我想根据两个数据帧中存在的列（“gridcell”列）中的值来匹配两个数据帧。如果不是因为需要为数据框中的每个唯一日期单独完成，这将是一项简单的任务。
以下是一些示例数据：
> dput(df1)
structure(list(index = 1:7, date = c("13/04/2011", "13/04/2011",
"04/04/2011", "04/04/2011", "04/04/2011", "28/03/2011", "28/03/2011"
), yrday = c(103L, 103L, 94L, 94L, 94L, 87L, 87L), gridcell = c(6L,
9L, 2L, 5L, 8L, 3L, 4L), dist = c(178L, 158L, 137L, 116L, 95L,
135L, 115L), ang = c(148, 147.6, 163.6, 159.7, 152.5, 152.2,
121.9)), .Names = c("index", "date", "yrday", "gridcell", "dist",
"ang"), class = "data.frame", row.names = c(NA, -7L))
> dput(df2)
structure(list(date = c("28/03/2011", "28/03/2011", "28/03/2011",
"28/03/2011", "28/03/2011", "28/03/2011", "28/03/2011", "28/03/2011",
"28/03/2011", "29/03/2011", "29/03/2011", "29/03/2011", "29/03/2011",
"29/03/2011", "29/03/2011", "29/03/2011", "29/03/2011", "29/03/2011",
"04/04/2011", "04/04/2011", "04/04/2011", "04/04/2011", "04/04/2011",
"04/04/2011", "04/04/2011", "04/04/2011", "04/04/2011", "13/04/2011",
"13/04/2011", "13/04/2011", "13/04/2011", "13/04/2011", "13/04/2011",
"13/04/2011", "13/04/2011", "13/04/2011"), yrday = c(87L, 87L,
87L, 87L, 87L, 87L, 87L, 87L, 87L, 88L, 88L, 88L, 88L, 88L, 88L,
88L, 88L, 88L, 94L, 94L, 94L, 94L, 94L, 94L, 94L, 94L, 94L, 103L,
103L, 103L, 103L, 103L, 103L, 103L, 103L, 103L), gridcell = c(1L,
2L, 3L, 4L, 5L, 6L, 7L, 8L, 9L, 1L, 2L, 3L, 4L, 5L, 6L, 7L, 8L,
9L, 1L, 2L, 3L, 4L, 5L, 6L, 7L, 8L, 9L, 1L, 2L, 3L, 4L, 5L, 6L,
7L, 8L, 9L), r = c(161L, 162L, 162L, 164L, 167L, 168L, 169L,
170L, 170L, 171L, 170L, 169L, 168L, 158L, 160L, 162L, 164L, 165L,
263L, 258L, 255L, 250L, 246L, 242L, 239L, 238L, 228L, 235L, 234L,
231L, 230L, 229L, 228L, 227L, 243L, 242L)), .Names = c("date",
"yrday", "gridcell", "r"), class = "data.frame", row.names = c(NA,
-36L))
> head(df1)
index date yrday gridcell dist ang
1 1 13/04/2011 103 6 178 148.0
2 2 13/04/2011 103 9 158 147.6
3 3 04/04/2011 94 2 137 163.6
4 4 04/04/2011 94 5 116 159.7
5 5 04/04/2011 94 8 95 152.5
> head(df2)
date yrday gridcell r
1 28/03/2011 87 1 161
2 28/03/2011 87 2 162
3 28/03/2011 87 3 162
4 28/03/2011 87 4 164
5 28/03/2011 87 5 167
我想最终得到一个新的df1数据框，其中包含来自df2的匹配行，基于每个日期内相同的'gridcell'值（如下所示）：
index date_1 yrday_1 gridcell_1 dist ang yrday_2 gridcell_2 r
1 1 13/04/2011 103 6 178 148.0 103 6 228
2 2 13/04/2011 103 9 158 147.6 103 9 242
3 3 04/04/2011 94 2 137 163.6 94 2 258
4 4 04/04/2011 94 5 116 159.7 94 5 246
5 5 04/04/2011 94 8 95 152.5 94 8 238
6 6 28/03/2011 87 3 135 152.2 87 3 162
7 7 28/03/2011 87 4 115 121.9 87 4 164
到目前为止，我已经尝试通过'date'列合并两个数据帧，这给出了一个新的数据框，其中df1中的每一行都以df2中匹配日期的行数重复（即每个可能的来自df2的'gridcell'值与唯一的df1行匹配）。
df1$date = as.Date(df1$date, format="%d/%m/%Y")
df2$date = as.Date(df2$date, format="%d/%m/%Y")
nw.df = merge(df1,df2, by="date")
我确信具有唯一值的'index'列可以在ddply之类的函数中使用，以压缩新数据框，只留下具有匹配'gridcell'列值的行每个唯一的'索引'值（即ddply(nw.df, .(index, …), summarise, …)
）。我只是想不通怎么做！任何建议/帮助将不胜感激！感谢。
答案 0 :(得分：1)
您可以同时指定＆＃39; date＆＃39;和＆＃39; gridcell＆＃39;作为用于合并的列：
merge(df1, df2, by = c("date", "gridcell"))
此代码的缺点是＆＃39; yr.day＆＃39;列是重复的。因此，您可能希望将“df2＆＃39;只包括用于合并的列以及您要添加的列（此处＆＃39; r＆＃39;）：
merge(x = df1, y = df2[ , c("date", "gridcell", "r")])
# date gridcell index yrday dist ang r
# 1 04/04/2011 2 3 94 137 163.6 258
# 2 04/04/2011 5 4 94 116 159.7 246
# 3 04/04/2011 8 5 94 95 152.5 238
# 4 13/04/2011 6 1 103 178 148.0 228
# 5 13/04/2011 9 2 103 158 147.6 242
# 6 28/03/2011 3 6 87 135 152.2 162
# 7 28/03/2011 4 7 87 115 121.9 164
请注意，我们不需要在此处指定by列。如果未给出by，则merge会在by = intersect(names(x), names(y))（?merge）找到用于合并的列，此处为：intersect(names(df1), names(df2[ , c("date", "gridcell", "r")]))
但是，如果您希望明确（有时候有用......），这将得到相同的结果：
merge(x = df1, y = df2[ , c("date", "gridcell", "r")], by = c("date", "gridcell"))