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Returns the index of the last element for which object == element.
When argument object is given but no block, returns the index of the last such element found:
a = [:foo, 'bar', 2, 'bar']
a.rindex('bar') # => 3
Returns nil if no such object found.
When a block is given but no argument, calls the block with each successive element; returns the index of the last element for which the block returns a truthy value:
a = [:foo, 'bar', 2, 'bar']
a.rindex {|element| element == 'bar' } # => 3
Returns nil if the block never returns a truthy value.
When neither an argument nor a block is given, returns a new Enumerator:
a = [:foo, 'bar', 2, 'bar']
e = a.rindex
e # => #<Enumerator: [:foo, "bar", 2, "bar"]:rindex>
e.each {|element| element == 'bar' } # => 3
Related: index.
 
               static VALUE
rb_ary_rindex(int argc, VALUE *argv, VALUE ary)
{
    VALUE val;
    long i = RARRAY_LEN(ary), len;

    if (argc == 0) {
        RETURN_ENUMERATOR(ary, 0, 0);
        while (i--) {
            if (RTEST(rb_yield(RARRAY_AREF(ary, i))))
                return LONG2NUM(i);
            if (i > (len = RARRAY_LEN(ary))) {
                i = len;
            }
        }
        return Qnil;
    }
    rb_check_arity(argc, 0, 1);
    val = argv[0];
    if (rb_block_given_p())
        rb_warn("given block not used");
    while (i--) {
        VALUE e = RARRAY_AREF(ary, i);
        if (rb_equal(e, val)) {
            return LONG2NUM(i);
        }
        if (i > RARRAY_LEN(ary)) {
            break;
        }
    }
    return Qnil;
}
            

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