R 条件式にもとづいて列の内容を変える

【目的】 条件式にもとづいて列の内容を変える
【方法】 df.x[dfx$x == 1, ]$new <- "x"

#サンプルデータ作成
v.x1 <- c(1,2,3,4,5,6,7,8,9,10)
v.x2 <- c("a","a","b","b","b","c","c","c","c","c")
v.x3 <- c(1000,900,800,700,600,500,400,300,200,100)
df.x <- data.frame(id = v.x1, item = v.x2, price =v.x3)

> df.x
   id item price
1   1    a  1000
2   2    a   900
3   3    b   800
4   4    b   700
5   5    b   600
6   6    c   500
7   7    c   400
8   8    c   300
9   9    c   200
10 10    c   100

#新たな列を追加
df.x$new <- "0"

> df.x
   id item price new
1   1    a  1000   0
2   2    a   900   0
3   3    b   800   0
4   4    b   700   0
5   5    b   600   0
6   6    c   500   0
7   7    c   400   0
8   8    c   300   0
9   9    c   200   0
10 10    c   100   0

 #条件式
df.x[df.x$item == "c" & df.x$price >= 500,]$new <- "ok"

> df.x
   id item price new
1   1    a  1000   0
2   2    a   900   0
3   3    b   800   0
4   4    b   700   0
5   5    b   600   0
6   6    c   500  ok
7   7    c   400   0
8   8    c   300   0
9   9    c   200   0
10 10    c   100   0