Error Matrix dimensions must agree

Question

jack carter 2017-7-5

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评论： jack carter 2017-7-6

I am getting "Matrix dimensions must agree" error while running the following codings in MatLab. From the first two equations I am getting error because of exp(f.*phi) part. If I remove this part, the code works fine. But I need to add this in both equations. In the last equation I am getting error due to the factor_c part. Could someone help me correcting the mistakes? Thanks

function myfunction
n=0;Ef=40*10^6;df=0.014;rou=3*10^6;S=0:600;f=0.5;phi=0:0.01:pi/2;Le=2;Vf=0.02;
P_friction=(pi/2)*sqrt((1+n)*Ef*df^3*rou*S).*exp(f.*phi);
Pslip=pi*rou*Le*df*(1-S/Le)*exp(f.phi);
factor_a=(4*Vf/(pi*df^2));
factor_b=Pslip.*P_friction;
factor_c=p_phi*p_z;
sigmab=factor_a.*factor_b.*factor_c;
end

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Answer 1

Star Strider 2017-7-5

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First, you need an operator here, I assume multiplication:

Pslip=pi*rou*Le*df*(1-S/Le)*exp(f.*phi);
                                  ↑ ← INSERT ‘*’ HERE

What are ‘p_phi’ and ‘p_z’ here:

factor_c=p_phi*p_z;

I do not see where you have defined them, and your function does not have any arguments. A function file will not pick up variables from your workspace as an anonymous function would.

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jack carter 2017-7-5

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It is working fine with your codings. May I ask in case if we modify the value of p(z) while keeping the rest of the codings same,

z=0:Lf./(2*cos(phi));
p(z)=2*z/Lf;

Star Strider 2017-7-5

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If you want all the vectors to be the same lengths, you will have to define ‘z’ as:

z = bsxfun(@rdivide, [0:Lf]', 2*cos(phi));
p = 2*z/Lf;

This creates a (3x100) matrix for ‘p’.

You cannot use 0 as a subscript. In MATLAB, indexing begins with 1.

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Answer 2

KSSV 2017-7-5

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Your S and phi should be arrays with equal dimensions. In your code they are not same. You need to proceed like below.

N = 100 ;   % can be changed 
n=0;
Ef=40*10^6;
df=0.014;
rou=3*10^6;
S = linspace(0,600,N) ;% S=0:600;
f=0.5;
phi = linspace(0,pi/2,N) ;% phi=0:0.01:pi/2;
Le=2;
Vf=0.02;
P_friction=(pi/2)*sqrt((1+n)*Ef*df^3*rou*S).*exp(f*phi);
Pslip=pi*rou*Le*df*(1-S/Le).*exp(f*phi);