How to reference a variable saved as .mat file to execute code

Question

JD 2019-9-18

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/480854-how-to-reference-a-variable-saved-as-mat-file-to-execute-code

评论： Walter Roberson 2019-9-18

采纳的回答： Walter Roberson

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So I have a variable named cyc_mph that is a 1370x2 double. This data is stored as a .mat file.

The first column in the variable file gives me 't' (which goes from 0 to 1369) and the second column gives me 'Vmph'

I am trying to calculate 'a' by the formula given below. But what I want the code to do, is to get the value of 'Vmph' that corresponds to its 't+1' or 't-1' in the data file and give me 'a' for t=2:1:1369.

I do not think my code is gathering the correct 'Vmph' corresponding to the correct 't+1' or 't-1' in my code below. Do you know what I am doing wrong?

CODE:

t = cyc_mph(:,1);
Vmph = cyc_mph(:,2); 
for t=2:1:1369
    a = (Vmph(t+1)-Vmph(t-1))/(2*((t+1)-(t-1)));
    
A = ((1/2)*Rhoa*Cd*Af*(Vmph(t).^3));
G = (Mv*g*cos(0).*Vmph(t));
I = (1.1*Mv.*a.*Vmph(t));
Pw(t-1) = (A+G+I)/1000;
end

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Walter Roberson 2019-9-18

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t = cyc_mph(:,1);

That extracts t values from the array.

for t=2:1:1369

That overwrites that vector of t values with integer constants 2, 3, up to 1369, one at a time.

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Answer 1

Walter Roberson 2019-9-18

1
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/480854-how-to-reference-a-variable-saved-as-mat-file-to-execute-code#answer_392355

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t = cyc_mph(:,1);
Vmph = cyc_mph(:,2); 
for tidx=2:length(t)-1
    a = (Vmph(tidx+1)-Vmph(tidx-1))/(2*(t(tidx+1)-t(tidx-1)));
    A = ((1/2)*Rhoa*Cd*Af*(Vmph(tidx).^3));
    G = (Mv*g*cos(0).*Vmph(tidx));
    I = (1.1*Mv.*a.*Vmph(tidx));
    Pw(tidx-1) = (A+G+I)/1000;
end

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JD 2019-9-18

Thank you Walter! If you don't mind, can you please explain the logic behind your code so I can understand for future reference?

Walter Roberson 2019-9-18

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You had the statement

t = cyc_mph(:,1);

so in one place you are expecting t to refer to some input data . But right after you had

for t=2:1:1369

so your t now referring to datapoint numbers instead.

From there I traced the logic of the code and figured out which references to t were likely to be datapoint numbers and which ones were likely to need the input times, and then I changed the datapoint number version to variable named tidx and changed the input times to t(tidx) to refer to the current input data value.

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