# BoundsError: attempt to access 0-element Vector{Int64} at index \[1\]
**URL:** https://itensor.discourse.group/t/boundserror-attempt-to-access-0-element-vector-int64-at-index-1/2299
**Category:** ITensor Julia Questions
**Tags:** julia, mps, mpo
**Created:** [March 12, 2025, 7:05am UTC](https://itensor.discourse.group/t/boundserror-attempt-to-access-0-element-vector-int64-at-index-1/2299 "2025-03-12T07:05:06Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@zipeilee](https://itensor.discourse.group/u/zipeilee)
#### Post date: [March 12, 2025, 7:05am UTC](https://itensor.discourse.group/t/boundserror-attempt-to-access-0-element-vector-int64-at-index-1/2299/1 "2025-03-12T07:05:06Z")
I encountered this error while using `apply` for TEBD, but I have already “grad” the sites of the MPS as described in [this link](https://itensor.org/support/3645/the-time-evolution-for-a-h5-file).
```Julia
function measure_pauli(mps::MPS, basis::Vector{Int})
psi = copy(mps)
N = length(psi)
sites = siteinds(psi)
x_inds = findall(x -> x == 1, basis)
y_inds = findall(x -> x == 2, basis)
gates_x = [op("H", sites[i]) for i in x_inds]
gates_y1 = [op("Phase", sites[i])' for i in y_inds]
gates_y2 = [op("H", sites[i]) for i in y_inds]
if !isempty(gates_x)
psi = apply(gates_x, psi)
end
if !isempty(gates_y1)
psi = apply(gates_y1, psi)
end
if !isempty(gates_y2)
psi = apply(gates_y2, psi)
end
result = sample!(psi)
return result
end
function classical_shadows(mps::MPS, num_samples=500)
N = length(mps)
bases = Matrix{Int}(undef, num_samples, N)
snapshots = Matrix{Int}(undef, num_samples, N)
for t in 1:num_samples
basis = rand(1:3, N)
bases[t, :] = basis
snapshots[t, :] = measure_pauli(mps, basis)
end
return snapshots, bases
end
N = 14
T = 100
sites = siteinds("Qubit", N)
psi = randomMPS(sites, 2)
snapshots, bases = classical_shadows(psi, T)
```
here is the error report
```auto
Stacktrace:
[1] getindex
@ ./essentials.jl:13 [inlined]
[2] product(o::ITensor, ψ::MPS, ns::Vector{Int64}; move_sites_back::Bool, apply_dag::Bool, kwargs::@Kwargs{})
@ ITensors.ITensorMPS ~/.julia/packages/ITensors/oOwvi/src/lib/ITensorMPS/src/abstractmps.jl:2120
[3] product (repeats 2 times)
@ ~/.julia/packages/ITensors/oOwvi/src/lib/ITensorMPS/src/abstractmps.jl:2105 [inlined]
[4] product(As::Vector{ITensor}, ψ::MPS; move_sites_back_between_gates::Bool, move_sites_back::Bool, kwargs::@Kwargs{})
@ ITensors.ITensorMPS ~/.julia/packages/ITensors/oOwvi/src/lib/ITensorMPS/src/abstractmps.jl:2247
[5] product
@ ~/.julia/packages/ITensors/oOwvi/src/lib/ITensorMPS/src/abstractmps.jl:2238 [inlined]
[6] measure_pauli(mps::MPS, basis::Vector{Int64})
@ Main ~/CSViT/classcal_shadow.jl:42
[7] classical_shadows(mps::MPS, num_samples::Int64)
@ Main ~/CSViT/classcal_shadow.jl:62
[8] top-level scope
@ ~/CSViT/classcal_shadow.jl:73
```
---
### Author:  [@VinceNeede](https://itensor.discourse.group/u/VinceNeede)
#### Post date: [March 12, 2025, 9:37am UTC](https://itensor.discourse.group/t/boundserror-attempt-to-access-0-element-vector-int64-at-index-1/2299/2 "2025-03-12T09:37:38Z")
Hi @zipeilee,
since this is your first post, welcome in the community.
The problem is how you are writing the dagger, the symbol `'` has a different meaning when applied to an `ITensor`, it primes each index of the tensor.
The function `op` supports an optional boolean argument `adjoint` that you can set to true, so your code would work by changing how you construct the phase gates to this:
```julia
gates_y1 = [op("Phase",sites[i]; adjoint=true) for i in y_inds]
```
Mind that the `adjoint` keyword is not bulletproof, I think it hasn’t been included in the documentation yet cause it is equivalent to `swapprime(dag(op),1=>0)`, so it only works if your operator maps `s -> s'`(which is the most common case), but if it has different prime levels it would fail. Anyway for your code works fine.
---
### Author:  [@zipeilee](https://itensor.discourse.group/u/zipeilee)
#### Post date: [March 15, 2025, 12:16pm UTC](https://itensor.discourse.group/t/boundserror-attempt-to-access-0-element-vector-int64-at-index-1/2299/3 "2025-03-15T12:16:20Z")
very useful， thanks a lot！
---
### Author:  [@system](https://itensor.discourse.group/u/system)
#### Post date: [March 25, 2025, 12:16pm UTC](https://itensor.discourse.group/t/boundserror-attempt-to-access-0-element-vector-int64-at-index-1/2299/4 "2025-03-25T12:16:54Z")
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