R：在满足条件之前和之后替换向量中的值

Question

考虑一个带有缺失值的向量：

myvec <- c('C', NA, 'test', NA, 'D')

[1] "C"   NA   "test"  NA   "D"

如何将 'test' 之前的向量中的所有元素替换为 'good' 以及之后的 'bad'？结果应该是：

[1] "good" "good" "test" "bad"  "bad"

我下面的尝试只成功地用 'good' 替换了所有内容，这不太好：

replace(myvec, is.na(myvec) | myvec!='test', 'good')

[1] "good" "good" "test" "good" "good"

Answer 1

一个选项是使用 diff 和 cumsum

c('good', 'test', 'bad')[cumsum(c(TRUE, abs(diff(myvec == "test" & !is.na(myvec)))==1))]
#[1] "good" "good" "test" "bad"  "bad"

或使用which

i1 <- which(myvec=='test')
myvec[1:(i1-1)] <- 'good'
myvec[(i1+1):length(myvec)] <- 'bad'

或者@nicola在评论中提到，sign和match可以一起使用

c("good","test","bad")[sign(seq_along(myvec)-match("test",my‌vec))+2]

R: replace values in a vector before and after a condition is met