Graphing .m file returning blank figure

Question

Andre Metzger 2019-9-18

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评论： Ankit 2019-9-19

I have a .m file that gives a euler approximation ,

function yout=euler1(t0,y0,tfinal,h,f)
y=y0;
yout=y;
for n=t0:0.01:tfinal-h;
   y=y+(h.*f(n,y));
end
yout=y;
end

I am trying to graph the approximated y values over t (attempt is below) , but running it, I always get a blank figure.

t=-3:0.01:4;
t0=-3;
y0=2.5;
tfinal=4;
h=0.01;
f=@(t,y)t.*(y-2).*(log(abs(y))-1);
figure;hold on
plot(t,euler1(t0,y0,tfinal,h,f))

Is there a way to make such a graph where the y values that are approximated by the .m file are plotted to their corresponding t values?

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Andre Metzger 2019-9-19

Adam, how would you save the value on wach iteration to get the vector?

Adam Danz 2019-9-19

在 MATLAB Online 中打开

I'm not sure what your eurler1 function is supposed to be doing. 'y' changes on each iteration and I'm not sure how that's supposed to produce your output vector.

This is generally how you save variables within a loop but it's very likely you need additional changes in that function.

function yout=euler1(. . .)
n = t0:0.01:tfinal-h;     
yout = zeros(size(n)); 
for i = 1:numel(n)
    yout(i)= . . .;
end
end

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Answer 1

Ankit 2019-9-19

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/481062-graphing-m-file-returning-blank-figure#answer_392552

编辑：Ankit 2019-9-19

在 MATLAB Online 中打开

Hello Andre,

There is problem in your code. You are not storing y values.

function yout=euler1(t0,y0,tfinal,h,f)
y=y0;
yout(1)=y;
i = 1;
for n=t0:0.01:tfinal-h
   y=y+(h.*f(n,y));
   yout(i+1)=y;
   i=i+1;
end
end

t=-3:0.01:4;
t0=-3;
y0=2.5;
tfinal=4;
h=0.01;
f=@(t,y)t.*(y-2).*(log(abs(y))-1);
figure;hold on
plot(t,euler1(t0,y0,tfinal,h,f))

Are you expecting similar curve?

Regards

Ankit

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Adam Danz 2019-9-19

在 MATLAB Online 中打开

Don't forget to pre-allocate the loop variables. The i=i+1 iteration is unnecessary in a for-loop.

function yout=euler1(t0,y0,tfinal,h,f)
y=y0;
n=t0:0.01:tfinal-h; 
yout = [y,zeros(size(n))]; 
for i = 1:numel(n)
    y=y+(h.*f(n(i),y));
    yout(i+1)=y;
end
end