r 数据表 dcast 新列的顺序

Question

下面的 csv 来自一个更长的数据表，称之为temp。我想将其转换为temp.wide，region_code 作为列和，垂直顺序为region_code（SAS、SSA、EUR、...）作为列的顺序。我刚刚注意到 dcast 按字母顺序排列新列。

         scenario region_code                  region_name    value
 1:          2010         SAS                   South Asia 61.17716
 2:          2010         SSA   Africa south of the Sahara 62.08588
 3:          2010         EUR                       Europe 63.76123
 4:          2010         LAC  Latin America and Caribbean 68.84806
 5:          2010         FSU          Former Soviet Union 59.04499
 6:          2010         EAP        East Asia and Pacific 64.00579
 7:          2010         NAM                North America 66.18235
 8:          2010         MEN Middle East and North Africa 58.03167
 9: SSP2-NoCC-REF         SAS                   South Asia 57.29973
10: SSP2-NoCC-REF         SSA   Africa south of the Sahara 65.14987
11: SSP2-NoCC-REF         EUR                       Europe 63.99204
12: SSP2-NoCC-REF         LAC  Latin America and Caribbean 68.21118
13: SSP2-NoCC-REF         FSU          Former Soviet Union 60.10807
14: SSP2-NoCC-REF         EAP        East Asia and Pacific 63.86103
15: SSP2-NoCC-REF         NAM                North America 65.97859
16: SSP2-NoCC-REF         MEN Middle East and North Africa 58.98356

temp = setDT(structure(list(scenario = c("2010", "2010", "2010", "2010", "2010", 
"2010", "2010", "2010", "SSP2-NoCC-REF", "SSP2-NoCC-REF", "SSP2-NoCC-REF", 
"SSP2-NoCC-REF", "SSP2-NoCC-REF", "SSP2-NoCC-REF", "SSP2-NoCC-REF", 
"SSP2-NoCC-REF"), region_code = c("SAS", "SSA", "EUR", "LAC", 
"FSU", "EAP", "NAM", "MEN", "SAS", "SSA", "EUR", "LAC", "FSU", 
"EAP", "NAM", "MEN"), region_name = c("South Asia", "Africa south of the Sahara", 
"Europe", "Latin America and Caribbean", "Former Soviet Union", 
"East Asia and Pacific", "North America", "Middle East and North Africa", 
"South Asia", "Africa south of the Sahara", "Europe", "Latin America and Caribbean", 
"Former Soviet Union", "East Asia and Pacific", "North America", 
"Middle East and North Africa"), value = c(61.1771623260257, 
62.0858809906661, 63.7612306428217, 68.84805628195, 59.0449875464304, 
64.0057851485101, 66.182351351389, 58.0316719859857, 57.299725759211, 
65.1498720847705, 63.9920412193261, 68.2111842947542, 60.1080745513644, 
63.86103368494, 65.9785850777114, 58.9835574681585)), .Names = c("scenario", 
"region_code", "region_name", "value"), row.names = c(NA, -16L
), class = "data.frame"))

这是我使用的代码。

formula.wide <- "scenario ~ region_code"
  temp.wide <- data.table::dcast(
    data = temp,
    formula = formula.wide,
    value.var = "value")


        scenario      EAP      EUR      FSU      LAC      MEN      NAM      SAS      SSA
1:          2010 64.00579 63.76123 59.04499 68.84806 58.03167 66.18235 61.17716 62.08588
2: SSP2-NoCC-REF 63.86103 63.99204 60.10807 68.21118 58.98356 65.97859 57.29973 65.14987

新的列名是scenario, EAP, EUR, FSU, LAC, MEN, NAM, SAS, SSA。

我可以从temp 获取正确的顺序，然后使用setcolorder 为temp.wide 提供正确的列顺序。但我想知道是否有某种方法可以不按字母顺序排列新的列顺序。

另外，dcast 的帮助文本说

正在转换的列的名称以相同的顺序生成 （由下划线 _ 分隔）与每个中的（唯一）值 公式 RHS 中提到的列。

如果我理解正确，我认为它并没有描述 dcast 的实际作用。但是我不明白括号中的短语（用下划线分隔，_）是什么意思。

Answer 1

以 region_code 的垂直顺序（SAS、SSA、EUR、...）作为列的顺序

只需通过适当水平的因素：

dcast(temp, scenario ~ factor(region_code, levels=unique(region_code)))

        scenario      SAS      SSA      EUR      LAC      FSU      EAP      NAM      MEN
1:          2010 61.17716 62.08588 63.76123 68.84806 59.04499 64.00579 66.18235 58.03167
2: SSP2-NoCC-REF 57.29973 65.14987 63.99204 68.21118 60.10807 63.86103 65.97859 58.98356

OP 中引用的文档对我来说是正确的； in z ~ x + y -- x 的唯一值在 y 的唯一值之前，按照生成的列名的顺序。