# CSV output (1D FDTD)

**CSV output** records the grid state in a text file for plotting or later continuation. `Fdtd_EzSimWriteCSV(sim, path)` writes the current state; the command-line driver calls it after completing the requested run.

Comment lines beginning with `#` store the grid width, spatial step, Courant number, and current step counter. Each following row describes one cell, with these columns:

```text
k,Hy,Ez,mu_r,eps_r,sigma_m,sigma_e,ch1,ch2,ce1,ce2
```

The file includes both field components, relative permeability and permittivity, electric and magnetic conductivities, and the four update coefficients. Material values are written relative to the vacuum constants where indicated by the `_r` suffix.

A path of `-` sends the CSV to standard output. A regular path produces a file that the supplied plotting script can read:

```bash
$ dist/bin/tools/plot_sim.py simulation.csv
```

The output describes the grid at the end of a run, rather than a history of every time step. The [[fdtd-openmp-restoring-a-simulation|CSV loader]] reads fields and material back and recomputes coefficients. Source registrations and the selected execution mode are not stored in the file.