如何“反向融化”一个data.frame？

Question

我有data.frame df1（见下面的代码）。我想把它转换成什么df2样子（见下面的代码）。

也许这可以用reshape cast或来完成reverse melt？但我不明白这些功能。请问有人可以帮忙吗？

 df1 <- data.frame(
   stringsAsFactors = FALSE,
                    sample = c("a","a","a",
                               "a","b","c","c","c","c","c","c","c","c",
                               "d","d","e","e","e","g","g"),
                    LETTER = c("P","R","V",
                               "Y","Q","Q","R","S","T","U","W","X","Z",
                               "Q","X","Q","V","X","Q","T")
        )

 df2 <- data.frame(
   stringsAsFactors = FALSE,
             sample = c("a", "b", "c", "d", "e", "f", "g"),
                  P = c(1L, 0L, 0L, 0L, 0L, 0L, 0L),
                  Q = c(0L, 1L, 1L, 1L, 1L, 0L, 1L),
                  R = c(1L, 0L, 1L, 0L, 0L, 0L, 0L),
                  S = c(0L, 0L, 1L, 0L, 0L, 0L, 0L),
                  T = c(0L, 0L, 1L, 0L, 0L, 0L, 1L),
                  U = c(0L, 0L, 1L, 0L, 0L, 0L, 0L),
                  V = c(1L, 0L, 0L, 0L, 1L, 0L, 0L),
                  W = c(0L, 0L, 1L, 0L, 0L, 0L, 0L),
                  X = c(0L, 0L, 1L, 1L, 1L, 0L, 0L),
                  Y = c(1L, 0L, 0L, 0L, 0L, 0L, 0L),
                  Z = c(0L, 0L, 1L, 0L, 0L, 0L, 0L)
        )

编辑

有人建议我看看这篇文章： How to reshape data from long to wide format。不幸的是，这并不能回答我的问题。等效代码如下所示，并引发以下错误。

 df2 <- reshape(df, idvar = "sample", timevar = "LETTER", direction = "wide")
 Error in data[, timevar] : object of type 'closure' is not subsettable

首先添加第三个变量 usingdf1$value <- 1L也不能解决它。

请注意，与上述帖子不同，在我的数据中，数据的长度和宽度之间没有完全匹配。任何帮助仍然不胜感激，请。