# Function to fill fespace
**URL:** https://community.freefem.org/t/function-to-fill-fespace/1297
**Category:** General Discussion
**Created:** [November 4, 2021, 4:04pm UTC](https://community.freefem.org/t/function-to-fill-fespace/1297 "2021-11-04T16:04:52Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@ivan](https://community.freefem.org/u/ivan)
#### Post date: [November 4, 2021, 4:04pm UTC](https://community.freefem.org/t/function-to-fill-fespace/1297/1 "2021-11-04T16:04:52Z")
Hello all,
I would like to know what are the available methods to assign values to a `fespace` variable.
I am trying to solve a transient diffusion problem where the diffusivity depends upon the variable of interest. For this purpose I have code which looks as follows:
```cpp
func real diff(real X, real T) {
real Deff;
Deff = 2.4e-7 * ( 1 - exp(-7.9e14*(X^15.7)) + X^0.686 ) * exp(-3156/(T+273));
return Deff;
}
real u0 = 0.3, T = 40;
fespace Vh(Th,P1);
Vh u = u0, v, uold, diffu;
// ...
problem diffusion(u,v) =
int2d(Th) (u*v/dt + diffu*(dx(u)*dx(v) + dy(u)*dy(v)))
- int2d(Th) (uold*v/dt)
+ on(2,u=0.1);
// ...
for (int i = 0; i < nsteps; i++) {
uold = u;
// Evaluate diffusivity
for (int i = 0; i < Vh.ndof; i++) {
diffu[][i] = diff(uold[][i],T);
}
diffusion;
t += dt;
}
```
Is the preferred solution to use a macro like
```cpp
macro diff(X,T)
2.4e-7 * ( 1 - exp(-7.9e14*(X^15.7)) + X^0.686 ) * exp(-3156/(T+273)) // EOM
```
in place of my current loop calling the scalar function?
---
### Author:  [@prj](https://community.freefem.org/u/prj)
#### Post date: [November 4, 2021, 5:43pm UTC](https://community.freefem.org/t/function-to-fill-fespace/1297/2 "2021-11-04T17:43:55Z")
Using a `macro` instead of a `func` is a better, you are correct.
Also, this loop will be slow:
```auto
for (int i = 0; i < Vh.ndof; i++) {
diffu[][i] = diff(uold[][i],T);
}
```
Best to use:
```auto
for [i, v : diffu[]] {
v = diff(uold[][i],T);
}
```
You can lookup “Implicit loop” in the documentation.
---
### Author:  [@ivan](https://community.freefem.org/u/ivan)
#### Post date: [November 4, 2021, 6:17pm UTC](https://community.freefem.org/t/function-to-fill-fespace/1297/3 "2021-11-04T18:17:48Z")
Thanks for the tip about implicit loops. I haven’t seen these used very often in examples.
They appear to be kind of a hybrid between Python:
```python
for i, ai in enumerate(a):
...
```
and the access iterator in C++:
```cpp
for (auto &ai: a) {
...
}
```