Question

I would like to match the elements of a column in a dataframe against another dataframe.

Consider these dataframes:

A=data.frame(par=c('long A story','long C story', 'blabla D'),val=1:3) 
B=data.frame(par=c('Z','D','A'),val=letters[1:3])

Each element of B column ‘par’ should be matched against A column par.
If there is a match, it should be labeled in A.
[This then gives a column of common values for merging A and B].

The desired result is therefore:

A=transform(A,label=c('A','NA','D'))

How can this be done?

Henk

Answer 1

To do what you’re asking for, try

A=data.frame(par=c('long A story','long C story', 'blabla D'),val=1:3) 
B=data.frame(par=c('Z','D','A'),val=letters[1:3])
A$label <- NA
for (x in B$par){
    is.match <- lapply(A$par,function(y) grep(x, y))
    A$label[which(is.match > 0)] <- x
}

(I assumed you meant a capital A in your example A=transform(a,label=c('A','NA','D')); in that case, these match exactly). EDIT: I see you made that edit. They do match then.

The above method will work only if there is exactly one B that fits every A (in other words, there can be multiple As to a B but not multiple Bs to an A). This is because of the structure you want in the output.