Plot inside a function

Question

Oskar Mevik Päts 2019-11-12

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/490702-plot-inside-a-function

评论： Oskar Mevik Päts 2019-11-14

采纳的回答： Jess Lovering

在 MATLAB Online 中打开

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!

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Answer 1

Jess Lovering 2019-11-12

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/490702-plot-inside-a-function#answer_401169

在 MATLAB Online 中打开

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
    

2 个评论
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Adam Danz 2019-11-12

在 MATLAB Online 中打开

@ 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