How to translate this function into matlab?

Hello everyone, I am trying to translate this R function to compute the pseudo-R-squared in MATLAB. I did

rho <- function(u,tau=.5)u*(tau - (u < 0))

V <- sum(rho(f$resid, f$tau))

I did the following, but I get the error: "Array indices must be positive integers or logical values."

function rho = TickFunction(u,tau)
%Tick function to evaluate a quantile regression model performance
%u is the residuals of the fitted model
%tau is the quantile of interest

rho = u(tau - (u<0));

end 

Anyone could help?

Thanks!

[Torsten]

Seems the function is

u.*(tau-(u<0))

[Sara Boni]

Ok, great, thank you!