It would help if you provided more information. Here is an example:
df <- data.frame(x=1:26, y=as.factor(LETTERS))
paste(df$x, df$y)
[1] "1 A" "2 B" "3 C" "4 D" "5 E"...
paste(df$x, df$y, sep="")
[1] "1A" "2B" "3C" "4D" "5E"...
It doesn’t matter what class the elements are, the engine will convert them to character class.
If this does not solve it, include a sample of your data to get more help.
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solved Concatenating two string variables in dataframe in R