Why this error?

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new user 00
new user 00 2018-7-12
回答： Dennis 2018-7-13
if true
% code
for j1 = 2:length(t)
q1(j1) = (rho2(j1) - rho2(j1-1))*v2(j1-1)./((rho1_final - rho2(j1))*dt);
v2(j1) = v2(j1-1) + (q1(j1)-q2)*dt;
v1(j1) = v1(j1-1) - q1(j1)*dt;
end
end
I ended up getting this error in the second line : In an assignment A(:) = B, the number of elements in A and B must be the same. What is the mistake?
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Dennis
Dennis 2018-7-12
It is way easier to answer your question if you provide the needed information. You are probably trying to assign a vector to q1(j1),v2(j1) or v1(j1). If dt is a scalar you should check if rho1_final is a scalar und if rho2() does return scalars.
In general it helps to provide the code with possible inputs, related functions (like rho2) and the complete error message - yes, knowing in which line an error occurs does sometimes help to solvee the problem.
new user 00
new user 00 2018-7-12
Okay. rho_final is a scalar. rho2() is a vector which has the same number of elements as q1.

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回答（2 个）

Image Analyst
Image Analyst 2018-7-12
Try
v2(j1) = v2(j1-1) + (q1(j1) - q2(j1)) .* dt;
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new user 00
new user 00 2018-7-12
You could have told me that the information provided in my question is insufficient instead of sending me links.
Image Analyst
Image Analyst 2018-7-12
But Dennis already did that "It is way easier to answer your question if you provide the needed information" so I didn't think I need to repeat that, but since you didn't provide it I thought I'd give a link that is more helpful than Dennis's request. It's a link that we often give that spells out all the various things that one might provide, lest one provide only part of the things that would help.
Anyway, since you didn't read the link or follow its helpful suggestions, we still don't have all your code and can't even get to the error you mention because of the missing variables, so we're left to guess at possible reasons instead of knowing what the problem is.
Still willing to help, if you let us.

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Dennis
Dennis 2018-7-13
If rho2() returns a vector your calculation will result in a vector aswell.
q1(j1,:) = (rho2(j1) - rho2(j1-1))*v2(j1-1)./((rho1_final - rho2(j1))*dt);
Please consider initializing q1 so its size doesn't change in every loop iteration.

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