r - 使用 R 中的 data.table 识别和注释分组最小值

Question

我需要为示例数据中的每个 class 确定最小的 value (忽略 NAs) 并将其标记为“min”使用 data.table

如下所示的新列

示例数据:

df = structure(list(class = c("apple", "apple", "apple", "banana", 
"banana", "berry", "berry", "grape", "grape", "grape", "grape", 
"grape", "melon", "melon", "melon"), value = c(108816872, 108851837, 
108890411, 108784778, NA, 108784778, 108816872, 108816872, 108850460, 
NA, NA, NA, NA, NA, NA)), .Names = c("class", "value"), class = "data.frame", row.names = c(NA, 
-15L))

期望的输出:

#    class     value anno
#1   apple 108816872  min
#2   apple 108851837   NA
#3   apple 108890411   NA
#4  banana 108784778  min
#5  banana        NA   NA
#6   berry 108784778  min
#7   berry 108816872   NA
#8   grape 108816872  min
#9   grape 108850460   NA
#10  grape        NA   NA
#11  grape        NA   NA
#12  grape        NA   NA
#13  melon        NA   NA
#14  melon        NA   NA
#15  melon        NA   NA

Answer 1

我打算建议@eddies 方法，但这里有一个替代方法

setDT(df)[order(value), min := c("min", rep(NA, .N - 1)), by = class]

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编辑，如果你想要实际值而不是“min”，你可以修改为

setDT(df)[order(value), min := c(value[1L], rep(NA, .N - 1L)), by = class]