Producing a NaN only where there is a NaN, zero otherwise

This interesting question came up for me today. I was looking for a simple expression that can be used in a function handle, one that produces a NaN only for elements that are already NaN, but I want it to return 0 for any other element, including +/- inf.

X = [-inf, -1, -eps, 0, realmin, 2, 1+i, pi, flintmax, realmax, inf, NaN];

That should qualify as a good list of the possible numbers in MATLAB that might be of interest.

I want my function to generate a result as the vector: [0 0 0 0 0 0 0 0 0 0 0 NaN].

For example, this fails:

broken1 = @(X) 0*X;
broken1(X)

ans = 1×12

NaN 0 0 0 0 0 0 0 0 0 NaN NaN

Though it is close. It suffers because 0*inf generates a NaN. An as pretty one is:

broken2 = @(X) X - X;
broken2(X)

ans = 1×12

NaN 0 0 0 0 0 0 0 0 0 NaN NaN

It fails for a similar reason, because inf-inf produces NaN.

A valid solution should work for arrays of any shape or size of course. Yes, it is trivial to write if I do it in an m-file. Thus...

nanZ(X)

ans = 1×12

0 0 0 0 0 0 0 0 0 0 0 NaN

function res = nanZ(X)
  % returns a NaN ONLY for elements of X that are NaN. All other elements will generate zero.
  res = zeros(size(X));
  res(isnan(X)) = NaN;
end

As I said, trivial if I use an m-flle. More difficult if I wish to use a function handle. It may be a blind spot on my part. But as a puzzle, can you write a simple, robust one line expression to produce my desired result, expressed as a function handle? Vectorized, of course.

And yes, I'll admit this question has essentially zero value, since a valid solution exists in an m-file form. Have fun! (I'll post a spoiler as an answer if people cannot find a better solution than the one I found. My final solution required only 9 characters, but it was definitely non-obvious.)

[the cyclist]

Reminds me of the old MATLAB Golf contests!

[John D'Errico]

Yes indeed.

Answer 1

[Stephen23]

X = [-inf, -1, -eps, 0, realmin, 2, 1+i, pi, flintmax, realmax, inf, NaN];
Y = 0./(X==X)

Y = 1×12

0 0 0 0 0 0 0 0 0 0 0 NaN

[the cyclist]

elegant

[Stephen23]

@the cyclist: thank you. I thought there must be a reason why @John D'Errico used NaN... nothing is by chance!

[the cyclist]

0./eq(X,X)

is a nice variant of this, but one additional character

[John D'Errico]

NIce. And it ties my solution of 9 characters. It will probably be more efficient than mine too.

Answer 2

[Gustavo Lunardon]

Added as an answer rather than a comment now

X = [-inf, -1, -eps, 0, realmin, 2, 1+i, pi, flintmax, realmax, inf, NaN];
broken1 = @(X) 0./X.^0;
broken1(X)

Also works, 7 characters

X = [-inf, -1, -eps, 0, realmin, 2, 1+i, pi, flintmax, realmax, inf, NaN];
broken1 = @(X) 0*X.^0;
broken1(X)

Works as well with 6 characters

Advantage is that you can put any number different than zero in the other elements by changing the multiplier

[the cyclist]

Very nice!

It's a darn shame that

not(NaN)

does not yield NaN, because "NaN cannot be converted to logical". If it did, then

~X.^0

would be a sublime 5-character solution.

Answer 3

[the cyclist]

First attempt:

X = [-inf, -1, -eps, 0, realmin, 2, 1+i, pi, flintmax, realmax, inf, NaN];
broken = @(x) 1./((isnan(x)-1)/0); % 19 characters
broken(X)

ans = 1×12

0 0 0 0 0 0 0 0 0 0 0 NaN

Answer 4

[the cyclist]

Second attempt ...

X = [-inf, -1, -eps, 0, realmin, 2, 1+i, pi, flintmax, realmax, inf, NaN];
broken = @(x) 0./(isnan(x)-1); % 15 characters
broken(X)

ans = 1×12

0 0 0 0 0 0 0 0 0 0 0 NaN

[John D'Errico]

Also nice.

[the cyclist]

I overlooked the obvious improvement to this one:

broken = @(x) 0./~isnan(x); % 12 characters

I like the fact that this one avoids the annoying denominator parentheses

Answer 5

[Jan]

X = [-inf, -1, -eps, 0, realmin, 2, 1+i, pi, flintmax, realmax, inf, NaN];
V = [0, NaN];
F = @(X) V(isnan(X) + 1);
F(X)

Answer 6

[John D'Errico]

I'm always amazed at how many ways there are to accomplish anything in MATLAB. Here was my effort. It took me some time to think of my solution - why I posted the question. I had to find a call that would return a real number for any inputs, including inf and -inf. But I still needed it to return NaN for NaN input. Then i could just multiply by 0. atan fit perfectly. And since 0*NaN is still NaN, this was my solution:

X = [-inf, -1, -eps, 0, realmin, 2, 1+i, pi, flintmax, realmax, inf, NaN];

unbroken = @(X) 0*atan(X);
unbroken(X)

ans = 1×12

0 0 0 0 0 0 0 0 0 0 0 NaN

Only 9 characters. But I am sure there is a better solution yet. And mine might not be terribly computationally efficient.

(Sadly, things like 0*sin(X) fails, because sin(inf) is NaN. But that would have saved a character.)

[Gustavo Lunardon]

X = [-inf, -1, -eps, 0, realmin, 2, 1+i, pi, flintmax, realmax, inf, NaN];
broken1 = @(X) 0./X.^0;
broken1(X)

Also works, 7 characters

X = [-inf, -1, -eps, 0, realmin, 2, 1+i, pi, flintmax, realmax, inf, NaN];
broken1 = @(X) 0*X.^0;
broken1(X)

Works as well with 6 characters

Advantage is that you can put any number different than zero in the other elements by changing the multiplier

[Stephen23]

@Gustavo Lunardon: very neat. Please add this as an answer!