l3fp: sign() and ?

[wspr]

From Andrew on LaTeX-L:

Two functions which I think could be beneficially added to l3fp without straining its role as a kernel module (in the way that, say, the gamma function would) are the signature function (sgn) and a unit step function (Heaviside? -- the difficulty is what value to assign at 0). Given the numerical values assigned to true and false in l3fp, it is easy to mimic these with simple inequalities (e.g. (#1 > 0) - (#1 < 0) for sgn) but perhaps the functions should be there in the first place?

I found myself wanting these when considering a table of fp values where it made sense to blank cells containing negative values which "got in the way". The code creating the table contained the option of blanking zero-value cells, so I multiplied the tabulated formula by 1 - (#1 < 0).

[wspr]

sign() or sgn() definitely makes sense to me, but I’m not sure about step() / heaviside() / whatever. Can you give another example for the use case there?

[blefloch]

I'm adding sign as soon as tests finish running. I'm not adding step/heaviside because it will be impossible to decide whether +0 and -0 should give 1 or 0. Also, there is already #1>0, or the more explicit #1>0?1:0.

[wspr]

Thanks Bruno, I agree.

[aparsloe]

The use case instanced was the one that prompted the suggestion. For that, the step function was more relevant, but the mathematician in me felt uncomfortable including a relation like (#1 < 0) in a formula, where it is an accident of implementation that it gives the right values. I had overlooked the ?: operation, but I see it could ease my discomfort.
Andrew

[blefloch]

@aparsloe I suppose I can close the issue, then.