R/normq.R

In TruncatedNormal: Truncated Multivariate Normal and Student Distributions

normq <-
  function(p,l,u){
    # computes with precision the quantile function
    # of the standard normal distribution,
    # truncated to the interval [l,u];
    # normq assumes 0<l<u and 0<p<1
    x=rep(NaN,length(l));
    I=(l>1e5); # if x is too big, no need for Newton iteration
    if (any(I)){
      x[I]=sqrt(l[I]^2-2*log(1+p[I]*expm1(l[I]^2/2-u[I]^2/2)))
    }
    I=!I # otherwise refine guess via Newton iterations
    if (any(I)){
      x[I]=newton(p[I],l[I],u[I])
    }
    return(x)
  }