Output argument "XXX" (and maybe others) not assigned during call to "function".

Not sure what this means. Code here:

global T d t v
T = 10;
d = 0.3;
[t,v]= ode23(@pwm,[0:1e-7:0.1], [0 0], options);

v = pwm(t,T,d);

figure,plot(t,v); %%Plot the datas

%Below is the function declaration, in a different file in the same folder.

function v = pwm(t,T,d)
    if T <= t <d*T
        v = 1;
    end
    if d*T <= t <T
        v = 0;
    end
end

Answer 1

[Star Strider]

First — Please do not use global variables! They create more problems than they aolve, and can make code very difficult to debug. Instead pass them as extra parameters as decribed in Passing Extra Parameters.

Second — If neither of the if conditions are satisfied, ‘v’ may not be assigned. One way to deal with that is to assign it as NaN in the beginning, then over-write that assignment if at least one of the if blocks are satisfied:

function v = pwm(t,T,d)
    v = NaN;
    if T <= t <d*T
        v = 1;
    end
    if d*T <= t <T
        v = 0;
    end
end

[Steven Lord]

Third -- the idiom "T <= t < d*T" doesn't do what you think it does. It is equivalent to "(T <= t) < d*T". Since (T <= t) can only take on the values false (0) or true (1), if d*T is greater than 1 the result of that expression will always be true.

Code Analyzer in the MATLAB Editor should have underlined that code in orange, suggesting that you break that up into two pieces:

if (T <= t) & (t < d*T)

Note that this is different from what your original version does:

if (T <= t) < d*T