# Undefined variable of function side while loop
**URL:** https://discourse.julialang.org/t/undefined-variable-of-function-side-while-loop/32668
**Category:** General Usage
**Tags:** question
**Created:** [December 24, 2019, 9:48pm UTC](https://discourse.julialang.org/t/undefined-variable-of-function-side-while-loop/32668 "2019-12-24T21:48:51Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@locluong09](https://discourse.julialang.org/u/locluong09)
#### Post date: [December 24, 2019, 9:48pm UTC](https://discourse.julialang.org/t/undefined-variable-of-function-side-while-loop/32668/1 "2019-12-24T21:48:51Z")
I have a function inside a while loop like the code below, but get problem ERROR: UndefVarError: Pre\_predict not defined.
```julia
Pre_predict = zeros(Row, Col)
while true
N,S,W,E,C,Q = construct_Tran_matrix(Pre_init, **Pre_predict** , Gamma, Z_init, Z_predict, Order)
LHS, RHS = construct_Coefficient_matrix(N,S,W,E,C,Q,Order,cnt)
Residual_matrix = Residual(LHS, RHS, Pre_predict, Order, cnt)
J = Jacobian_matrix(Pre_predict, Order, Gamma, Z_predict, cnt)
Delta_p = J\Residual_matrix
if maximum(broadcast(abs, Pre_predict-Pre_init)) < 0.01
break
end
end
```
BUT when I change to variable X = 5, it works.
```julia
X = 5
while true
N,S,W,E,C,Q = construct_Tran_matrix(Pre_init, **X** , Gamma, Z_init, Z_predict, Order)
LHS, RHS = construct_Coefficient_matrix(N,S,W,E,C,Q,Order,cnt)
Residual_matrix = Residual(LHS, RHS, Pre_predict, Order, cnt)
J = Jacobian_matrix(Pre_predict, Order, Gamma, Z_predict, cnt)
Delta_p = J\Residual_matrix
if maximum(broadcast(abs, Pre_predict-Pre_init)) < 0.01
break
end
end
```
Can some one explain the problem with my code?
---
### Author:  [@bashonubuntu](https://discourse.julialang.org/u/bashonubuntu)
#### Post date: [December 24, 2019, 10:11pm UTC](https://discourse.julialang.org/t/undefined-variable-of-function-side-while-loop/32668/2 "2019-12-24T22:11:51Z")
It looks like you need a `let` block. See also [Scope of Variables · The Julia Language](https://docs.julialang.org/en/v1/manual/variables-and-scoping/)
```julia
julia> y = 10
10
julia> while y < 100
y += y
end
ERROR: UndefVarError: y not defined
Stacktrace:
[1] top-level scope at .\REPL[8]:2
julia> let y = 10
while y < 100
y += y
end
return y
end
160
```
---
### Author:  [@locluong09](https://discourse.julialang.org/u/locluong09)
#### Post date: [December 24, 2019, 10:13pm UTC](https://discourse.julialang.org/t/undefined-variable-of-function-side-while-loop/32668/3 "2019-12-24T22:13:29Z")
Thank you. I got it.
---
### Author:  [@bashonubuntu](https://discourse.julialang.org/u/bashonubuntu)
#### Post date: [December 24, 2019, 10:15pm UTC](https://discourse.julialang.org/t/undefined-variable-of-function-side-while-loop/32668/4 "2019-12-24T22:15:42Z")
Perfect, you’re most welcome.