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如何aggregate通过传递条件和值列表来总结函数？

# This works fine:
x <- data.frame(cond1 = sample(letters[1:3], 500, replace=TRUE), 
                cond2 = sample(LETTERS[1:7], 500, replace = TRUE), 
                cond3 = sample(LETTERS[1:4], 500, replace = TRUE), 
                value1 = rnorm(500), 
                value2 = rnorm(500))

aggregate(cbind(value1,value2) ~ cond1 + cond2, data = x, FUN=sum)

需要创建列名列表:(显示3个选项)然后调用函数:

c1 <- c("cond1","cond2","cond3"); v1 <- c("value1","value2")
c1 <- c("cond2","cond3");         v1 <- c("value2")
c1 <- c("cond3");                 v1 <- c("value1")

aggregate(cbind(v1) ~ c1, data = x, FUN=sum)

我已经回顾了许多替代方案,但还没有发现这种抽象的关键.