Help with dot operator

Question

Emma 2011-11-16

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/21387-help-with-dot-operator

评论： John D'Errico 2020-3-11

I keep getting the error code:

??? Error using ==> rdivide
Matrix dimensions must agree.
Error in ==> Design_Principles_3_MATLAB_Assignment at 41
b=Z./A

I thought that adding the dot operator would stop the error but it hasn't. Could someone please help? TThanks in advance!

My script is:

%Design_Principles_3_MATLAB_Assignment.m
%Script to find minimum peak force required from ram
%
%Emma Rhodes, 16/11/2001
%Variable Dictionary
clear all;
clc;
phi=0:5:180;
gamma=phi+20; 
theta=-20:1:80;
thetamin=-20; %Minimum Theta
thetamax=80; %Maximum Theta
m=4000; %Load 
mg=m*9.81; 
r=1.2:0.2:2.2; %Variable Ram Length (AB)
rmin=1.2; %Retracted Ram Length (AB)
rmax=2.2; %Extended Ram Length (AB)
L=4; %Boom Length (OC)
c2=((rmax^2)-(rmin^2))./((cos(gamma+thetamin))-(cos(gamma+thetamax)))
c1=(rmax^2)+(c2.*(cos(gamma+thetamax)))
C_minus=c1-c2 
X=C_minus(C_minus>0) 
C_plus=c1+c2 
Y=C_plus(C_plus>0)
Z=c2(c2>0)
a=(((Y).^(1/2))+((X).^(1/2)))/2
A=2.*a
b=Z./A

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Shanuka Jayasinghe 2020-3-9

编辑：Walter Roberson 2020-3-11

there's a table that outlines rules for the dot operator.

https://math.boisestate.edu/~calhoun/teaching/matlab-tutorials/lab_31/html/lab_31.html

John D'Errico 2020-3-11

By the way, your next anxious question will be why do I get the wrong answers?

cos and sin (along with the other trig functions) use radians, NOT degrees. However, your variables appear to be in degrees. S you will get what you consider to be strange results.

If you insist on the use of degrees, then you must alsu use sind and cosd. Or, you could convert the arguments to to radians from degrees. Take your pick.

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Answer 1

Walter Roberson 2011-11-16

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There is no "dot operator". "./" is a single operator whose name is two characters long, not a dot operator applied to the "/" operator.

When you use "./" then the size of the left side must be exactly the same as the size of the right side, unless the right side is a scalar.

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Emma 2011-11-16

Ok, thanks. How would I solve this then? Is there no way of finding b?

Walter Roberson 2011-11-16

I do not know what your code is intended to do, so I do not know whether there is any way of finding b.

Sometimes it is easier to do the calculation on the entire array, including locations it will not produce meaningful answers for, and then filter out the locations that would be meaningless.

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Answer 2

Thomas 2011-11-16

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https://ww2.mathworks.cn/matlabcentral/answers/21387-help-with-dot-operator#answer_28159

在 MATLAB Online 中打开

In your case Z=[1x19] and A=[1x23], both the matrices are of different dimensions and hence you get a dimension mismatch error..

More information on rdivide: http://www.mathworks.com/help/toolbox/fixedpoint/ref/rdivide.html

Also Emma, are you sure your equation are correct:

The code works correctly if:

Z=c1(c1>0)

Just check your equations again..

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Walter Roberson 2011-11-16

Note: that is the link for rdivide for the fixed point toolbox. The general reference is http://www.mathworks.com/help/techdoc/ref/ldivide.html which covers both ldivide and rdivide

Emma 2011-11-16

I am basically trying to find a and b using these equations and they are definitely right. If I don't use only the positive answers for c1-c2 and c1+c2 then I get complex numbers for a

c2=((rmax^2)-(rmin^2))./((cos(gamma+thetamin))-(cos(gamma+thetamax)));

c1=(rmax^2)+(c2.*(cos(gamma+thetamax)));

a=(((c1+c2).^(1/2))+((c1-c2).^(1/2)))./2

b=c2./(2.*a)

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Answer 3

Thomas 2011-11-16

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/21387-help-with-dot-operator#answer_28177

在 MATLAB Online 中打开

hmm,, this code works fine now:

%Design_Principles_3_MATLAB_Assignment.m
%Script to find minimum peak force required from ram
%
%Emma Rhodes, 16/11/2001
 %Variable Dictionary
 clear all;
 clc;
 phi=0:5:180;
 gamma=phi+20; 
 theta=-20:1:80;
 thetamin=-20; %Minimum Theta
 thetamax=80; %Maximum Theta
 m=4000; %Load 
 mg=m*9.81; 
 r=1.2:0.2:2.2; %Variable Ram Length (AB)
 rmin=1.2; %Retracted Ram Length (AB)
 rmax=2.2; %Extended Ram Length (AB)
 L=4; %Boom Length (OC)
 c2=((rmax^2)-(rmin^2))./((cos(gamma+thetamin))-(cos(gamma+thetamax)))
 c1=(rmax^2)+(c2.*(cos(gamma+thetamax)))
 % C_minus=c1-c2 
 % X=C_minus(C_minus>0) 
 % C_plus=c1+c2 
 % Y=C_plus(C_plus>0)
 % Z=c2(c2>0)
 a=(((c1+c2).^(1/2))+((c1-c2).^(1/2)))./2 
 A=2.*a
 b=c2./(2.*a)