??? In an assignment A(I) = B, the number of elements in B and I must be the same.

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Mariela Flores 2017-9-24

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https://ww2.mathworks.cn/matlabcentral/answers/358080-in-an-assignment-a-i-b-the-number-of-elements-in-b-and-i-must-be-the-same

评论： Image Analyst 2017-9-24

Hi, I don't get why my code is incorrect. I hope you guys can help please.

clc;
A=10;
m=1;
k=40;
c=10;
xi=c/(2*sqrt(k*m));
w=1;
wn=sqrt(k/m);
b=(1-(w/wn)^2);
d=(2*xi*w/wn)^2;
Z=A/(b^2+d)^1/2;
fi=atan((2*xi*w/wn)/(1-(w/wn)^2));
muestras=1000;
t0=0;
tf=15;
for i=1:muestras+1
t(i)=t0+(i-1)*(tf-t0)/muestras;
x(i)=Z*sin(w*t-fi);
end
plot(t,x,'g')
xlabel('t')
ylabel('x')

In the command window, it says:

???  In an assignment  A(I) = B, the number of elements in B and
 I must be the same.
Error in ==> AVER at 19
x(i)=Z*sin(w*t-fi);

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Answer 1

Image Analyst 2017-9-24

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https://ww2.mathworks.cn/matlabcentral/answers/358080-in-an-assignment-a-i-b-the-number-of-elements-in-b-and-i-must-be-the-same#answer_282873

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The FAQ explains it completely: http://matlab.wikia.com/wiki/FAQ#How_do_I_fix_the_error_.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22.3F

But in short, since t is a vector of lots of values, then so is the result of sin(), and this means that you are trying to put a bunch of values into a single element of x, and you can't do that.

Try using t(i), which is only one single value, instead of the whole t array, which has "i" elements in it:

x(i) = Z * sin(w * t(i) - fi);

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Mariela Flores 2017-9-24

编辑：Mariela Flores 2017-9-24

I really appreciate your help, thank you so much, that was the error in my code. If now, the program says that there is an error in plot, will you think it is because I do not including the (i) with the variables?

Image Analyst 2017-9-24

在 MATLAB Online 中打开

The code with my fix works perfectly fine and runs with no errors:

clc;
A=10;
m=1;
k=40;
c=10;
xi=c/(2*sqrt(k*m));
w=1;
wn=sqrt(k/m);
b=(1-(w/wn)^2);
d=(2*xi*w/wn)^2;
Z=A/(b^2+d)^1/2;
fi=atan((2*xi*w/wn)/(1-(w/wn)^2));
muestras=1000;
t0=0;
tf=15;
for i=1:muestras+1
  t(i)=t0+(i-1)*(tf-t0)/muestras;
  x(i)=Z*sin(w*t(i)-fi);
end
plot(t,x,'g')
xlabel('t')
ylabel('x')