dataframe - Transposing two fields to one unique key in R -

i have dataframe has productid, seller1name, seller1price, seller2name, seller2price below. table (df) unique productid:

productid   seller1name    seller1price    seller2name     seller2price 1                         $1             x                $3 2           b               $3             y                $6 3           c               $2             z                $1 

the desired output should df:

productid    seller  price 1                   $1 1             x       $3 2             b       $3 2             y       $6 3             c       $2 3             z       $1 

i tried using reshape package results funky:

output <-melt(df, id = c("productid")) 

is there better way this?

in data.table v1.9.5, current devel version, melt data.tables has gained new feature -- able melt on multiple columns..

require(data.table) ## v1.9.5+ ans = melt(setdt(df), measure=patterns("name$", "price$"),                  value.name=c("seller", "price")) 

we provide columns grouped while melting list in measure.vars argument.

now, can remove variable column , reorder follows:

setorder(ans[, variable := null], productid)[] #    productid seller price # 1:         1         $1 # 2:         1      x    $3 # 3:         2      b    $3 # 4:         2      y    $6 # 5:         3      c    $2 # 6:         3      z    $1 

hth