Method: Array#combination

Defined in:

array.c

#combination(count) {|element| ... } ⇒ self #combination(count) ⇒ Object

When a block and a positive integer-convertible object argument count (0 < count <= self.size) are given, calls the block with each combination of self of size count; returns self:

a = %w[a b c]                                   # => ["a", "b", "c"]
a.combination(2) {|combination| p combination } # => ["a", "b", "c"]

Output:

["a", "b"]
["a", "c"]
["b", "c"]

The order of the yielded combinations is not guaranteed.

When count is zero, calls the block once with a new empty array:

a.combination(0) {|combination| p combination }
[].combination(0) {|combination| p combination }

Output:

[]
[]

When count is negative or larger than self.size and self is non-empty, does not call the block:

a.combination(-1) {|combination| fail 'Cannot happen' } # => ["a", "b", "c"]
a.combination(4)  {|combination| fail 'Cannot happen' } # => ["a", "b", "c"]

With no block given, returns a new Enumerator.

Related: Array#permutation; see also Methods for Iterating.

Overloads:

• #combination(count) {|element| ... } ⇒ self

  Yields:

  • (element)

  Returns:

  • (self)

# File 'array.c', line 7218

static VALUE
rb_ary_combination(VALUE ary, VALUE num)
{
    long i, n, len;

    n = NUM2LONG(num);
    RETURN_SIZED_ENUMERATOR(ary, 1, &num, rb_ary_combination_size);
    len = RARRAY_LEN(ary);
    if (n < 0 || len < n) {
        /* yield nothing */
    }
    else if (n == 0) {
        rb_yield(rb_ary_new2(0));
    }
    else if (n == 1) {
        for (i = 0; i < RARRAY_LEN(ary); i++) {
            rb_yield(rb_ary_new3(1, RARRAY_AREF(ary, i)));
        }
    }
    else {
        VALUE ary0 = ary_make_shared_copy(ary); /* private defensive copy of ary */
        volatile VALUE t0;
        long *stack = ALLOCV_N(long, t0, n+1);

        RBASIC_CLEAR_CLASS(ary0);
        combinate0(len, n, stack, ary0);
        ALLOCV_END(t0);
        RBASIC_SET_CLASS_RAW(ary0, rb_cArray);
    }
    return ary;
}