Undefined function or variable 'v'.??

t=-5:50;
if t<=8
    v=10.*(t.^2)-(5.*t);
elseif (t>=8) & (t<=16)
    v=624-(5.*t);
elseif (t>=16) & (t<=26)
    v=(36.*t)+12.*(t-16).^2;
elseif (t>26)
    v=2136.*exp(-0.1.*(t-26));
elseif (t<0)
    v=0;
end
plot(t,v)

Answer 1

[Rik]

You made the classic mistake of thinking that this set of logical statements would be handled for each element of your vector. But Matlab doesn't work that way, unless you tell it with a for-loop.

The code below uses logical indexing to get the result.

t=-5:50;
v=zeros(size(t));

L=t<=8;
v(L)=10.*(t(L).^2)-(5.*t(L));
L=(t>=8) & (t<=16);
v(L)=624-(5.*t(L));
L=(t>=16) & (t<=26);
v(L)=(36.*t(L))+12.*(t(L)-16).^2;
L=t>26;
v(L)=2136.*exp(-0.1.*(t(L)-26));
L=t<0;
v(L)=0;

plot(t,v)

[Libby Goodes]

What does the second line do?

[Rik]

The second line pre-allocates the vector v by filling it with zeros. This way we can use the same logical vector for v as for t.

Have you tried reading the documentation for zeros and size?

[Rik]

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