# plot() and fplot() give different results for the same function

18 views (last 30 days)
Tamara Macadam on 15 Feb 2024
Edited: Bruno Luong on 15 Feb 2024
I'm trying to plot the function
between 0 and 100 (not including zero of course), but am running into a problem. When I use
x = linspace(0, 100, 10000);
y = 2.*sqrt(1+x)+((sin(sqrt(x).*pi).*sin(sqrt(1+x).*pi).*(2.*x+1))/(sqrt(x))) - 2.*sqrt(1+x).*cos(sqrt(x).*pi).*cos(sqrt(1+x).*pi);
plot(x,y);
I don't get the result I expected, which is the graph occasionally intersecting y = 0. However, by doing
fplot(@(x) 2.*sqrt(1+x)+((sin(sqrt(x).*pi).*sin(sqrt(1+x).*pi).*(2.*x+1))/(sqrt(x))) - 2.*sqrt(1+x).*cos(sqrt(x).*pi).*cos(sqrt(1+x).*pi), [0 100]);
I do get the result I expected.
I can't just use fplot, as I do need to use the variable y for other things, so I'm not sure what to do or what is even happening here. Any help would be greatly appreciated.

Bruno Luong on 15 Feb 2024
Edited: Bruno Luong on 15 Feb 2024
Your coding is wrong: Replace "/" with "./"
Tamara Macadam on 15 Feb 2024
Oh god I'm silly. Thanks, I should've realised.

Torsten on 15 Feb 2024
Edited: Torsten on 15 Feb 2024
x = linspace(0, 100, 10000);
y = 2*sqrt(1+x)+sin(sqrt(pi)*x).*sin(sqrt(1+x)*pi).*(2*x+1)./sqrt(x) - 2*sqrt(1+x).*cos(sqrt(x)*pi).*cos(sqrt(1+x)*pi);
plot(x,y);
hold on
f = @(x)2*sqrt(1+x)+sin(sqrt(pi)*x).*sin(sqrt(1+x)*pi).*(2*x+1)./sqrt(x) - 2*sqrt(1+x).*cos(sqrt(x)*pi).*cos(sqrt(1+x)*pi);
fplot(f,[0 100])
Bruno Luong on 15 Feb 2024
Edited: Bruno Luong on 15 Feb 2024
I believe fplot ezplot replace "*" "/" "^" etc... with elementwise operators ".*" "./', ".^". It interprets as standard math notation.
This how charming or quirking MATLAB is, depending on your taste.

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