如果元素为 x 且前一个元素为 na，则在 R 中替换向量中的元素

Question

foo <- c("a","a",NA,"b","a","a","b","b")

如果前一个元素是 NA，如何用 whatever 替换“b”？

foo[foo=="b" & "previous-element"==NA] <- "whatever"

所以预期的输出是:

result <- c("a","a",NA,"whatever","a","a","b","b")

因此只有以 NA 开头的“b”(实际数据中很多)会被更改。

谢谢你的帮助!

Answer 1

一个简单的解决方案:

foo[-1][foo[-1] == "b" & is.na(head(foo, -1))] <- "whatever"

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更新:

zoo 包中的 rollapply 解决方案:

library(zoo)
foo[-1][rollapply(foo, 2, identical, c(NA, "b"))] <- "whatever"