# UndefVarError in for loop
**URL:** https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999
**Category:** General Usage
**Tags:** question
**Created:** [October 18, 2021, 7:52pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999 "2021-10-18T19:52:32Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@Amro](https://discourse.julialang.org/u/Amro)
#### Post date: [October 18, 2021, 7:52pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/1 "2021-10-18T19:52:32Z")
Below is my MWE. I do not know why I am having the below error though I am defining it at the beginning?
```julia
using LinearAlgebra;
using BenchmarkTools
function curMatRLC(NodeVoltage, dt)
if NodeVoltage[1,1]==11
return s = ones(3*5) - dt;
else
return s = ones(3*5) + dt;
end
end
NodeVoltage = zeros(3,5);
A_Mat = ones(3*5,3*5);
I_Mat = ones(3*5);
@btime begin
global iter = 0;
for t in 0:1e-6:0.03
global iter += 1;
I_Mat[1:3*5] = curMatRLC(NodeVoltage,dt);
V_Mat = lu(A_Mat) \ I_Mat;
NodeVoltage = reshape(V_Mat[1:3*5], 3, :);
end
end
ERROR: LoadError: UndefVarError: NodeVoltage not defined
Stacktrace:
[1] macro expansion
```
---
### Author:  [@jlperla](https://discourse.julialang.org/u/jlperla)
#### Post date: [October 18, 2021, 8:00pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/2 "2021-10-18T20:00:24Z")
If you are ever benchmarking, then you should never have the `global` identifier anywhere in sight. Put everything in a function (taking `A_mat` and `I_mat` as inputs, and tell call it as `@btime my_function($A_mat, $I_mat)`
---
### Author:  [@Nathan\_Boyer](https://discourse.julialang.org/u/Nathan_Boyer)
#### Post date: [October 18, 2021, 8:34pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/3 "2021-10-18T20:34:23Z")
Global variables are not in local scope without the `global` keyword. See [Scope of Variables · The Julia Language](https://docs.julialang.org/en/v1/manual/variables-and-scoping/#scope-of-variables). Adding the line `global NodeVoltage` to your innermost `for` loop should remove the error. However, it is better to write a function for that part of the code and pass `NodeVoltage` to that function. Then you won’t have to worry about scope and your code will be faster.
---
### Author:  [@Amro](https://discourse.julialang.org/u/Amro)
#### Post date: [October 18, 2021, 8:39pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/4 "2021-10-18T20:39:04Z")
Got it, thank you!
---
### Author:  [@Amro](https://discourse.julialang.org/u/Amro)
#### Post date: [October 18, 2021, 8:45pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/6 "2021-10-18T20:45:04Z")
Thank for your reply. But why the error is only for `NodeVoltage` not for `A_mat` and `I_mat`, though they are all supposed to be global variables being used in local scope of for loop?
---
### Author:  [@Nathan\_Boyer](https://discourse.julialang.org/u/Nathan_Boyer)
#### Post date: [October 18, 2021, 8:47pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/7 "2021-10-18T20:47:03Z")
Sorry, I didn’t actually test. You will need to declare those as globals too. I was just writing based on the current error.
---
### Author:  [@Nathan\_Boyer](https://discourse.julialang.org/u/Nathan_Boyer)
#### Post date: [October 18, 2021, 8:55pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/8 "2021-10-18T20:55:53Z")
But again, do `@btime myfunction!(NodeVoltage, A_Mat, and I_Mat)` instead of adding the `global` statements.
---
### Author:  [@jlperla](https://discourse.julialang.org/u/jlperla)
#### Post date: [October 18, 2021, 9:46pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/9 "2021-10-18T21:46:32Z")
> [@Nathan\_Boyer](#):
>
> `@btime myfunction!(NodeVoltage, A_Mat, and I_Mat)`
`@btime myfunction!($NodeVoltage,$A_Mat,$I_Mat)` or it will end up global.
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [October 19, 2021, 1:02am UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/10 "2021-10-19T01:02:30Z")
> [@Amro](#):
>
> But why the error is only for `NodeVoltage` not for `A_mat` and `I_mat` , though they are all supposed to be global variables being used in local scope of for loop?
Reading from global variables, is always allowed, but re-assigning them is only allowed (if you explicitly declared them as `global`. (Note that `I_Mat[1:3*5] = ...` is _not_ re-assigning `I_Mat` itself, but rather is mutating the _contents_ of `I_Mat`.)
---
### Author:  [@Amro](https://discourse.julialang.org/u/Amro)
#### Post date: [October 19, 2021, 1:21pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/11 "2021-10-19T13:21:02Z")
@stevengj
Thanks for your valuable reply. It seems that I have a lack in differentiating between `re-assigning` and `mutating`. Is it possible to explain this to me or direct me to its documentation?
---
### Author:  [@Amro](https://discourse.julialang.org/u/Amro)
#### Post date: [October 19, 2021, 1:23pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/12 "2021-10-19T13:23:08Z")
> [@jlperla](#):
>
> `@btime myfunction!($NodeVoltage,$A_Mat,$I_Mat)` or it will end up global.
@jlperla Thanks for your reply. What does the `$` really do be before the variables?
---
### Author:  [@Nathan\_Boyer](https://discourse.julialang.org/u/Nathan_Boyer)
#### Post date: [October 19, 2021, 1:32pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/13 "2021-10-19T13:32:58Z")
It is just because you are benchmarking. You do not need the `$` otherwise.
From [BenchmarkTools](https://juliaci.github.io/BenchmarkTools.jl/dev/):
> If the expression you want to benchmark depends on external variables, you should use [`$` to “interpolate”](https://github.com/JuliaCI/BenchmarkTools.jl/blob/master/doc/manual.md#interpolating-values-into-benchmark-expressions) them into the benchmark expression to [avoid the problems of benchmarking with globals](https://docs.julialang.org/en/v1/manual/performance-tips/#Avoid-global-variables). Essentially, any interpolated variable `$x` or expression `$(...)` is “pre-computed” before benchmarking begins:
---
### Author:  [@Nathan\_Boyer](https://discourse.julialang.org/u/Nathan_Boyer)
#### Post date: [October 19, 2021, 1:58pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/14 "2021-10-19T13:58:12Z")
Your `I_Mat` is a `Vector` which is a type of mutable container meaning you can change what is held inside of `I_Mat`. When you write `I_Mat[1:3*5] = new_value` the values stored at those locations are changed in place (mutated). Whereas, if you write `I_Mat = new_value`, a new location in memory is allocated and assigned the name `I_Mat` (re-assignment). The difference is likely not important for you at this stage as long as you avoid global variables.
---
### Author:  [@Amro](https://discourse.julialang.org/u/Amro)
#### Post date: [October 19, 2021, 2:15pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/15 "2021-10-19T14:15:58Z")
> [@Nathan\_Boyer](#):
>
> When you write `I_Mat[1:3*5] = new_value` the values stored at those locations are changed in place (mutated). Whereas, if you write `I_Mat = new_value` , a new location in memory is allocated and assigned the name `I_Mat` (re-assignment).
@Nathan_Boyer
1- So is it always good to use indexes for the elements of vector (or matrix) as in `I_Mat[1:3*5] = new_value`, to avoid re-assignment?
2- After this new assignment of `I_Mat = new_value` , what will happen to the old location of `I_Mat` in memory? will be still occupied (but useless) of freed?
---
### Author:  [@Jeff\_Emanuel](https://discourse.julialang.org/u/Jeff_Emanuel)
#### Post date: [October 19, 2021, 2:19pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/16 "2021-10-19T14:19:00Z")
> [@Amro](#):
>
> 2- After this new assignment of `I_Mat = new_value` , what will happen to the old location of `I_Mat` in memory? will be still occupied (but useless) of freed?
It depends on whether you have other variables bound to that array. If there are no other references to the array, it will be garbage collected (eventually) .
---
### Author:  [@Amro](https://discourse.julialang.org/u/Amro)
#### Post date: [October 19, 2021, 2:23pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/17 "2021-10-19T14:23:27Z")
> [@Jeff\_Emanuel](#):
>
> It depends on whether you have other variables bound to that array. If there are no other references to the array, it will be garbage collected (eventually)
@Jeff_Emanuel The other variables that are bound to that array `I_Mat` are actually bound to the name not location, correct? So, by using the re-assignment, these variable will be bound to the name of the re-assignment, right?
If not right, could you please give me an example?
---
### Author:  [@Jeff\_Emanuel](https://discourse.julialang.org/u/Jeff_Emanuel)
#### Post date: [October 19, 2021, 2:25pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/18 "2021-10-19T14:25:24Z")
```julia
a=ones(5)
b=a
a=zeros(3)
```
After this code runs, b is bound to the original result of `ones`. It gets bound to the same value that a is bound to. It is not an alias for a. a then gets bound to the new array created by `zeros`.
---
### Author:  [@Amro](https://discourse.julialang.org/u/Amro)
#### Post date: [October 19, 2021, 2:36pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/19 "2021-10-19T14:36:56Z")
Thank you, I got it.
---
### Author:  [@Nathan\_Boyer](https://discourse.julialang.org/u/Nathan_Boyer)
#### Post date: [October 19, 2021, 3:12pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/20 "2021-10-19T15:12:09Z")
I updated your MWE to illustrate the general form of what I was describing above. I chose not to mutate for clarity. You may want to mutate if speed is more important.
```julia
using LinearAlgebra, BenchmarkTools
function curMatRLC(NodeVoltage, dt)
if NodeVoltage[1,1]==11
return s = ones(3*5) .- dt;
else
return s = ones(3*5) .+ dt;
end
end
function march!(NodeVoltage, A_Mat)
dt = 1e-6
for t in 0:dt:0.03
local I_Mat = curMatRLC(NodeVoltage, dt)
local V_Mat = lu(A_Mat) \ I_Mat
NodeVoltage = reshape(V_Mat, 3, :)
end
return NodeVoltage, A_Mat, I_Mat, V_Mat
end
NodeVoltage = zeros(3,5)
A_Mat = rand(3*5, 3*5)
(NodeVoltage, A_Mat, I_Mat, V_Mat) = march!(NodeVoltage, A_Mat)
@btime march!($NodeVoltage, $A_Mat)
```
Below are some issues I encountered getting your code to run with some explanations that you may find helpful:
1. `dt` wasn’t previously defined. I defined it as a scalar, so I could use it in the definition of `t`, but then I had to add a broadcasting `.` to the addition inside `curMatRLC`.
2. The result of `curMatRLC` doesn’t change with time right now. I assume you will need to pass another variable to it to represent whatever ones(3\*5) is supposed to be, so that `s` changes.
3. The `V_Mat` calculation gave me an error because everything was 1’s, so I had to change `A_Mat` to be random numbers.
4. Scope became an issue again when returning values from my function since `I_Mat` and `V_Mat` are only defined inside the `for` loop. I added the `local` keyword to fix this. If you care about the values before the end time though, you probably want to define a vector to store your results at every timestep outside of the loop and then update the elements of that vector in the loop instead of using `local`.
---
### Author:  [@Amro](https://discourse.julialang.org/u/Amro)
#### Post date: [October 19, 2021, 6:50pm UTC](https://discourse.julialang.org/t/undefvarerror-in-for-loop/69999/21 "2021-10-19T18:50:40Z")
@Nathan_Boyer Thank you very much for your time and explanations.
Why you put `!` in `march!`?
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