| identical {base} | R Documentation |
The safe and reliable way to test two objects for being
exactly equal. It returns TRUE in this case,
FALSE in every other case.
identical(x, y)
x, y |
any R objects. |
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 as expected.
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.
As from R 1.6.0, identical sees NaN as different from
as.double(NA), but all NaNs are equal (and all NA
of the same type are equal).
A single logical value, TRUE or FALSE, never NA
and never anything other than a single value.
John Chambers
Chambers, J. M. (1998) Programming with Data. A Guide to the S Language. Springer.
all.equal for descriptions of how two objects differ;
Comparison for operators that generate elementwise comparisons.
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 x <- 1.0; y <- 0.99999999999 ## 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 # even for unusual R objects : identical(.GlobalEnv, environment())