xLOGy

Generalized logarithm

Description

XLOGY - Logarithm

XLOGY(X,Y) returns the N for which X ^ N = Y.

XLOGY is defined as LOG(Y) elementswise divided by LOG(X).

N is defined as one for all elements where X equals one, and also for elements where X and Y are equal.

The log of X = 0 for any Y is defined as Inf.

• MATLAB release: MATLAB 6.5 (R13)

[urs (us) schwarz]

could the engine not simply read

ix=x~=1&y~=1&x~=y;
N=log(y(ix))./log(x(ix));

us

[urs (us) schwarz]

could the engine not simply read

ix=x~=1&y~=1&x~=y;
N=zeros(size(ix));
N(ix)=log(y(ix))./log(x(ix));

us

[Jasper Menger]

You're absolutely right. I've just submitted an update.

[John D'Errico]

The documentation and the help both suggest that xlogy(1,y) is defined as 1. This makes no sense, and it is also inconsistent with the code itself, since xlogy(1,y) actually returns 0 in this code.

Perhaps a better choice for xlogy(1,y) should be +/- inf., depending on the magnitude of y, with xlogy(1,1) == 0.