Modulo operator in brms

Interfaces brms
1

In exploring non-linear regression in brms I tried to fit a particular function to the data:

x <- seq(1,64)/64
prior1 <- prior(normal(1, 3), nlpar = "alpha") + prior(normal(1, 1.5), nlpar = "beta")
form = bf(y ~ alpha * (  (( beta*x + .5)  %% 1 )  - .5) , alpha + beta ~ 1, nl = TRUE)
fit1 <- brm(formula = form, data = Df, prior = prior1)

However, I get an error from the compiler about the modulo operator.
How to use the modulo operator in a brms formula?

33:     for (n in 1:N) {
34:       // compute non-linear predictor values
35:       mu[n] = nlp_alpha[n] * (((nlp_beta[n] * C_1[n] + 0.5) %  % 1) - 0.5);
                                                                                               ^
36:     }
2

We don’t have the modulo operator defined in Stan, but it is implemented as the fmod function. So you’ll just need to change your formula definition to use fmod(numerator, denominator) instead of numerator %% denominator:

form <- bf(y ~ alpha * (fmod(beta*x + 0.5, 1)  - 0.5),
          alpha + beta ~ 1,
          nl = TRUE)
3

thanks!