Plot inside a function

Hi,

I want to plot Distance (x) vs. Velocity (y).

With my code

function y = fp(x);
d = 75;
a = 1 / 3;
    for i = 1: length(x);
        if x(i) <= 0;
            y(i) = 0;
        elseif x(i) > 0 & x(i) <= d;
            y(i) = a .* x(i);
        elseif x(i) > d;
            y(i) = a .* d;
        end       
end

I execute for example

x = [0:1:100];
plot(x,fp(x))     

and it works. But what I want is to just execute fp(x) and get the plot from the function,

but I get the error "Vectors must of same length" with the code

function y = fp(x);
d = 75;
a = 1 / 3;
    for i = 1: length(x);
        if x(i) <= 0;
            y(i) = 0;
        elseif x(i) > 0 & x(i) <= d;
            y(i) = a .* x(i);
        elseif x(i) > d;
            y(i) = a .* d;
        end
plot(x,y)
end
    

Whats the deal? Thanks!

Answer 1

[Jess Lovering]

In the code it looks like you have the plot within your for loop. You need an additional end before the plot line. Does that fix your issue?

function y = fp(x);
d = 75;
a = 1 / 3;
    for i = 1: length(x);
        if x(i) <= 0;
            y(i) = 0;
        elseif x(i) > 0 & x(i) <= d;
            y(i) = a .* x(i);
        elseif x(i) > d;
            y(i) = a .* d;
        end
    end
plot(x,y)
end
    

[Adam Danz]

@ Oskar Mevik Päts, note that these types of problems that Jess Lovering identified can often be avoided by using proper indentation.

To smart-indent your code, from the editor, select all of your code (ctrl + a) and then smart-indent (ctrl+i).

On another note, here's a cleaner and faster version of your function.

function y = jff(x)
d = 75;
a = 1 / 3;
y  = nan(size(x));            
y(x <= 0) = 0; 
y(x > 0 & x <= d) = a .* x(x > 0 & x <= d); 
y(x > d) = a .* d;
end

But if you insist on using the slower loop method, at least pre-allocate your loop variable and follow these other suggestions:

function y = fp(x) %remove ;
d = 75;
a = 1 / 3;
y = nan(size(x));     % pre-allocate your loop var!
    for i = 1: numel(x) %use numel instead of length() &  remove ;
        if x(i) <= 0 %remove ;
            y(i) = 0;
        elseif x(i) > 0 & x(i) <= d %remove ;
            y(i) = a .* x(i);
        elseif x(i) > d %remove ;
            y(i) = a .* d;
        end
    end
plot(x,y)
end

[Oskar Mevik Päts]

Muchos gracias!