Objects in R can have many properties associated with them, called attributes. These properties explain what an object represents and how it should be interpreted by R. Quite often, the only difference between two similar objects is that they have different attributes.[22] Some important attributes are shown in Table 7-2. Many objects in R are used to represent numerical data—in particular, arrays, matrices, and data frames. So many common attributes refer to properties of these objects.

Table 7-2. Common attributes

classThe class of the object.
commentA comment on the object; often a description of what the object means.
dimDimensions of the object.
dimnamesNames associated with each dimension of the object.
namesReturns the names attribute of an object. Results depend on object type; for example, it returns the name of each data column in a data frame or each named object in an array.
row.namesThe name of each row in an object (related to dimnames).
tspStart time for an object. Useful for time series data.
levelsLevels of a factor.

There is a standard way to query object attributes in R. For an object x and attribute a, you refer to the attribute through a(x). In most cases, there is a method to get the current value of the attribute and a method to set a new value of the attribute. (Changing attributes with these methods will alter the attributes in the current environment but will not affect the attributes in an enclosing environment.)

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