R: Test Objects for Exact Equality

identical {base} | R Documentation

## 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

`x, y` | Any R objects.

### 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

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