# Understanding 'MethodError'

**URL:** <https://discourse.julialang.org/t/understanding-methoderror/1994>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [February 9, 2017, 12:25am UTC](https://discourse.julialang.org/t/understanding-methoderror/1994 "2017-02-09T00:25:30Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![leiteiro](https://avatars.discourse-cdn.com/v4/letter/l/b5e925/32.png) [@leiteiro](https://discourse.julialang.org/u/leiteiro)\
**Post date:** [February 9, 2017, 12:25am UTC](https://discourse.julialang.org/t/understanding-methoderror/1994/1 "2017-02-09T00:25:30Z")

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Please consider my very first Julia function below.  
It throws a “MethodError: no method matching zero(::Type{Any})”  
and gives a list of three “Closest candidates”.  
I do not understand how I can correct this error. Any help appreciated.

```
function b(u::Int, o::Int, t::Int)
    if u == -o 
        return 1 
    end
    if t == 0 
        return sum(b(u-j, o+j-1, (t+1) % 2) for j in 1:u) 
    end
    return sum(b(u+j-1, o-j, (t+1) % 2) for j in 1:o)
end
function B(n::Int)
    return b(n, 0, 0)
end
B(5)

```

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**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [February 9, 2017, 12:33am UTC](https://discourse.julialang.org/t/understanding-methoderror/1994/2 "2017-02-09T00:33:08Z")

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Your function ends up trying to sum over an empty collection, which seems like it should be fine – the answer is zero, right? – until you ask the question “which zero?” The `sum` function answers that question by calling `zero(Any)` which is not defined. You can avoid this problem by avoiding the empty cases, e.g. by adding explicit recursion cases instead. E.g.:

```julia
julia> function b(u::Int, o::Int, t::Int)
           if t == 0 && u == 0 || o == 0
               return 0
           end
           if u == -o
               return 1
           end
           if t == 0
               return sum(b(u-j, o+j-1, (t+1) % 2) for j in 1:u)
           end
           return sum(b(u+j-1, o-j, (t+1) % 2) for j in 1:o)
       end
b (generic function with 1 method)

julia> function B(n::Int)
           return b(n, 0, 0)
       end
B (generic function with 1 method)

julia> B(5)
0

```

I’m not sure if that was the answer you were expecting, but that’s what I got.

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<div class="post-metadata">

**Author:** ![leiteiro](https://avatars.discourse-cdn.com/v4/letter/l/b5e925/32.png) [@leiteiro](https://discourse.julialang.org/u/leiteiro)\
**Post date:** [February 9, 2017, 12:21pm UTC](https://discourse.julialang.org/t/understanding-methoderror/1994/3 "2017-02-09T12:21:58Z")

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“…the answer is zero, right?”

No. As you can see from the almost identical  
Python function below I expect

```
[B(n) for n in (0..6)] 
[1, 1, 1, 2, 5, 16, 61]

def b(u, o, t):
    if u ==-o: return 1
    if t == 0: return sum(b(u-j, o+j-1, (t+1) % 2) for j in (1..u))
    return sum(b(u+j-1, o-j, (t+1) % 2) for j in (1..o))

B = lambda n: b(n, 0, 0)

```

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<div class="post-metadata">

**Author:** ![tkelman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkelman/32/692_2.png) [@tkelman](https://discourse.julialang.org/u/tkelman)\
**Post date:** [February 9, 2017, 12:50pm UTC](https://discourse.julialang.org/t/understanding-methoderror/1994/4 "2017-02-09T12:50:46Z")

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By “the answer is zero, right?” I don’t think Stefan was referring to your overall function, but rather to the question of summing over an empty collection.

The version Stefan posted returns 0 in more cases than your original. You want something more like

```julia
function b(u::Int, o::Int, t::Int)
    if u == -o
        return 1
    end
    if t == 0
        return u == 0 ? 0 : sum(b(u-j, o+j-1, (t+1) % 2) for j in 1:u)
    end
    return o == 0 ? 0 : sum(b(u+j-1, o-j, (t+1) % 2) for j in 1:o)
end

```

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<div class="post-metadata">

**Author:** ![leiteiro](https://avatars.discourse-cdn.com/v4/letter/l/b5e925/32.png) [@leiteiro](https://discourse.julialang.org/u/leiteiro)\
**Post date:** [February 9, 2017, 2:05pm UTC](https://discourse.julialang.org/t/understanding-methoderror/1994/5 "2017-02-09T14:05:53Z")

</div>

“You want something more like …”

Yes, this is what I was looking for. Basically it takes care that  
the sum over an empty set is 0, which is common practice.

Still I wonder why in this case the “sum function answers  
by calling zero(Any) which is not defined” since the  
function b(u::Int, o::Int, t::Int) specifies the types of  
the parameters. If I had written only written b(u, o, t)  
I would understand it immediately. In other words: Where  
does this ‘Any’ in ‘zero(Any)’ comes from?

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<div class="post-metadata">

**Author:** ![tkelman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkelman/32/692_2.png) [@tkelman](https://discourse.julialang.org/u/tkelman)\
**Post date:** [February 9, 2017, 2:09pm UTC](https://discourse.julialang.org/t/understanding-methoderror/1994/6 "2017-02-09T14:09:57Z")

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I think there were some type inference issues limiting the compiler’s ability to correctly guess the element type of generators on 0.5. On Julia 0.6-dev I get a different error, `ArgumentError: reducing over an empty collection is not allowed`. I’m not sure whether the PR that fixed that is backportable (or if it has been marked for backporting) but should find out shortly as I work through that list.

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<div class="post-metadata">

**Author:** ![tkelman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkelman/32/692_2.png) [@tkelman](https://discourse.julialang.org/u/tkelman)\
**Post date:** [February 9, 2017, 2:12pm UTC](https://discourse.julialang.org/t/understanding-methoderror/1994/7 "2017-02-09T14:12:53Z")

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You could also change your `sum(generator)` calls to `reduce(+, 0, generator)` which specifies the return value for empty collections. Looks like that works fine on both 0.5 and 0.6-dev.