tapply function complains that args are unequal length yet they appear to match
Here is the failing call, error messages and some displays to show the lengths in question:
it <- tapply(molten, c(molten$Activity, molten$Subject, molten$variable), mean)
# Error in tapply(molten, c(molten$Activity, molten$Subject, molten$variable), :
# arguments must have same length
length(molten$Activity)
# [1] 679734
length(molten$Subject)
# [1] 679734
length(molten$variable)
# [1] 679734
dim(molten)
# [1] 679734 4
str(molten)
# 'data.frame': 679734 obs. of 4 variables:
# $ Activity: Factor w/ 6 levels "WALKING","WALKING_UPSTAIRS",..: 5 5 5 5 5 5 5 5 5 5 ...
# $ Subject : Factor w/ 30 levels "1","2","3","4",..: 2 2 2 2 2 2 2 2 2 2 ...
# $ variable: Factor w/ 66 levels "tBodyAcc-mean()-X",..: 1 1 1 1 1 1 1 1 1 1 ...
# $ value : num 0.257 0.286 0.275 0.27 0.275 ...
Answer
If you have a look at ?tapply you will see that X should be "an atomic object, typically a vector". You feed tapply with a data frame ("molten"), which is not an atomic object. See is.atomic, and try is.atomic(molten). Furthermore, your grouping variables should be provided as a list (see INDEX argument).
Something like this works:
tapply(X = warpbreaks$breaks, INDEX = list(warpbreaks$wool, warpbreaks$tension), mean)
# L M H
# A 44.55556 24.00000 24.55556
# B 28.22222 28.77778 18.77778