# Simple for loop woth if condition in Julia

**URL:** <https://discourse.julialang.org/t/simple-for-loop-woth-if-condition-in-julia/49430>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [November 1, 2020, 9:56pm UTC](https://discourse.julialang.org/t/simple-for-loop-woth-if-condition-in-julia/49430 "2020-11-01T21:56:52Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![discourserogu](https://avatars.discourse-cdn.com/v4/letter/d/919ad9/32.png) [@discourserogu](https://discourse.julialang.org/u/discourserogu)\
**Post date:** [November 1, 2020, 9:56pm UTC](https://discourse.julialang.org/t/simple-for-loop-woth-if-condition-in-julia/49430/1 "2020-11-01T21:56:52Z")

</div>

I’m trying to implement the following:

```julia
res = 0.0 
res0= 0.0 

for i = 1:10
    
    res = 0.5^i
    
    if i==1 
       res0 = copy(res)
    end 
    
   res = copy(res)/res0
   println(res)
end

```

but when I run it I get the following error:

```julia
1.0
ERROR: LoadError: UndefVarError: res0 not defined
Stacktrace:
 [1] top-level scope at /home/rohit/Desktop/cfd_julia/sample.jl:12
 [2] include(::Module, ::String) at ./Base.jl:377
 [3] exec_options(::Base.JLOptions) at ./client.jl:288
 [4] _start() at ./client.jl:484
in expression starting at /home/rohit/Desktop/cfd_julia/sample.jl:4

```

Fortran Equivalent I’m attempting to recode in Julia:

```julia
program main
real(8) :: res,res0
integer :: i

res = 0.0 
res0= 0.0 

do i = 1,10
   res = 0.5d0**i
   if(i.eq.1) then
     res0 = res 
   endif
   res = res/res0
   print*, res 
enddo
endprogram

```

I seem to be misunderstanding how copy, value assignment and scope of variables work in Julia, what am I missing? Thanks in advance!

---

<div class="post-metadata">

**Author:** ![Mason](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mason/32/2423_2.png) [@Mason](https://discourse.julialang.org/u/Mason)\
**Post date:** [November 1, 2020, 10:00pm UTC](https://discourse.julialang.org/t/simple-for-loop-woth-if-condition-in-julia/49430/2 "2020-11-01T22:00:35Z")

</div>

Put your code in a function or `let` block and it’ll work (it will also avoid being painfully slow) [Scope of Variables · The Julia Language](https://docs.julialang.org/en/v1.4/manual/variables-and-scoping/#scope-of-variables-1)

Alternatively, you can upgrade to version 1.5, where your code will no longer error but will still be inefficient.

---

<div class="post-metadata">

**Author:** ![discourserogu](https://avatars.discourse-cdn.com/v4/letter/d/919ad9/32.png) [@discourserogu](https://discourse.julialang.org/u/discourserogu)\
**Post date:** [November 1, 2020, 10:02pm UTC](https://discourse.julialang.org/t/simple-for-loop-woth-if-condition-in-julia/49430/3 "2020-11-01T22:02:12Z")

</div>

Thanks for the advice I’ll look into the `let` block. So what was my problem, just the version? I mean did the scope of variables change over versions?

---

<div class="post-metadata">

**Author:** ![tomerarnon](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tomerarnon/32/3170_2.png) [@tomerarnon](https://discourse.julialang.org/u/tomerarnon)\
**Post date:** [November 1, 2020, 10:06pm UTC](https://discourse.julialang.org/t/simple-for-loop-woth-if-condition-in-julia/49430/4 "2020-11-01T22:06:50Z")

</div>

Scoping rules changed for the REPL. You can take a look at the julia manual page on scope, in particular this section:  
[https://docs.julialang.org/en/v1/manual/variables-and-scoping/#On-Soft-Scope](https://docs.julialang.org/en/v1/manual/variables-and-scoping/#On-Soft-Scope)  
And the release notes:  
[https://docs.julialang.org/en/v1/NEWS/#Language-changes](https://docs.julialang.org/en/v1/NEWS/#Language-changes)

---

<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [November 1, 2020, 10:15pm UTC](https://discourse.julialang.org/t/simple-for-loop-woth-if-condition-in-julia/49430/5 "2020-11-01T22:15:36Z")

</div>

> [@discourserogu](#):
>
> `copy(res)`

`res` is a scalar of an immutable value type. It doesn’t make sense to copy them.

Remove all the `copy`s, and put your code in a function.

---

<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [November 1, 2020, 10:37pm UTC](https://discourse.julialang.org/t/simple-for-loop-woth-if-condition-in-julia/49430/6 "2020-11-01T22:37:10Z")

</div>

Not sure what you actually need to accomplish, but here are two alternatives; one that returns a vector of the values you are trying to calculate, and one that just prints the values one-by-one:

```julia
function foo(N)
    res = 2.0
    return [res *= 0.5 for i in 1:N]
end

function bar(N)
    res = 2.0
    for i in 1:N
        res *= 0.5
        println(res)
    end
end

```

```julia
julia> foo(10)
10-element Array{Float64,1}:
 1.0
 0.5
 0.25
 0.125
 0.0625
 0.03125
 0.015625
 0.0078125
 0.00390625
 0.001953125

julia> bar(10)
1.0
0.5
0.25
0.125
0.0625
0.03125
0.015625
0.0078125
0.00390625
0.001953125

```