r语言数据分组，如何将数据更具要求分组

请问如何将length以month来进行分组
比如：
把1-6月的length存为一个数组，6-12月的length存为另外一个数组
这两个数组的用途是进行双样本假设检验从而得出长度是否会因为季节变化

最好是发一段可参考的代码，能跑就采纳

choose_month<-c('January','February','March','April','May','June')
choose_index<-which(file$Month %in% choose_month)#这个file是你的dataframe，识别在上半年的索引
before<-file$Length[choose_index]#在上半年的
after<-file$Length[-choose_index]#不在上半年的

电子表格 (Open XML SDK) | Microsoft Docs Use the Open XML SDK to programmatically create Office Word, Excel, and PowerPoint documents, and manipulate their content. https://docs.microsoft.com/zh-cn/office/open-xml/spreadsheets

代码可以这样写：

numMonth<-function(x){months=list(January=1,Febrary=2,March=3,Appril=4,May=5,June=6,July=7,August=8,September=9,October=10,November=11,December=12)
sapply(x,function(x) months[[x]])}

a<-data.frame(Length=c(1,0.7,1.5,2.2,1.7,1.5),Water.Temp=c(27,27,27,26,25,22),Month=c('January','February','May','June','October','November'))
month=numMonth(a$Month)
#print(month)
halfyear1<-a[(month>=1 & month<=6),]
halfyear1
halfyear2<-a[(month>=7 & month<=12),]
halfyear2

运行结果：

[1] "F:/2021/qa"
  Length Water.Temp   Month
1    1.0         27 January
2    0.7         27 February
3    1.5         27     May
4    2.2         26    June
  Length Water.Temp    Month
5    1.7         25  October
6    1.5         22 November