Method: Array#permutation

Defined in:
array.c

#permutation(count = self.size) {|permutation| ... } ⇒ self #permutation(count = self.size) ⇒ Object

Iterates over permutations of the elements of self; the order of permutations is indeterminate.

With a block and an in-range positive integer argument count (0 < count <= self.size) given, calls the block with each permutation of self of size count; returns self:

a = [0, 1, 2]
perms = []
a.permutation(1) {|perm| perms.push(perm) }
perms # => [[0], [1], [2]]

perms = []
a.permutation(2) {|perm| perms.push(perm) }
perms # => [[0, 1], [0, 2], [1, 0], [1, 2], [2, 0], [2, 1]]

perms = []
a.permutation(3) {|perm| perms.push(perm) }
perms # => [[0, 1, 2], [0, 2, 1], [1, 0, 2], [1, 2, 0], [2, 0, 1], [2, 1, 0]]

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

perms = []
a.permutation(0) {|perm| perms.push(perm) }
perms # => [[]]

When count is out of range (negative or larger than self.size), does not call the block:

a.permutation(-1) {|permutation| fail 'Cannot happen' }
a.permutation(4) {|permutation| fail 'Cannot happen' }

With no block given, returns a new Enumerator.

Related: Methods for Iterating.

Overloads:

  • #permutation(count = self.size) {|permutation| ... } ⇒ self

    Yields:

    Returns:

    • (self) 

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# File 'array.c', line 7106

static VALUE
rb_ary_permutation(int argc, VALUE *argv, VALUE ary)
{
    long r, n, i;

    n = RARRAY_LEN(ary);                  /* Array length */
    RETURN_SIZED_ENUMERATOR(ary, argc, argv, rb_ary_permutation_size);   /* Return enumerator if no block */
    r = n;
    if (rb_check_arity(argc, 0, 1) && !NIL_P(argv[0]))
        r = NUM2LONG(argv[0]);            /* Permutation size from argument */

    if (r < 0 || n < r) {
        /* no permutations: yield nothing */
    }
    else if (r == 0) { /* exactly one permutation: the zero-length array */
        rb_yield(rb_ary_new2(0));
    }
    else if (r == 1) { /* this is a special, easy case */
        for (i = 0; i < RARRAY_LEN(ary); i++) {
            rb_yield(rb_ary_new3(1, RARRAY_AREF(ary, i)));
        }
    }
    else {             /* this is the general case */
        volatile VALUE t0;
        long *p = ALLOCV_N(long, t0, r+roomof(n, sizeof(long)));
        char *used = (char*)(p + r);
        VALUE ary0 = ary_make_shared_copy(ary); /* private defensive copy of ary */
        RBASIC_CLEAR_CLASS(ary0);

        MEMZERO(used, char, n); /* initialize array */

        permute0(n, r, p, used, ary0); /* compute and yield permutations */
        ALLOCV_END(t0);
        RBASIC_SET_CLASS_RAW(ary0, rb_cArray);
    }
    return ary;
}