r - Calculating item frequencies from single column -

i have set of data transaction id has @ least 1 item both , b. want calculate frequencies items category tabulated respect of category b.

input looks like:

id = c(1,1,2,2,2,2,3,3,3,3) cat = c("a","b","a","a","a","b","b","a","a","b") item = c("item 1","item 30","item 2","item 3",          "item 1","item 30","item 31","item 1","item 2","item 32") df = data.frame(id,cat,item)  id  cat item 1     item 1 1   b   item 30 2     item 2 2     item 3 2     item 1 2   b   item 30 3   b   item 31 3     item 1 3     item 2 3   b   item 32 

the output i'm looking

        item30  item31  item 32 item1   2       1       1 item2   1       1       1 item3   1    

i have solution can loop through unique values of each category, there cleaner solution avoids loop entirely?

edit - fixed example missing row. , clarify, category {a,b} relates category item belongs to

you appear doing

with(merge(df[cat=='a',],df[cat=='b',],by='id',all=true),   table(droplevels(item.x),droplevels(item.y))) 

          item 30 item 31 item 32   item 1       2       1       1   item 2       1       1       1   item 3       1       0       0