# Routine sometimes not returning what i expect

**URL:** <https://www.ruby-forum.com/t/routine-sometimes-not-returning-what-i-expect/79925>\
**Category:** Ruby\
**Created:** [December 20, 2006, 6:25pm UTC](https://www.ruby-forum.com/t/routine-sometimes-not-returning-what-i-expect/79925 "2006-12-20T18:25:44Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![james-calivar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/2bfe46/32.png) [@james-calivar](https://www.ruby-forum.com/u/james-calivar)\
**Post date:** [December 20, 2006, 6:25pm UTC](https://www.ruby-forum.com/t/routine-sometimes-not-returning-what-i-expect/79925/1 "2006-12-20T18:25:44Z")

</div>

As part of my first-ever submission to the ruby quiz, I wrote  
(translated from Python, really) a recursive routine to return all  
n-combinations of an array. So that:

combinations([1, 2, 3], 2)

would return

[[1, 2], [1, 3], [2, 3]]

and combinations([1, 2, 3], 2, false)

would return

[[1, 2], [1, 3], [2, 3], [2, 1], [3, 1], [3, 2]]

My original code was as follows (with some stupid corrections):

def combinations sequence, n, unique=true  
return [] if n == 0  
return sequence if n == 1  
result = []

0.upto(sequence.length - 1) do |i|  
sub\_sequence = sequence[(i + 1)…-1]  
sub\_sequence += sequence[0…i] if not unique

```
combinations(sequence[(i + 1)..-1], n - 1).each do |smaller|
  result << [sequence[i]] + smaller
end

```

end

result  
end

But this gives a “TypeError: can’t convert Fixnum into Array”, from the  
line “result \<\< [sequence[i]] + smaller”. After a few minutes of  
scratching my head, it became clear that sometimes the result of  
combinations(…) was an Array (which is expected) and sometimes it was  
of type whatever the array elements were (which is unexpected),  
whenever the array would be a singleton.

I ended up changing it to “result \<\< ([sequence[i]] +  
[smaller]).flatten”, but after I turned it in I realized that won’t do

- I’d want the code to handle cases where elements of the array are  
themselves arrays. I added the check “smaller = [smaller] if  
smaller.class != Array”, but it doesn’t _seem_ like I should have to do  
that.

Where am I going wrong here?

Best,  
James C.

---

<div class="post-metadata">

**Author:** ![james-calivar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/2bfe46/32.png) [@james-calivar](https://www.ruby-forum.com/u/james-calivar)\
**Post date:** [December 20, 2006, 6:29pm UTC](https://www.ruby-forum.com/t/routine-sometimes-not-returning-what-i-expect/79925/2 "2006-12-20T18:29:46Z")

</div>

On 2006-12-19 22:10:13 -0500, James C.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) said:

> and combinations([1, 2, 3], 2, false)  
> result = []  
> result  
> [smaller]).flatten", but after I turned it in I realized that won’t do
> 
> - I’d want the code to handle cases where elements of the array are  
> themselves arrays. I added the check “smaller = [smaller] if  
> smaller.class != Array”, but it doesn’t _seem_ like I should have to do  
> that.
> 
> Where am I going wrong here?
> 
> Best,  
> James C.

Okay. It occurs to me that I was wrong in two respects.

Even with the change of adding “smaller = [smaller] if smaller.class !=  
Array”, I still get weird flattening.

combinations([1, 2, [3, 4]], 2)

should return

[[1, 2], [1, [3, 4]], [2, [3, 4]]]

but instead returns

[[1, 2], [1, 3, 4], [2, 3, 4]]

Also, combinations([1, 2, 3], 2, false) doesn’t do what I’d expect: as  
far as I can tell it should collect all 2-permutations, but it has the  
same effect as combinations([1, 2, 3], 2).

For reference, here’s the Python code that seems to work in the way I  
describe:

def combinations(sequence, n, unique=True):  
if not n: yield []

```
for i in range(len(sequence)):
    sub_sequence = sequence[i + 1:]
    if not unique: sub_sequence += sequence[:i]

    for smaller in combinations(sub_sequence, n - 1, unique):
        yield [sequence[i]] + smaller

```

Best,  
James