€™s slower in this case):

```
x$c2 = ifelse(duplicated(x$c1), 0, 1)
```