如何使用字符串变量为ddply创建变量列表? [英] How to use string variables to create variables list for ddply?

问题描述

使用R的内置ToothGrowth示例数据集，此方法有效:

Using R's builtin ToothGrowth example dataset, this works:

ddply(ToothGrowth, .(supp,dose), function(df) mean(df$len)) 

但是我希望子集因子是变量，例如

But I would like to have the subsetting factors be variables, something like

factor1 = 'supp'
factor2 = 'dose'
ddply(ToothGrowth, .(factor1,factor2), function(df) mean(df$len)) 

那是行不通的.应该怎么做?

That doesn't work. How should this be done?

我想也许是这样的:

factorCombo = paste('.(',factor1,',',factor2,')', sep='') 
ddply(ToothGrowth, factorCombo, function(df) mean(df$len)) 

但是它也不起作用.我想我很亲密，但不确定执行此操作的正确方法.我想可以将整个命令放入一个字符串中，然后再调用该字符串的eval()，但是希望有一种更优雅的方法吗?

But it doesn't work either. I think I am close, but not sure the proper way to do it. I suppose the entire command could be put into a string, followed by an eval() call of the string, but hopefully is there a more elegant way?

推荐答案

尝试:

x <- c("supp", "dose") 
ddply(ToothGrowth, x, function(df) mean(df$len)) 

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