# Requesting help for piecewise ploting

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Jonathan Hawkins on 24 Oct 2020 at 23:11
Commented: Star Strider on 26 Oct 2020 at 1:16
clc
clear
syms r I % I = current, r = radius
a = 2;
b = 5;
c = 7;
B = piecewise(r<=0 & r>=a,((r/a)^2)*I, r>=a & r<=b, I, r>=b & r<=c, (c^2-r^2)/(c^2-b^2), r>c, 0);
fplot(B)
ax = gca;
ax.XAxisLocation = 'origin';
ax.YAxisLocation = 'origin';
grid on
title('\color[rgb]{0,0.5,0.5} Magentic Flux Density Magnitude')
xlabel('\sl \color[rgb]{0,0.5,0.5} r Radial Distance (cm)')
ylabel('\sl \color[rgb]{0,0.5,0.5} H Magnetic field magnitude (A/cm)')
Above is a what I would like to plot, but I recieve errors plotting. Below are my errors, I would like some guidance on how to correct my errors.
piecewise(2 <= r & r <= 5, I, 5 <= r & r <= 7, sym(49/24) - r^2/24, 7 < r, 0)
Error using fplot>singleFplot (line 240)
Input must be a function or functions of a single variable.
Error in fplot>@(f)singleFplot(cax,{f},limits,extraOpts,args) (line 200)
hObj = cellfun(@(f) singleFplot(cax,{f},limits,extraOpts,args),fn{1},'UniformOutput',false);
Error in fplot>vectorizeFplot (line 200)
hObj = cellfun(@(f) singleFplot(cax,{f},limits,extraOpts,args),fn{1},'UniformOutput',false);
Error in fplot (line 166)
hObj = vectorizeFplot(cax,fn,limits,extraOpts,args);

Star Strider on 25 Oct 2020 at 1:27
The Symbolic Math Toolbox and here particularly the piecewise funciton may not be the best option for this problem.
Use ordinary MATLAB plotting fucntions and code the piecewise expression and plot it as demonstrated here (not your function):
f = @(x) (2-2*x).*(x <= 0) + (2+2*x).*(x>0);
t = linspace(-3, 3);
figure
plot(t, f(t))
grid
ylim([0 max(ylim)])
Your function has two variables, ‘r’ and ‘I’, so it would have to be coded as:
B = @(r,I) (r<=0 & r>=a) .*(((r/a)^2)*I) + (r>=a & r<=b).*I + (I).*((c^2-r^2)/(c^2-b^2)) + (r>c).*(0);
both of variables would be generated fro their appropriate vectors with with ndgrid or meshgrid., then plotted with a 2D or 3D plotting function, depending on the result you want.

Star Strider on 25 Oct 2020 at 23:55
Walter — Thank you!
Jonathan Hawkins on 26 Oct 2020 at 0:12
My apologies for the lack of a better explanation, I was trying to use r as my x-axis while trying to magnetic flux density for all regions while plotting the abs(B) vs. r. I chose arbitrary values of a=2, b=5, c=7. My result is B={(a(phi)*((u0*I*r)/2pi*a^2),0<=r<=a) ; (a(phi)*((u0*I*r)/2pi*r),a<r<b); (a(phi)*((u0*I*r)/2pi*r)*((c^2-r^2)/(c^2-b^2)),b<=r<=c); (0, r>c)}
Star Strider on 26 Oct 2020 at 1:16
Our pleasure!
.

R2020b

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