Re: Plotting an ELU function with GNU Octave

On Thu, Feb 4, 2021 at 2:27 PM Alexandru Munteanu <
[email protected]> wrote:

> Hello,
>
> I am trying to create plots for an ELU (Exponential Liniear Unit)
> funciton. The function is defined as follows:
>
> function e = elu(z, alpha)
>   if z >= 0
>     e = z
>   else
>     e = alpha * ((exp(z)) - 1)
>   endif
> endfunction
>
> So this is pretty simple so far (alpha is a constant of values 0.01),
> and I can test to evaluate it in the interpreter and everything is
> working as expected.
>
>
> However, I want to plot the values of elu(x), where x belongs to the
> interval (-5, 5).
>
> function plot_elu
>   x = -5:0.01:5
>   y = elu(x, 0.01)
>
>   plot(x, y, "linewidth", 5)
>   set(gca,"fontsize", 25)
>   axis([-6, 6, -1.2, 1.2])
>   grid on;
> endfunction
>
> This is where the plot goes wrong: it creates the plot as if the values
> for elu are all going on the second branch of the if-then-else
> statement.
>
> I have attached what the plot looks like. Any help in this direction is
> appreciated.
>
> --
> Alexandru Munteanu
>
>
The if statement does not operate elementwise, which is the cause of your
problem. The expression

y = (x>=0).*x + (x<0).*alpha.*(exp(x)-1)

is essentially an elementwise if-else statement.

Try something more like this.

x = -5:0.01:5
alpha = 0.01
y = (x>=0).*x + (x<0).*alpha.*(exp(x)-1)
plot(x, y, "linewidth", 5)
set(gca,"fontsize", 25)
axis([-6, 6, -1.2, 1.2])
grid on

- Brett Green