# Operation on files 

This example use [comodo-calculator](../../data-model-processing/comodo-calculator.md) command is order to realized some operations on files. Keep in mind that this method realized the the described operation on the entire field uniformaly, optimized with parallel computation.

## Basic operations

### Add, Substract, Multiply, Divide

The most simple operators can be used to raised the topography: 

```bash
rep=/ecola/share/tmp/training/bathymetry/
comodo-calculator -i $rep/etopo-37_IBI.nc =lon =lat z=topography \
                  -f "z_raised=z+50" \
                  -o etopo-37_IBI-raised.nc
```

or invert it:

```bash
rep=/ecola/share/tmp/training/bathymetry/
comodo-calculator -i $rep/etopo-37_IBI.nc =lon =lat z=topography \
                  -f "z_inv=-z" \
                  -o etopo-37_IBI-inverted.nc
```

## Functions

Some functions are already coded and can be directly used, as some exemple are presented below ([list of available functions](#function)). 

### Norm and phase for complex variables
HRET is a internal tides atlas that provide the complex coefficient for several tidal waves. As the coefficient are given in Real and Imaginary part, you can convert to Amplitude and Phase lag: 

```bash
rep=/ecola/share/tmp/training/HRET/
comodo-calculator -i $rep/HRET_v8.1.nc =longitude =latitude =M2re =M2im \
                  -f "M2a=abs(M2re+1j*M2im); M2g=-r2d*arg(M2re+1j*M2im); M2g?atted({units},{degrees})" \
                  -o M2-ag.nc
ncatted -a "units,ITg,o,c,deg" tmp/M2-ag.nc
```

The use of "?atted" can force the value of the attributes in the saved files. Here, the attribute "units" is set to "degrees" for the phase lag variable "M2g".

### Norm for velocity vector


```bash
rep=/ecola/share/tmp/training/FES2014/
comodo-calculator -i $rep/M2.FES2014.nc =longitude =latitude =Ua =Va -f "velocity_norm=ellipseM(Ua,Va)" -o M2-velocity.nc
```