## The Run Function Is Not A Loop

In Meep, there are functions run-until and similar that run the simulation, and take arguments allowing custom actions to be performed on every time step, or on some subset of the time steps. Many users misunderstand this, however, and make the same mistake: they think the run function is a "looping" construct of some kind, and that you can just put any code you want into it and it will get executed for every time step. This mistake and how to correct it are described in this article.

## Hello World

Let's consider a "Hello World" example. Suppose we start with a control file that runs for 200 time units and outputs $E_z$ on each time step:

(run-until 200 output-efield-z)


and now we want to modify it to also print "Hello World!" for every time step, as it is running.

## The Wrong Way

Many users will naively write:

(run-until 200 output-efield-z (print "Hello World!\n"))


This is wrong. It will output "Hello World!" once, then give an error. What is going on? The problem is that you are thinking of run-until in the wrong way, as if it were a loop:

for time < 200 do
output-efield-z
(print "Hello World!\n")


This is not what is happening. Instead, run-until is just a function that runs the simulation, and its arguments should be functions that are called for each time step. That is, it is really doing something like:

evaluate the arguments: 200: a number
output-efield-z: a function
(print "Hello World!\n"): prints output and returns #<unspecified>;
call run-until:
time-step until t=200
at each time step, call the arguments:
call (output-efield-z)
call (#<unspecified>)


Two things went wrong. First, the arguments are evaluated before calling the function, which means that the print statement is executed before run-until even starts! Second, run-until then tries to call the result of (print ...) as if it were a function, which causes an error because (print ...) does not return a function. The print returns a special Scheme code #<unspecified> that means it doesn't really return anything at all.

## The Right Way

What we should have passed to run-until, instead of the result of calling (print ...), is a function that calls (print ...). There are two ways to do it.

First, we could explicitly define a function, call it my-hello, that does what we want:

(define (my-hello) (print "Hello World!\n"))
(run-until 200 output-efield-z my-hello)


Notice two things. First, my-hello is a function of no arguments, which means that it is just called at every time step. Another, more complicated, possibility is described in the User Interface. Second, when we call run-until, we just pass the name of the function my-hello, and not the result of calling the function (my-hello).

A second possibility is that we could use Scheme's lambda construct to define our function in-line. The lambda syntax in Scheme allows you to define anonymous functions without assigning them a name via define, and to stick the function definition right into another expression. It works like this:

(run-until 200 output-efield-z (lambda () (print "Hello World!\n")))


Here, the (lambda () ...) defines a function of no arguments () that, when called, executes the ... statements.