[R] Best way to extract values from an aov object ?

[Paul]

I'm trying to write a function to automate doing a variance analysis, 
part of which involves doing some further calculations. The method I've 
been using isn't very robust, if variable names change then it stops 
working.

For this dummy data

 > dput(assayvar,"")
structure(list(Run = structure(c(1L, 1L, 1L, 2L, 2L, 2L, 3L,
3L, 3L, 4L, 4L, 4L), .rk.invalid.fields = list(), .Label = c("1",
"2", "3", "4"), class = "factor"), Actual = c(246.3, 253.6, 247.6,
249, 249, 251.3, 254.9, 254.1, 253.2, 250, 248.9, 250.3)), .Names = 
c("Run",
"Actual"), row.names = c(NA, 12L), class = "data.frame")

 > assayaov<-aov(Actual~Run+Error(Run),data=assayvar)
 > str(summary(assayaov))
List of 2
$ Error: Run :List of 1
..$ :Classes ‘anova’ and 'data.frame': 1 obs. of 3 variables:
.. ..$ Df : num 3
.. ..$ Sum Sq : num 46.5
.. ..$ Mean Sq: num 15.5
..- attr(*, "class")= chr [1:2] "summary.aov" "listof"
$ Error: Within:List of 1
..$ :Classes ‘anova’ and 'data.frame': 1 obs. of 5 variables:
.. ..$ Df : num 8
.. ..$ Sum Sq : num 36.4
.. ..$ Mean Sq: num 4.55
.. ..$ F value: num NA
.. ..$ Pr(>F) : num NA
..- attr(*, "class")= chr [1:2] "summary.aov" "listof"
- attr(*, "class")= chr "summary.aovlist"

But for this dummy data

 > dput(BGBottles,"")
structure(list(Machine = structure(c(1L, 1L, 1L, 2L, 2L, 2L,
3L, 3L, 3L, 4L, 4L, 4L), .rk.invalid.fields = structure(list(), .Names = 
character(0)), .Label = c("1",
"2", "3", "4"), class = "factor"), Weight = c(14.23, 14.96, 14.85,
16.46, 16.74, 15.94, 14.98, 14.88, 14.87, 15.94, 16.07, 14.91
)), .Names = c("Machine", "Weight"), row.names = c(NA, 12L), class = 
"data.frame")

 > bgaov<-aov(Weight~Machine+Error(Machine),data=BGBottles)
 > str(summary(bgaov))
List of 2
$ Error: Machine:List of 1
..$ :Classes ‘anova’ and 'data.frame': 1 obs. of 3 variables:
.. ..$ Df : num 3
.. ..$ Sum Sq : num 5.33
.. ..$ Mean Sq: num 1.78
..- attr(*, "class")= chr [1:2] "summary.aov" "listof"
$ Error: Within :List of 1
..$ :Classes ‘anova’ and 'data.frame': 1 obs. of 5 variables:
.. ..$ Df : num 8
.. ..$ Sum Sq : num 1.45
.. ..$ Mean Sq: num 0.182
.. ..$ F value: num NA
.. ..$ Pr(>F) : num NA
..- attr(*, "class")= chr [1:2] "summary.aov" "listof"
- attr(*, "class")= chr "summary.aovlist"

So the code to get the mean square value

machine<-summary(bgaov)$"Error: Machine"[[1]]$"Mean Sq"

Doesn't work, because the Machine but won't always be Machine.

Any better way to do this ?

Thanks

Paul.