Anonymous functions and conditional logic? (one line step function)

Hi Folks,

It looks like you can't use conditional logic in an anonymous function -- which is too bad because I'm trying to do a step function as an anon function. I'd like to do it as a one liner, something like:

step = @(t) if t>t0 return a, elseif t<=t0 return b, end,

but MATLAB doesn't like that of course. In my case, a=10e-4 and b=10e-6, just in case anyone is wondering. Is there something similar that I can do? I've been trying to modify tanh(x) to go from a to b, but I can't quite get the syntax right. I'd like an anon fun because it would be an element in a structure array if I get this coded right.

Thanks for the help! Adam

Answer 1

[Fangjun Jiang]

It can be done in a twisted way. I am not sure how much value it provides. step is a built-in function so avoid using it.

h=@(t) a*(t>5)+b*(t<=5)

or with the complete parameters

h=@(t,t0,a,b) a*(t>t0)+b*(t<=t0)

[Adam Attarian]

Thanks -- this totally does the trick. And vectorization for free!

[Walter Roberson]

Warning: this trick does not work if the "unselected" calculation happens to be infinite. For example if you were to try

1/x * (x~=0) + 1 * (x==0)

then for x=0, aslthough the x~=0 would be false, it would be false multiplied by 1/0 which is 0 * infinity which is defined as NaN.

There is a safer version using subsref.

[hosein Javan]

this way you can define a piecewise anonymous function:

f=@(x) [x(x<1).^2 , x(x>=1)]

the function is x^2 for x<1 and x for x>=1. use fplot to check it out.

[Walter Roberson]

Interesting approach, hosein Javan, but for vector x, that potentially re-orders the outputs. For example let x = rand(1,20)*2; then your code would put all of the results for x in (0,1) into the left side and the results for [1,2) into the right side instead of having them in order of x.

[Piyush Kant]

What if the value to be returned is a string?

[Walter Roberson]

In the case of string objects I would suggest having a string array being indexed by (2 minus logical value). This would select the first string for true and the second for false