How to compute equation only for x(y>0)

Hello,

I have time, t on the x axis and power, Pw on the y axis. I want to compute the following equations for only times, t where Pw is positive. How can I do that?

% calculating alphar dependent on t
for tidx = 2:length(t)-1
alphar = Cr(tidx).*Mv*g
end

% calculating three summation terms
x = 0;
for tidx = 2:length(t)-1
    x = x+(V(tidx)).^3;
end

y=0;
for tidx = 2:length(t)-1
    y = y+V(tidx);
end

z=0;
for tidx=2:length(t)-1
    z = z+(V(tidx)*(V(tidx+1)-V(tidx-1)));
end

% Want to calculate below equation only for t at Pw>0 (Pw calculation shows below)

Ew = (1369*((alphaa*x)+(alphar.*y)+(alphai*z)))/1000 % alphaa and alphai are constants. alphar is only one dependent on t (shown first line above)

Pw calculation shows below

for tidx=2:length(t)-1
    a = (V(tidx+1)-V(tidx-1))/(2*(t(tidx+1)-t(tidx-1)));
    A = ((1/2)*Rhoa*Cd*Af*(V(tidx).^3));
    G = (Cr(tidx).*Mv*g*cos(0).*V(tidx));
    I = (1.1*Mv.*a.*V(tidx));
    Pw(tidx-1) = (A+G+I)/1000;
end

[darova]

What to do with those t (Pw<=0)?

[JD]

Want to ignore t(pw<0). just want to use t(pw>=0)

Answer 1

[darova]

Take Ew where Pw>0

Ew = (1369*((alphaa*x)+(alphar.*y)+(alphai*z)))/1000 % alphaa and alphai are constants. alphar is only one dependent on t (shown first line above)
Ew1 = Ew(Pw>0);

[JD]

When I do that, I am getting the following error message

"The logical indices contain a true value oustide of the array bounds"

so then I tried:

E1=Ew(Pw(tidx-1)>0)

But that gives me an empty [ ]

[darova]

My bed, sizes are different

Ew = (1369*((alphaa*x)+(alphar.*y)+(alphai*z)))/1000 % alphaa and alphai are constants. alphar is only one dependent on t (shown first line above)
Ew_temp = Ew(2:end-1);
Ew1 = Ew_temp(Pw>0)

[JD]

That still gives me the same error message

"The logical indices contain a true value oustide of the array bounds"

[darova]

x is only of size [1x1]

x = 0;
for tidx = 2:length(t)-1
    x = x+(V(tidx)).^3;
end

Then you use it here

Ew = (1369*((alphaa*x)+(alphar.*y)+(alphai*z)))/1000 % alphaa and alphai are constants. alphar is only one dependent on t (shown first line above)

Ew is [1,1] but Pw is [ length(t)-2, 1 ]

Maybe you mean

[x,y,z,alphar] = deal( zeros(1,length(t)-2) );    % preallocate
for tidx = 2:length(t)-1
    x(tidx-1) = (V(tidx)).^3;
    % y ...
    % z ...
    % alphar
end
Ew = (1369*((alphaa*x)+(alphar.*y)+(alphai*z)))/1000 % alphaa and alphai are constants. alphar is only one dependent on t (shown first line above)
% now Ew is of size [1,length(t)-2]
% Pw is [1, length(t-2)]

Finally

Ew1 = Ew(Pw>0);

[JD]

Hi Darova,

What you are saying makes sense and works. However, for x, y and z, i was calculating the way I have them because they are summation terms.

x is summation from n=1 to n=t of Vn^3

y is summation from n=1 to n=t of Vn

z is summation from n=1 to n=t of Vn(t+1)-Vn(t-1)

If I change x to the way you have it above, that isn't performing the summation?

[darova]

That isn't performing the summation

% Want to calculate below equation only for t at Pw>0 (Pw calculation shows below)

The question: do x,y and z depend on t? What is value of x at specific t?

[JD]

Yes, x, y and z all depend on t given in the summation equation above....