method

combination

v2_1_10 - Show latest stable - Class: Array

combination(p1)

public

When invoked with a block, yields all combinations of length n of elements from the array and then returns the array itself.

The implementation makes no guarantees about the order in which the combinations are yielded.

If no block is given, an Enumerator is returned instead.

Examples:

a = [1, 2, 3, 4]
a.combination(1).to_a  #=> [[1],[2],[3],[4]]
a.combination(2).to_a  #=> [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
a.combination(3).to_a  #=> [[1,2,3],[1,2,4],[1,3,4],[2,3,4]]
a.combination(4).to_a  #=> [[1,2,3,4]]
a.combination(0).to_a  #=> [[]] # one combination of length 0
a.combination(5).to_a  #=> []   # no combinations of length 5

static VALUE
rb_ary_combination(VALUE ary, VALUE num)
{
    long n, i, 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 < len; 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);
        long lev = 0;

        RBASIC_CLEAR_CLASS(ary0);
        MEMZERO(stack+1, long, n);
        stack[0] = -1;
        for (;;) {
            for (lev++; lev < n; lev++) {
                stack[lev+1] = stack[lev]+1;
            }
            if (!yield_indexed_values(ary0, n, stack+1)) {
                rb_raise(rb_eRuntimeError, "combination reentered");
            }
            do {
                if (lev == 0) goto done;
                stack[lev--]++;
            } while (stack[lev+1]+n == len+lev+1);
        }
    done:
        ALLOCV_END(t0);
        RBASIC_SET_CLASS_RAW(ary0, rb_cArray);
    }
    return ary;
}

combination

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