R object identification

Question

I am often ending up with a function producing output for which I don\'t understand the output data type. I\'m expecting a list and it ends up being a list of lists or a dat

Answer 1

If I get 'someObject', say via

someObject <- myMagicFunction(...)

then I usually proceed by

class(someObject)
str(someObject)

which can be followed by head(), summary(), print(), ... depending on the class you have.

Answer 2

str(x)

It's all you need to remember for 99% of cases.

Answer 3

attributes(someObject) 

Can also be useful

Answer 4

I usually start out with some combination of:

typeof(obj)
class(obj)
sapply(obj, class)
sapply(obj, attributes)
attributes(obj)
names(obj)

as appropriate based on what's revealed. For example, try with:

obj <- data.frame(a=1:26, b=letters)
obj <- list(a=1:26, b=letters, c=list(d=1:26, e=letters))
data(cars)
obj <- lm(dist ~ speed, data=cars)

..etc.

If obj is an S3 or S4 object, you can also try methods or showMethods, showClass, etc. Patrick Burns' R Inferno has a pretty good section on this (sec #7).

EDIT: Dirk and Hadley mention str(obj) in their answers. It really is much better than any of the above for a quick and even detailed peek into an object.