For 循环返回 NA：只返回 ifelse 语句的第一个匹配项

Question

如何让我的 for 循环只返回 ifelse 语句的第一个匹配项？

这是我的数据框：

df <- structure(
  list(
x0 = c("0", "0", "0", "0", "0", "0", "0", "0",
       "0", "0"),
x1 = c("0", "0", "0", "0", "0", "0", "0", "0", "0",
       "0"),
x2 = c("0", "0", "0", "0", "0", "0", "0", "0", "0", "0"),
x3 = c("0", "0", "0", "0", "1", "0", "0", "0", "0", "0"),
x4 = c("0", "0", "0", "0", NA, "0", "0", "0", NA, "0"),
x5 = c("0",
       "0", "0", "0", NA, "0", "0", "0", NA, "0"),
x6 = c("0", "0",
       "0", "0", NA, "0", "0", "0", NA, "0"),
x7 = c("0", "0", "0",
       "0", NA, "0", "0", "0", NA, "0"),
x8 = c("0", "0", "0", "0",
       NA, "0", "0", "0", NA, "0"),
x9 = c("0", "0", "0", "0", NA,
       "0", "0", "0", NA, "1"),
x10 = c("0", "0", "0", "0", NA,
        "0", "0", "1", NA, "1"),
x11 = c("0", "0", "0", "0", NA,
        "0", "0", "1", NA, "2"),
x12 = c("0", "0", "0", "0", NA,
        "0", "0", "0", NA, "2"),
x13 = c("0", "0", "0", "0", NA,
        "0", "0", "1", NA, "3"),
x14 = c(NA, NA, "0", "0", NA, NA,
        NA, "2", NA, NA),
x15 = c(
  NA_character_,
  NA_character_,
  NA_character_,
  NA_character_,
  NA_character_,
  NA_character_,
  NA_character_,
  NA_character_,
  NA_character_,
  NA_character_
)
  ),
  .Names = c(
"x0",
"x1",
"x2",
"x3",
"x4",
"x5",
"x6",
"x7",
"x8",
"x9",
"x10",
"x11",
"x12",
"x13",
"x14",
"x15"
  ),
  row.names = c(NA, 10L),
  class = "data.frame"
)

这是我想要实现的预期输出：

df$expectedoutput <- c(NA, NA, NA, NA, NA, NA, NA, 'x14', NA, 'x11')

这是我获得上述输出的尝试：

df$newvar <- NA
for (i in 1:16) {
   df$newvar <- ifelse(df[,i] == "2" & is.na(df$newvar), 
                                  colnames(df[i]), NA)
}

我相信我的问题是我创建的 for 循环一直持续到最后并覆盖了发生的第一个匹配项，例如对于第 10 行，而不是完成 df[10,12] 的列名，循环继续到 df[10,16]，因此返回 NA 但是，我可能是错的，也许这不是我的问题。

Answer 1

首先，使用apply获取前2的列索引。
接下来，获取该索引对应的名称

first2 <- apply(df, MARGIN = 1, FUN = function(x) head(which(x == 2), 1))
df$newvar <- 
  vapply(first2, 
         FUN = function(x) if (length(x)) names(df)[x] else NA_character_, 
         character(1))

for 循环等效项是

df$newvar <- NA
for (i in seq_len(nrow(df))){
  first2 <- head(which(df[i, ] == 2), 1)
  df$newvar[i] <- if (length(first2)) names(df)[first2] else NA
}