double for loop errors

%part 8
m=1.24;
td=10;
P0=1;
w_n=0:50/80:50;
t=0:30/80:30;
for ii=1:length(t); 
    jj=1:length(w_n);
    Tn(jj)=(2*pi)./(w_n(jj));
    %tt(ii)=(td/Tn(ii));
    if 0<t(ii)<td/2;
        xt(ii)=2*(t(ii)/td)-2*(Tn(jj)/2*pi*td)*sin(t(ii)/Tn(jj));
    elseif td/2<t(ii)<td;
        xt(ii)=2*(1-(t(ii)/td)+(Tn(jj)/2*pi*td)*(2*sin((2*pi/Tn(jj)*(t(ii)-0.5*td)))-sin(2*pi*t(ii)/Tn(jj))));
    elseif t(ii)>td;
        xt(ii)=2*((Tn(jj)/2*pi*td)*(2*sin((2*pi/Tn(jj)))*(t(ii)-td))-sin((2*pi/Tn(jj))*(t(ii)-td))-sin(2*pi*t(ii)/Tn(jj)));
    end
end

Error using /
Matrix dimensions must agree.

tt=(td./Tn);
M=max(xt(jj));
%derivative
figure(1)
plot(tt,M)

I am trying to get the maximum values for different values of w_n but it either the figure that's incorrect or problem wih the double for loop

Answer 1

[Walter Roberson]

%part 8
m=1.24;
td=10;
P0=1;
w_n=0:50/80:50;
t=0:30/80:30;
for ii=1:length(t); 
    jj=1:length(w_n);
    Tn(jj)=(2*pi)./(w_n(jj));
    %tt(ii)=(td/Tn(ii));
    if 0<t(ii) & t(ii)<td/2;
        xt(ii)=2*(t(ii)/td)-2*(Tn(jj)/2*pi*td)*sin(t(ii)/Tn(jj));
    elseif td/2<t(ii) & t(ii)<td;
        xt(ii)=2*(1-(t(ii)/td)+(Tn(jj)/2*pi*td)*(2*sin((2*pi/Tn(jj)*(t(ii)-0.5*td)))-sin(2*pi*t(ii)/Tn(jj))));
    elseif t(ii)>td;
        xt(ii)=2*((Tn(jj)/2*pi*td)*(2*sin((2*pi/Tn(jj)))*(t(ii)-td))-sin((2*pi/Tn(jj))*(t(ii)-td))-sin(2*pi*t(ii)/Tn(jj)));
    elseif t(ii) == 0
        xt(ii) = nan;
    else
        fprintf('td is %.16g, t(ii) is %.16g\n', td, t(ii));
        error('missed a t case')
    end
end

Error using /
Matrix dimensions must agree.

tt=(td./Tn);
M=max(xt(jj));
%derivative
figure(1)
plot(tt,M)

That error is because you are calculating xt(ii) [which is a single value] in terms of the vector Tn(jj)

You have not coded a double for loop. You have a single for loop, and inside that for loop you start with a vector assignment statement. If you want a double for loop, you have to use the keyword for each time.

for ii = 1 : length(t)

for jj = 1 : length(w_n)

and remember that in statements such as

xt(ii)=2*(t(ii)/td)-2*(Tn(jj)/2*pi*td)*sin(t(ii)/Tn(jj))

that if ii is scalar, you are overwriting **all* of xt(ii) for each different jj value.

You either need to output 2 dimensional arrays, or else you need something that "summarizes" the calculations over the jj range, such as a sum() statement, or else as you work through each jj entry you need to add (or subtract or multiply or whatever) to modify the current xt(ii) value, such as

xt(ii) = xt(ii) + 2*(t(ii)/td)-2*(Tn(jj)/2*pi*td)*sin(t(ii)/Tn(jj))