instance method

Enumerable#tally

(?::Hash[Elem, ::Integer] hash) -> ::Hash[Elem, ::Integer]

tally(hash = {}) → hash

static VALUE
enum_tally(int argc, VALUE *argv, VALUE obj)
{
    VALUE hash;
    if (rb_check_arity(argc, 0, 1)) {
        hash = rb_to_hash_type(argv[0]);
        rb_check_frozen(hash);
    }
    else {
        hash = rb_hash_new();
    }

    return enum_hashify_into(obj, 0, 0, tally_i, hash);
}

When argument hash is not given, returns a new hash whose keys are the distinct elements in self; each integer value is the count of occurrences of each element:

Example

%w[a b c b c a c b].tally # => {"a"=>2, "b"=>3, "c"=>3}

When argument hash is given, returns hash, possibly augmented; for each element ele in self:

Adds it as a key with a zero value if that key does not already exist:

Example
```
hash[ele] = 0 unless hash.include?(ele)
```
Increments the value of key ele:

Example
```
hash[ele] += 1
```

This is useful for accumulating tallies across multiple enumerables:

Example

h = {}                   # => {}
%w[a c d b c a].tally(h) # => {"a"=>2, "c"=>2, "d"=>1, "b"=>1}
%w[b a z].tally(h)       # => {"a"=>3, "c"=>2, "d"=>1, "b"=>2, "z"=>1}
%w[b a m].tally(h)       # => {"a"=>4, "c"=>2, "d"=>1, "b"=>3, "z"=>1, "m"=>1}

The key to be added or found for an element depends on the class of self; see Enumerable in Ruby Classes.

Examples:

Array (and certain array-like classes): the key is the element (as above).

Hash (and certain hash-like classes): the key is the 2-element array formed from the key-value pair:

Example

h = {}                        # => {}
{foo: 'a', bar: 'b'}.tally(h) # => {[:foo, "a"]=>1, [:bar, "b"]=>1}
{foo: 'c', bar: 'd'}.tally(h) # => {[:foo, "a"]=>1, [:bar, "b"]=>1, [:foo, "c"]=>1, [:bar, "d"]=>1}
{foo: 'a', bar: 'b'}.tally(h) # => {[:foo, "a"]=>2, [:bar, "b"]=>2, [:foo, "c"]=>1, [:bar, "d"]=>1}
{foo: 'c', bar: 'd'}.tally(h) # => {[:foo, "a"]=>2, [:bar, "b"]=>2, [:foo, "c"]=>2, [:bar, "d"]=>2}

Ruby version changes

3.1
- Enumerable#tally now accepts an optional hash to count. [Feature #17744]

See also #take_while #to_a #take #to_h also in 4.1 · 3.4 · 3.3 · 3.2 · 3.1 · 3.0

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