use multiple columns as variables with sapply

I have a `dataframe` and I would like to apply a function that takes the values of three columns and computes the minimum difference between the three values. ``` #dataset df <- data.frame(a= sample(1:100, 10),b = sample(1:100, 10),c= sample(1:100, 10)) #function minimum_distance <- function(a,b,c) { dist1 <- abs(a-b) dist2 <- abs(a-c) dist3 <- abs(b-c) return(min(dist1,dist2,dist3)) } ``` I am looking for something like: ``` df$distance <- sapply(df, function(x) minimum_distance(x$a,x$b,x$c) ) ## errormessage Error in x$a : $ operator is invalid for atomic vectors ``` While I can use ddply: ``` df2 <- ddply(df,.(a),function(r) {data.frame(min_distance=minimum_distance(r$a,r$b, r$c))}, .drop=FALSE) ``` This doesn't keep all of the columns. Any suggestions? Edit: I ended up using: ``` df$distance <- mapply(minimum_distance, df$a, df$b, df$c) ```