"在 R 中围绕标记创建组"

Question

新的 R 用户在这里我正在使用面板数据集，我需要围绕标记创建组。 数据集看起来像这样

structure(list(country = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L), .Label = c("Bangladesh", 
"Barbados", "Benin", "Burundi", "Cameroon", "Chile", "Cyprus", 
"Ecuador", "Equatorial Guinea", "Gabon", "Ghana", "Guatemala", 
"Guinea", "Guyana", "Haiti", "India", "Jordan", "Lebanon", "Liberia", 
"Madagascar", "Mali", "Mexico", "Morocco", "Mozambique", "Nepal", 
"Nicaragua", "Niger", "Oman", "Pakistan", "Panama", "Papua New Guinea", 
"Peru", "Rwanda", "Senegal", "Seychelles", "Sierra Leone", "Singapore", 
"Sri Lanka", "Sudan", "Togo", "Tunisia", "Turkey", "Uganda", 
"Zambia", "Zimbabwe"), class = c("pseries", "factor")), date = structure(12:31, .Label = c("1965", 
"1966", "1967", "1968", "1969", "1970", "1971", "1972", "1973", 
"1974", "1975", "1976", "1977", "1978", "1979", "1980", "1981", 
"1982", "1983", "1984", "1985", "1986", "1987", "1988", "1989", 
"1990", "1991", "1992", "1993", "1994", "1995", "1996", "1997", 
"1998", "1999", "2000", "2001", "2002", "2003", "2004", "2005", 
"2006", "2007", "2008", "2009", "2010", "2011", "2012", "2013", 
"2014", "2015", "2016", "2017", "2018"), class = c("pseries", 
"factor")), oda_gdp_cyclical = c(-0.0324935877598818, -0.00752600677202254, 
0.0189276427016587, 0.0157818139406085, 0.0124581749631083, -0.00540096623366293, 
0.0277213961736074, -0.0212109456439322, 0.0042184336739809, 
-0.00749022185426284, -0.00583126153715459, 0.0109793841538662, 
-0.00802428451793625, 0.00528596069421613, 0.0179370088331591, 
7.18645244752135e-05, 0.000703793610724401, -0.0111594103693246, 
0.00294080442969348, -0.0142329769770062), episode = c(0L, 0L, 
1L, 1L, 1L, 0L, 1L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 1L, 0L, 0L, 0L, 
0L, 0L), epi_id = c(0, 0, 1, 1, 1, 0, 2, 0, 0, 0, 0, 0, 0, 0, 
3, 0, 0, 0, 0, 0), output = c(1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 
2, 3, 3, 3, 3, 3, 3, 3, 3)), class = "data.frame", row.names = c(NA, 
20L))