Count, size, length...too many choices in Ruby?

Question

I can't seem to find a definitive answer on this and I want to make sure I understand this to the "n'th level" :-)

    a = { "a" => "Hello", "b" => "World" }
    a.count  # 2
    a.size   # 2
    a.length # 2

    a = [ 10, 20 ]
    a.count  # 2
    a.size   # 2
    a.length # 2

So which to use? If I want to know if a has more than one element then it doesn't seem to matter but I want to make sure I understand the real difference. This applies to arrays too. I get the same results.

Also, I realize that count/size/length have different meanings with ActiveRecord. I'm mostly interested in pure Ruby (1.92) right now but if anyone wants to chime in on the difference AR makes that would be appreciated as well.

Thanks!

Answer 1

For arrays and hashes size is an alias for length. They are synonyms and do exactly the same thing.

count is more versatile - it can take an element or predicate and count only those items that match.

> [1,2,3].count{|x| x > 2 }
=> 1

---

In the case where you don't provide a parameter to count it has basically the same effect as calling length. There can be a performance difference though.

We can see from the source code for Array that they do almost exactly the same thing. Here is the C code for the implementation of array.length:

static VALUE
rb_ary_length(VALUE ary)
{
    long len = RARRAY_LEN(ary);
    return LONG2NUM(len);
}

And here is the relevant part from the implementation of array.count:

static VALUE
rb_ary_count(int argc, VALUE *argv, VALUE ary)
{
    long n = 0;

    if (argc == 0) {
        VALUE *p, *pend;

        if (!rb_block_given_p())
            return LONG2NUM(RARRAY_LEN(ary));

        // etc..
    }
}

The code for array.count does a few extra checks but in the end calls the exact same code: LONG2NUM(RARRAY_LEN(ary)).

Hashes (source code) on the other hand don't seem to implement their own optimized version of count so the implementation from Enumerable (source code) is used, which iterates over all the elements and counts them one-by-one.

In general I'd advise using length (or its alias size) rather than count if you want to know how many elements there are altogether.

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Regarding ActiveRecord, on the other hand, there are important differences. check out this post:

• Counting ActiveRecord associations: count, size or length?