# StackOverflowError when contracting with specified cutoff

**URL:** <https://itensor.discourse.group/t/stackoverflowerror-when-contracting-with-specified-cutoff/225>\
**Category:** ITensor Julia Questions\
**Created:** [June 22, 2022, 10:18pm UTC](https://itensor.discourse.group/t/stackoverflowerror-when-contracting-with-specified-cutoff/225 "2022-06-22T22:18:28Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![nGn](https://avatars.discourse-cdn.com/v4/letter/n/d78d45/32.png) [@nGn](https://itensor.discourse.group/u/nGn)\
**Post date:** [June 22, 2022, 10:18pm UTC](https://itensor.discourse.group/t/stackoverflowerror-when-contracting-with-specified-cutoff/225/1 "2022-06-22T22:18:28Z")

</div>

I’ve been coming across a strange problem in ITensors v0.3.17.

```auto
s = [Index(2) for i in 1:3]
a = randomITensor(s[1], s[2])
b = randomITensor(s[2], s[3])

```

Contracting these two tensors without specifying the cutoff keyword works fine:

```auto
julia> contract([a, b])
ITensor ord=2 (dim=2|id=803) (dim=2|id=272)
NDTensors.Dense{Float64, Vector{Float64}}

```

But specifying the cutoff seems to make it infinitely recurse:

```auto
julia> contract([a, b]; cutoff=1e-13)
ERROR: StackOverflowError:
Stacktrace:
     [1] contract(::ITensor, ::Vararg{ITensor}; kwargs::Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}})
       @ ITensors ~/.julia/packages/ITensors/MUEOu/src/itensor.jl:2060
     [2] (::ITensors.var"#233#235"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}})(A::ITensor, B::ITensor)
       @ ITensors ~/.julia/packages/ITensors/MUEOu/src/itensor.jl:2050
     [3] (::Base.BottomRF{ITensors.var"#233#235"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}}})(acc::ITensor, x::ITensor)
       @ Base ./reduce.jl:81
     [4] afoldl(::Base.BottomRF{ITensors.var"#233#235"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}}}, ::Base._InitialValue, ::ITensor, ::ITensor)
       @ Base ./operators.jl:613
     [5] _foldl_impl(op::Base.BottomRF{ITensors.var"#233#235"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}}}, init::Base._InitialValue, itr::Tuple{ITensor, ITensor})
       @ Base ./tuple.jl:277
     [6] foldl_impl(op::Base.BottomRF{ITensors.var"#233#235"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}}}, nt::Base._InitialValue, itr::Tuple{ITensor, ITensor})
       @ Base ./reduce.jl:48
     [7] mapfoldl_impl(f::typeof(identity), op::ITensors.var"#233#235"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}}, nt::Base._InitialValue, itr::Tuple{ITensor, ITensor})
       @ Base ./reduce.jl:44
     [8] mapfoldl(f::Function, op::Function, itr::Tuple{ITensor, ITensor}; init::Base._InitialValue)
       @ Base ./reduce.jl:162
     [9] mapfoldl
       @ ./reduce.jl:162 [inlined]
    [10] #foldl#245
       @ ./reduce.jl:180 [inlined]
    [11] foldl(op::Function, itr::Tuple{ITensor, ITensor})
       @ Base ./reduce.jl:180
    [12] contract(As::Tuple{ITensor, ITensor}; sequence::String, kwargs::Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}})
       @ ITensors ~/.julia/packages/ITensors/MUEOu/src/itensor.jl:2050
--- the last 12 lines are repeated 4703 more times ---

```

Looking at line 2050 in `ITensors.jl`,

```auto
    return foldl((A, B) -> contract(A, B; kwargs...), As)

```

it isn’t immediately obvious to me how this could cause the infinite recursion. Am I using `contract` incorrectly somehow?

---

<div class="post-metadata">

**Author:** ![mtfishman](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/mtfishman/32/11_2.png) [@mtfishman](https://itensor.discourse.group/u/mtfishman)\
**Post date:** [June 23, 2022, 8:58pm UTC](https://itensor.discourse.group/t/stackoverflowerror-when-contracting-with-specified-cutoff/225/2 "2022-06-23T20:58:22Z")

</div>

I’m not sure why adding that keyword argument causes an infinite recursion, but `cutoff` isn’t supported for contracting ITensors anyway since approximate contraction of ITensors is not currently support through `contract`.