the variable appears to change size every loop iteration

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Nisreen Aljumaili
Nisreen Aljumaili 2021-9-30
回答： Image Analyst 2021-9-30
%(a)Write the following script to plot the magnetization curve.
clc
clear
If = [0 0.5 1.0 1.5 2.0 2.5];
Ea = [0 75 150 205 242 270];
Ra = 0.14;
RI = 2;
la = 100;
Ifield = linspace(0,2.5,100);
for n = 1:100
Ea_curve(n) = spline(If,Ea,lfield(n));
end
plot(Ifield, Ea_curve)
title ('Field current vs. Generated EMF')
xlabel('Field current [A]')
ylabel('Generated EMF [V]')

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回答（2 个）

Alan Stevens
Alan Stevens 2021-9-30
Ran in:
You don't need the loop:
If = [0 0.5 1.0 1.5 2.0 2.5];
Ea = [0 75 150 205 242 270];
Ra = 0.14;
RI = 2;
la = 100;
Ifield = linspace(0,2.5,100);
Ea_curve = spline(If,Ea,Ifield);
plot(Ifield, Ea_curve)
title ('Field current vs. Generated EMF')
xlabel('Field current [A]')
ylabel('Generated EMF [V]')
  2 个评论
Nisreen Aljumaili
Nisreen Aljumaili 2021-9-30
thank you so much:)
I am sorry i tried the same way with this code and did not use the loop but it did not work
clc
clear
If = [0 0.5 1.0 1.5 2.0 2.5];
Ea = [0 75 150 205 242 270];
Ra = 0.14
RI = 2;
Ifield = linspace(0,2.5,100);
for n = 1:100
Ea_curve(n) = spline(lf, Ea, Ifield(n));
la(n) = Ea_curve(n)/2;
Vt(n) = Ea_curve(n)-la(n)*Ra;
Pload(n) = Ea_curve(n)*la(n);
end
plot(Ifield, Vt)
title ('Field current vs. Terminal voltage')
xlabel('Field current [A]')
ylabel('Terminal voltage [V]')
Alan Stevens
Alan Stevens 2021-9-30
Ran in:
If = [0 0.5 1.0 1.5 2.0 2.5];
Ea = [0 75 150 205 242 270];
Ra = 0.14;
RI = 2;
Ifield = linspace(0,2.5,100);
Ea_curve = spline(If, Ea, Ifield); % If not lf
la = Ea_curve/2;
Vt = Ea_curve-la*Ra;
Pload = Ea_curve.*la; % Notice the dot in .*
plot(Ifield, Vt)
title ('Field current vs. Terminal voltage')
xlabel('Field current [A]')
ylabel('Terminal voltage [V]')

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Image Analyst
Image Analyst 2021-9-30
You're getting the warning because of this line
Ea_curve(n) = spline(If,Ea,lfield(n));
So every time you want to stuff a new number into the nth location of Ea_curve, it has to reallocation a new chunk of memory as big as the entire array, not just one 8 byte chunk for the additional element. To avoid the error you can preallocate Ea_curve with zeros before the loop
Ea_curve(n) = zeros(1, 100);
for n = 1 : 100

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