Exponential approximation for vector input

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I was double checking the behaviour of a sigmoid function used in my Simulink model and I noticed that I was getting incorrect approximations when I made the computation for a vector of values
vect = [-5.0000 -5.0000 -5.0000 1.0000 0.9000 0.8000 0.7000 -5.0000 -5.0000];
y_vect = 1/(1+exp(-2*(vect'-1)));
% Value calculated using the vector
y_vect(4)
ans = 0
% Value calculated alone
y_val = 1/(1+exp(-2*(vect(4)-1)))
y_val = 0.5000
This approximation in my case causes great confussion due to the magnitude of the quantity expected.
Is there any way to solve this?

Risposta accettata

Sulaymon Eshkabilov
Sulaymon Eshkabilov il 31 Gen 2023
You have overlooked one dot. Here is the corrected commands:
vect = [-5.0000 -5.0000 -5.0000 1.0000 0.9000 0.8000 0.7000 -5.0000 -5.0000];
y_vect = 1./(1+exp(-2*(vect-1)));
% Value calculated using the vector
y_vect(4)
ans = 0.5000
% Value calculated alone
y_val = 1/(1+exp(-2*(vect(4)-1)))
y_val = 0.5000
  1 Commento
Eduardo
Eduardo il 1 Feb 2023
Oh nice to know!
I wrongly thought the broadcasting would be done automatically since we just had a scalar in the numerator

Accedi per commentare.

Più risposte (1)

Voss
Voss il 31 Gen 2023
vect = [-5.0000 -5.0000 -5.0000 1.0000 0.9000 0.8000 0.7000 -5.0000 -5.0000];
Using / (matrix right division), as you have it now:
y_vect = 1/(1+exp(-2*(vect'-1)));
disp(y_vect)
1.0e-05 * 0.6144 0 0 0 0 0 0 0 0
Using ./ (element-wise right division):
y_vect = 1./(1+exp(-2*(vect'-1)));
disp(y_vect)
0.0000 0.0000 0.0000 0.5000 0.4502 0.4013 0.3543 0.0000 0.0000

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