[R] Potential problem with subset !!!

[Arnaud Mosnier]

Dear useRs,

Just a little post to provide the answer of a problem that took me
some time to resolve !
Hope that reading this will permit the others to avoid that error.

When using the subset function, writing

subset (data, data$columnname == X) or subset (data, columnname == X)

do the same thing.

thus, the function consider that argument name given after the coma
(like "columnname") is the name of a column of the data frame
considered.
A problem occur when other arguments such as X are the names of both a
column of the data frame and  an object !

Here is an example:

df <- data.frame(ID = c("a","b","c","b","e"), Other = 1:5)
ID <- unique (df$ID)
ID

## Now the potential problem !!

subset (df, df$ID == ID[4])

## BE CAREFUL subset function use the column ID of the data.frame
## and NOT the object ID containing unique value !!!!

Sorry if it seems obvious for some of you, but hope that others find
it useful !!

Arnaud