Matrix calculations in R--erroneous?
On Fri, 7 Oct 2005, Peter Muhlberger wrote:
Does anyone know how -log(x) can equal 743 but -log(x+0)=Inf? That's what the following stream of calculations suggest: Browse[2]> -log ( 1e-323+yMat2 - yMat1 * logitShape(matrix(parsList$Xs, nrow = numXs, ncol=numOfCurves), matrix(means, nrow = numXs, ncol=numOfCurves, byrow=TRUE), matrix(sigmas, nrow = numXs, ncol=numOfCurves, byrow=TRUE)) )[5,9] [1] Inf Yet: Browse[2]> logitShape(matrix(parsList$Xs, nrow = numXs, ncol=numOfCurves), matrix(means, nrow = numXs, ncol=numOfCurves, byrow=TRUE), matrix(sigmas, nrow = numXs, ncol=numOfCurves, byrow=TRUE))[5,9] [1] 1 So, the logitShape component equals 1.
to within 2e-16
Browse[2]> yMat1[5,9] [1] 1 So yMat1[5,9]*logitShape()[5,9]=1
to within 2e-16
Browse[2]> yMat2[5,9] [1] 1
to within 2e-16
So, yMat2[5,9]-yMat1[5,9]*logitShape()[5,9]=0
to within a few parts in 10^16 You haven't actually shown us yMat2[5,9]-yMat1[5,9]*logitShape()[5,9], though
Browse[2]> -log ( 1e-323) [1] 743.7469 So, -log( 1e-323)=743 while -log( 1e-323+0)=Inf ?
If "0" is really of the order of 1e-16 then this isn't surprising. If the only point of 1e-323 is as a guard value for 0 then use max(1e-323, yMat2[5,9]-yMat1[5,9]*logitShape()[5,9]) -thomas