test <- c(2,NA,5, 8, NA, 10, 5)
test

test2 <- as.integer(na.omit(test))  ## 清除NA
test2

test3 <- which(!is.na(test))        ## 返回非空的索引
test3

test4 <- which(test == 5)           ## 返回指定项的索引
test4