Test Objects for Exact Equality

Description

The safe and reliable way to test two objects for being exactly equal. It returns TRUE in this case, FALSE in every other case.

Usage

identical(x, y)

Arguments

Details

A call to identical is the way to test exact equality in if and while statements, as well as in logical expressions that use && or ||. In all these applications you need to be assured of getting a single logical value.

Users often use the comparison operators, such as == or !=, in these situations. It looks natural, but it is not what these operators are designed to do in R. They return an object like the arguments. If you expected x and y to be of length 1, but it happened that one of them wasn't, you will not get a single FALSE. Similarly, if one of the arguments is NA, the result is also NA. In either case, the expression if(x==y).... won't work.

The function all.equal is also sometimes used to test equality this way, but it was intended for something different. First, it tries to allow for “reasonable” differences in numeric results. Second, it returns a descriptive character vector instead of FALSE when the objects do not match. Therefore, it is not the right function to use for reliable testing either. (If you do want to allow for numeric fuzziness in comparing objects, you can combine all.equal and identical, as shown in the examples below.)

The computations in identical are also reliable and usually fast. There should never be an error. The only known way to kill identical is by having an invalid pointer at the C level, generating a memory fault. It will usually find inequality quickly. Checking equality for two large, complicated objects can take longer if the objects are identical or nearly so, but represent completely independent copies. For most applications, however, the computational cost should be negligible.

Value

A single logical value, TRUE or FALSE, never NA and never anything other than a single value.

Author(s)

John Chambers

See Also

all.equal for descriptions of how two objects differ; Comparison for operators that generate elementwise comparisons.

Examples

identical(1, NULL) ## FALSE -- don't try this with ==
identical(1, 1.)   ## TRUE in R (both are stored as doubles)
identical(1, as.integer(1)) ## FALSE, stored as different types

## how to test for object equality allowing for numeric fuzz
identical(all.equal(x, y), TRUE)
## If all.equal thinks the objects are different, it returns a
## character string, and this expression evaluates to FALSE