# Mysterious error: undefined variable

**URL:** <https://discourse.julialang.org/t/mysterious-error-undefined-variable/7819>\
**Category:** General Usage\
**Created:** [December 17, 2017, 6:29pm UTC](https://discourse.julialang.org/t/mysterious-error-undefined-variable/7819 "2017-12-17T18:29:27Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Stephen\_Vavasis](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stephen_vavasis/32/3389_2.png) [@Stephen\_Vavasis](https://discourse.julialang.org/u/Stephen_Vavasis)\
**Post date:** [December 17, 2017, 6:29pm UTC](https://discourse.julialang.org/t/mysterious-error-undefined-variable/7819/1 "2017-12-17T18:29:27Z")

</div>

Below is a simple program to integrate acceleration and velocity via Forward Euler. I received an error message (below) that `vel` is undefined; I can’t figure out what is causing this. The program is OK if I initialize all the scalars inside the `open...do..` block. I assume that the problem is caused by some subtle interaction of scoping rules, but it would be great if someone could spell it out.

```julia
module pushsq_direct

const load_table = [
0.00E+00 -2.041201359
5.00E-07 -1.661441147
1.00E-06 -0.311829107]

h = .02
w = .02
d = 1.
density = 1180.
totalmass = density / (h*w*d)
vel = 0.
pos = 0.
open("pushsq_direct.out", "w") do hnd
    for tind = 2 : size(load_table,1)
        deltat = load_table[tind,1] - load_table[tind-1,1]
        pressure = load_table[tind,2]*1e6
        force = pressure * h * d
        acc = force / totalmass
        vel += acc * deltat
        pos += vel * deltat
        println(hnd, "t = ", load_table[tind,1], " vel = ", vel, " pos = ", pos)
    end
end

end

```

Error message (Julia 0.6.1, Windows 10)

```julia
julia> include("pushsq_direct_mini.jl")
ERROR: LoadError: UndefVarError: vel not defined
Stacktrace:
 [1] (::pushsq_direct.##1#2)(::IOStream) at C:\Users\vavasis\ownCloud\Documents\Katerina\cohesive\conic_jl\pushsq_direct_mini.jl:21
 [2] open(::pushsq_direct.##1#2, ::String, ::String) at .\iostream.jl:152
 [3] include_from_node1(::String) at .\loading.jl:576
 [4] include(::String) at .\sysimg.jl:14
while loading C:\Users\vavasis\ownCloud\Documents\Katerina\cohesive\conic_jl\pushsq_direct_mini.jl, in expression starting on line 15

```

---

<div class="post-metadata">

**Author:** ![benelsen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/benelsen/32/1620_2.png) [@benelsen](https://discourse.julialang.org/u/benelsen)\
**Post date:** [December 17, 2017, 6:55pm UTC](https://discourse.julialang.org/t/mysterious-error-undefined-variable/7819/2 "2017-12-17T18:55:27Z")

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A do-block creates a [Hard Local Scope](https://docs.julialang.org/en/stable/manual/variables-and-scoping/#Hard-Local-Scope-1) which inherits all variables from the parent scope (here the global) if the variables are **not modified**.

In your example you could add the line `global pos, vel` inside the do-block to force `vel` and `pos` to be inherited and achieve the expected behaviour.

```julia
open("pushsq_direct.out", "w") do hnd
    global pos, vel
    for tind = 2 : size(load_table,1)
        ...

```

The for-loop creates a soft local scope and thus permits modifying the parent’s (do block) variables.

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<div class="post-metadata">

**Author:** ![Stephen\_Vavasis](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stephen_vavasis/32/3389_2.png) [@Stephen\_Vavasis](https://discourse.julialang.org/u/Stephen_Vavasis)\
**Post date:** [December 17, 2017, 10:31pm UTC](https://discourse.julialang.org/t/mysterious-error-undefined-variable/7819/3 "2017-12-17T22:31:34Z")

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Thanks! I didn’t know this, but it is in the manual in the section on Variable Scopes. It isn’t in the section on `do` blocks, I just submitted a PR to add a sentence to the latter section. Meanwhile, I discovered another bug in my code: it should be mass = density \* volume!

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<div class="post-metadata">

**Author:** ![Stephen\_Vavasis](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stephen_vavasis/32/3389_2.png) [@Stephen\_Vavasis](https://discourse.julialang.org/u/Stephen_Vavasis)\
**Post date:** [December 18, 2017, 1:38am UTC](https://discourse.julialang.org/t/mysterious-error-undefined-variable/7819/4 "2017-12-18T01:38:09Z")

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Further follow-up on my own post-- apparently the notions of hard and soft local scope are undergoing significant revision; see

[https://github.com/JuliaLang/julia/pull/19324#pullrequestreview-8520079](https://github.com/JuliaLang/julia/pull/19324#pullrequestreview-8520079)

And in fact, my posted code runs without error in the most recent 0.7 nightly.