# Help newbie to understand type stability please
**URL:** https://discourse.julialang.org/t/help-newbie-to-understand-type-stability-please/7119
**Category:** General Usage
**Created:** [November 17, 2017, 10:55am UTC](https://discourse.julialang.org/t/help-newbie-to-understand-type-stability-please/7119 "2017-11-17T10:55:23Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@Liso](https://discourse.julialang.org/u/Liso)
#### Post date: [November 17, 2017, 10:55am UTC](https://discourse.julialang.org/t/help-newbie-to-understand-type-stability-please/7119/1 "2017-11-17T10:55:23Z")
Could anybody please help me improve my understanding of what looks like strange behavior?
```julia
function good1(x::Array{T,1}) where T<:Float64
n = length(x)
x *= 1 # problem is not here
[i for i in x]
end
function good2(x::Array{T,1}) where T<:Float64
n = length(x)
[x[i] for i in 1:n] # problem is not here
end
function wrong(x::Array{T,1}) where T<:Float64
n = length(x)
x *= 1
[x[i] for i in 1:n]
end
@code_warntype good1([0.])
@code_warntype good2([0.])
@code_warntype wrong([0.]) # also n is interpreted as Any here!
```
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [November 17, 2017, 11:14am UTC](https://discourse.julialang.org/t/help-newbie-to-understand-type-stability-please/7119/2 "2017-11-17T11:14:36Z")
You make `x` change its type. Did you mean `x .*= 1`? Or you can use another variable, as in
```julia
function good3(x::Array{T,1}) where T<:Float64
n = length(x)
y = x * 1
[y[i] for i in 1:n]
end
```
Also see [this suggestion](https://docs.julialang.org/en/latest/manual/performance-tips/#Avoid-changing-the-type-of-a-variable-1) in the manual.
---
### Author:  [@Liso](https://discourse.julialang.org/u/Liso)
#### Post date: [November 17, 2017, 11:26am UTC](https://discourse.julialang.org/t/help-newbie-to-understand-type-stability-please/7119/3 "2017-11-17T11:26:11Z")
But why it is not a problem in good1?
---
### Author:  [@Liso](https://discourse.julialang.org/u/Liso)
#### Post date: [November 17, 2017, 11:40am UTC](https://discourse.julialang.org/t/help-newbie-to-understand-type-stability-please/7119/4 "2017-11-17T11:40:21Z")
BTW. I asked because I liked to give good answer on [SO](https://stackoverflow.com/questions/47346414/type-stability-for-a-function-involving-case-distinctions/47348191#47348191).
---
### Author:  [@Liso](https://discourse.julialang.org/u/Liso)
#### Post date: [November 17, 2017, 11:56am UTC](https://discourse.julialang.org/t/help-newbie-to-understand-type-stability-please/7119/5 "2017-11-17T11:56:43Z")
Another subquestion: Why is y “coreboxed” in next code? What does it mean? Or where could I find good documentation about it?
```julia
function tst(x::Array{T,1}) where T<:Float64
n = length(x)
n == 1 && return [1.0]
y = x * 1
[y[i] for i = 1:n]
end
@code_warntype tst([1.])
```
---
### Author:  [@GunnarFarneback](https://discourse.julialang.org/u/GunnarFarneback)
#### Post date: [November 17, 2017, 12:19pm UTC](https://discourse.julialang.org/t/help-newbie-to-understand-type-stability-please/7119/6 "2017-11-17T12:19:53Z")
Looks like something goes wrong. Somewhat simplified:
```julia
function f(x)
return [x[i] for i = 1:length(x)]
end
function g(x)
x = x
return [x[i] for i = 1:length(x)]
end
@code_warntype f([0])
@code_warntype g([0])
```
---
### Author:  [@sdanisch](https://discourse.julialang.org/u/sdanisch)
#### Post date: [November 17, 2017, 12:21pm UTC](https://discourse.julialang.org/t/help-newbie-to-understand-type-stability-please/7119/7 "2017-11-17T12:21:08Z")
[https://github.com/JuliaLang/julia/issues/15276](https://github.com/JuliaLang/julia/issues/15276)
---
### Author:  [@Liso](https://discourse.julialang.org/u/Liso)
#### Post date: [November 17, 2017, 12:48pm UTC](https://discourse.julialang.org/t/help-newbie-to-understand-type-stability-please/7119/8 "2017-11-17T12:48:24Z")
That (open!) issue has interesting milestone (0.5). (Probably label 1.0 is missing too?)
And yes - `let` could help avoid `Core.Box`. Thanks! 🙂