if statement for piecewise function

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Rana Önem
Rana Önem on 23 Nov 2021
Answered: Walter Roberson on 23 Nov 2021
I have a piecewise function like :
p(t) = ( 0.30*beta 0<t<10 s
-0.30*beta 10<t<20 s
0 t<20 s )
i write this on matlab like this:
if t>0,t<10;
p(t)=0.30*beta;
elseif t>10,t<20;
p(t)=-0.30*beta;
else
p(t)=0;
end
and i got an error like this " Not enough input arguments." so what am i doing wrong? can you help me?

Answers (2)

VBBV
VBBV on 23 Nov 2021
syms t
beta = 10;
p = piecewise(0<t<10,0.3*beta,10<t<20,-0.3*beta,0)
p = 
fplot(p,[0 20],'linewidth',2)
axis([0 20 -5 5])

Walter Roberson
Walter Roberson on 23 Nov 2021
p = zeros(size(t));
for K = 1 : numel(t)
if t(K)>0 && t(K)<10
p(K)=0.30*beta;
elseif t(K)>10 && t(K)<20
p(K)=-0.30*beta;
else
p(K)=0;
end
end
Notice that if you are going to use if statements and t is a vector, then you need to loop over all of the entries in the vector.
However, in many cases, it is better to use logical indexing instead of looping.
p = zeros(size(t));
mask = t > 0 & t < 10;
p(mask) = 0.30*beta;
mask = t > 10 & t < 20;
p(mask) = -0.30*beta;
No need for an else because all elements were initialized to 0 anyhow.
Caution: when t = 10 exactly your code falls through to the else and assigns 0 there.
Caution: your code does not implement the same as the function definition. The function definition does not define any value for complex-valued t or for t >= 20, but your code would assign 0 to those locations. When a function definition does not define a result for a case, the code should either error or generate NaN if that situation is encountered.

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