how we can define piecewise function in matlab?

in maple we can use comamnd:(

f:= piecewise(0 <= t and t <= T1, f1, T1 < t and t < T2, 0, T2 <=t and t <= T3, f1, T3 < t and t < T4, 0)

but in matlab i can not define this function.

please give me som information ...

thanks

Answer 1

[Sally Al Khamees]

If you have R2016b and the Symbolic Math Toolbox installed, you can just use the piecewise function:

https://www.mathworks.com/help/symbolic/piecewise.html

Answer 2

[Walter Roberson]

You cannot define that as a function in MATLAB. MATLAB functions need to have a defined value for the case where none of the conditions hold. If you were absolutely sure that one of the conditions will hold you would rewrite the Maple function without the final condition, as

piecewise(0 <= t and t <= T1, f1, T1 < t and t < T2, 0, T2 <=t and t <= T3, f1, 0)

This would be

function y = f(t, T1, T2, T3, f1)
  y = zeros(size(t));
  idx1 = 0 <= t & t <= T1;
  y(idx1) = f1;
  idx2 = T1 < t & t < T2;
  y(idx2) = 0;
  idx3 = T2 <= t & t <= T3;
  y(idx3) = f1;
  y(~(idx1 | idx2 | idx3)) = 0;
end

[ebi]

hello dear walter... thanks for your answer , but i think i couldn't express clearly. i want define a piecewise function like as picture below that t is variable and we have f(t)= picture below and i want to plot it and do fourier illustration of it and use fft function on it ant plot them.

[Walter Roberson]

You can only do fourier transforms symbolically, using the symbolic toolbox. For numeric values, you need to use fft (fast fourier transform) or the like.

If you want to use the symbolic toolbox to create the MuPAD equivalent of the Maple function, you need to change the syntax very slightly:

f_generic = sym('piecewise((0 <= t and t <= T1, f1), (T1 < t and t < T2, 0), (T2 <=t and t <= T3, f1), (T3 < t and t < T4, 0))');

Note: possibly the inner () should be [] instead.

Before using you would want to

f = subs(f_generic, {'T1', 'T2',, 'T3', 'f1'}, {0.02, 0.287, 0.307, 0.335});

Note that the result would be a symbolic formula, not a function. To get the function version, use

    f_generic = sym('t -> piecewise((0 <= t and t <= T1, f1), (T1 < t and t < T2, 0), (T2 <=t and t <= T3, f1), (T3 < t and t < T4, 0))');

[ebi]

thanks a lot i will try it ...

[Walter Roberson]

Note: my later experiments showed that using sym('t->...') cannot possibly create a symbolic function.

Answer 3

[sixwwwwww]

Dear ebi, You need something like this if I understood correctly:

syms f f1
t = input('Enter value of t: ');
% Assuming values of T1, T2 and T3 as follows
T1 = 10;
T2 = 20;
T3 = 30;
if ((t >= 0 && t <= T1)  || (t >= T2 && t <= T3))
    f = f1; % here you can define f1 as you wish
else
    f = 0;
end

Is it ok?

[ebi]

hello dear sixww... thanks for your answer , but i think i couldn't express clearly. i want define a piecewise function like as picture below that t is variable and we have f(t)= piecewise function and i want to plot it and do fourier illustration of this function and use fft function on it ant plot them.

[sixwwwwww]

Do you input 't' as a time vector? and get f(t) for which you can calculate fft? Is it right?

[ebi]

i plot f(t) from [0..0.336]s and for that time period i have to calculate the integral of (f(t).exp(-i.w.t)dt)=F(w) and then plot F - w .

Answer 4

[shashi raj]

https://in.mathworks.com/matlabcentral/answers/90014-how-we-can-define-piecewise-function-in-matlab#comment_430686