如何将变量的值分配给plyr中的列名？

Question

有没有办法将变量的值赋给plyr中的结果列名？所以在这段代码中......

column_name <- 'total'
df <- data.frame(a=c('a','b'), b=c(1,2))
ddply(df, .(a), summarise, column_name=sum(b))

如您所知,这会吐出一个由变量a和数据组成的数据框column_name.不过,我想这是变量a和total,其中total由一个变量的值动态分配的,因为实际上,我要处理它在循环中,我不能直接指定它ddply函数内.

对于您的信息,此代码不起作用.

ddply(df, .(a), summarise, get(column_name)=sum(b))

那么有没有解决方案呢？

Answer 1

有几种方法:

> column_names <- c('total', 'latot')
> df <- data.frame(a=c('a','b'), b=c(1,2))
> 
> # temporal variable
> for (cn in column_names) {
+   ret <- ddply(df, .(a), summarise, ..x=sum(b))
+   ret <- rename(ret, c(..x = cn))
+   print(ret)
+ }
  a total
1 a     1
2 b     2
  a latot
1 a     1
2 b     2
> 
> # ept solution
> for (cn in column_names) {
+   print(eval(parse(text = paste0("ret <- ddply(df, .(a), summarise,", cn, "=sum(b))"))))
+ }
  a total
1 a     1
2 b     2
  a latot
1 a     1
2 b     2
> 
> # dynamic generation of call
> for (cn in column_names) {
+   args <- alist(df, .(a), summarize, sum(b))
+   names(args) <- c("", "", "", cn)
+   print(do.call("ddply", args))   
+ }
  a total
1 a     1
2 b     2
  a latot
1 a     1
2 b     2