Hi,

I have this code:

```
using Pkg;
n = 1.0;
function yfc(t)
# x in here is actually t from before
cos(1/3 * t^3 + n*t)
end;
const t0 = 0;
const tf = 1000;
const N = 10000000;
const dt = 1000 / N;
t = t0:dt:tf;
const x = 0.0 ;
for i in 1:N;
k1 = dt * yfc(t[i]);
k2 = dt * yfc(t[i] + 1/2*dt);
k4 = dt * yfc(t[i+1]);
x = x + 1.0/6.0 * k1 + 2.0/3.0 * k2 + 1.0/6.0 * k4 ;
end
```

and I’m getting the error:

```
ERROR: LoadError: UndefVarError: x not defined
Stacktrace:
[1] top-level scope at /data/GitHub/mine/math/julia-scripts/airy-ai-approx.jl:24 [inlined]
[2] top-level scope at ./none:0
[3] include(::String) at ./client.jl:403
[4] top-level scope at none:0
in expression starting at /data/GitHub/mine/math/julia-scripts/airy-ai-approx.jl:20
```

which confuses me. x is defined, originally as a constant and then as a variable that is updated over the loop. What is the problem here?