Equation Error Argument to dynamic structure reference must evaluate to a valid field name

Question

Iplaysailing 2021-5-4

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/821745-equation-error-argument-to-dynamic-structure-reference-must-evaluate-to-a-valid-field-name

评论： Steven Lord 2021-5-5

  Ft.(kk)= log((k.*y.*r.^2).*(sqrt(((4*zetasq*(r.^2.)+1))./(((1-(r.^2).^2)+(4*zetasq*(r.^2.)))))));  %Force Transmissibility Equation
  

I have the following Error for this line of code:

Attached is the whole code:

  %Clear WorkSpace
clc;
clear ;close all
%Define Variables
v=[0:0.1:100]; %Velocity Range
y = 0.35; %Y value
Amplitude = 0.35; %Amplitude in ft
Peroid = 12; %Peroid in ft
Weight = 155.42; %Mass in Slugs orignal 5000lbf
c = 40; %c in Lbs-s/ft
k = 165/12; %k in lb/ft
Wn = sqrt(k/Weight); %Model Variable
W = (((2*pi)/12).*v); %Model Variable
r = W/Wn; %Model Variable 
zeta = (c/(2*(sqrt(Weight*k)))); %Model Variable
zetasq = zeta^2; %Model Variable Reduced
%Solve Equation
X = y*(sqrt(((4*zetasq*(r.^2.)+1))./(((1-(r.^2).^2)+(4*zetasq*(r.^2.)))))); % Displacement Transmissibilty Equation 
%Plot Equation 
figure(1)
plot(v,X)
%Make Look Pretty
grid on;
xlabel('velocity (Mph)');
ylabel('amplitude of X (ft)');
title('Amplitude of X vs velocity');
%Prep Second Plot
figure(2)
kk=1;
for v=[0.1:0.1:100] % velocity Range
    
%Solve Second Equation (Increments of 1) 
    Ft.(kk)= log((k.*y.*r.^2).*(sqrt(((4*zetasq*(r.^2.)+1))./(((1-(r.^2).^2)+(4*zetasq*(r.^2.)))))));  %Force Transmissibility Equation 
kk=kk+1;
end
v=[0.1:0.1:100]; %Velocity Range
plot(v,Ft)
%Make look Pretty
grid on;
xlabel('velocity (Mph)');
ylabel('amplitude of Ft(lb), log scale');
title('Amplitude of transmitted force Ft vs velocity');

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Answer 1

Steven Lord 2021-5-5

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/821745-equation-error-argument-to-dynamic-structure-reference-must-evaluate-to-a-valid-field-name#answer_692170

在 MATLAB Online 中打开

The number 1 is not a valid struct array field name.

S = struct('a', 1, 'b', 2)
S = struct with fields:
    a: 1
    b: 2
S.('c') = 42 % works
S = struct with fields:
    a: 1
    b: 2
    c: 42
S.(1) = -99 % does not work
Argument to dynamic structure reference must evaluate to a valid field name.

Immediately before this section of your code:

%Solve Second Equation (Increments of 1) 
    Ft.(kk)=

you define kk to be 1. You probably just want to get rid of the . there, use Ft(kk) = instead.

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Iplaysailing 2021-5-5

在 MATLAB Online 中打开

now i receive this error

Unable to perform assignment because the indices on the left side are not compatible with the size of the right side.
Error in VibrationsFinal (line 48)
    Ft(kk)= log((k.*y.*r.^2).*(sqrt(((4*zetasq.*(r.^2.)+1))./(((1-(r.^2).^2)+(4*zetasq.*(r.^2.)))))));  %#ok<SAGROW> %Force Transmissibility Equation

Steven Lord 2021-5-5

Okay, so you're trying to store multiple elements from the right hand side into one element on the left hand side. That wouldn't work if you were trying to put multiple eggs in one cup of an egg carton (without scrambling) and it won't work here.

So you need to use a cell array or write your code so it writes to a block of elements on the left hand side that is the same size as the block of elements returned by the right hand side. For the former search the documentation for "cell array" and for the latter don't just use kk alone, write up an index expression that's big enough to hold what's on the right.

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