MATLAB: In an assignment A(I) = B, the number of elements in B and I must be the same.

I need to compute runoff for 2 rainfall conditions
1. If rainfall > 5, then runoff = 1000/CN -10;
2. otherwise Runoff = (Rainfall -0.2 (1000/CN -10))^2 / (Rainfall + 0.80* (1000/CN -10)
My code is as follow
clear
>> Rainfall = randi(10,10,1);
>> CN = randi( 25,10,1);
>> idx = Rainfall > 5;
>> Runoff =size(Rainfall,1);
>> Runoff = zeros(Runoff,1);
>> Runoff(idx)= 1000/CN -10;
 Runoff(~idx) = (Rainfall(~idx) - 0.2 * (1000/CN -10)^2)./Rainfall(~idx) + 0.8 *(1000/CN -10);
I am getting the following error when running the 1st condition
???  In an assignment  A(I) = B, the number of elements in B and
 I must be the same.
Could any body help to solve this issue? I am not very familiar with matlab and trying to write script for runoff calculation using Soil Moisture Condition. Thanks and regards, Saleem

Best Answer

Hi,
your problem is, that the dimensions of Rainfall and CN don't match. Use the the index idx or ~idx on CN. Also the operators in the last line need tweaking to work elementwise.
Here are the changed lines:
Runoff(idx)= 1000/CN(idx) -10;
Runoff(~idx) = (Rainfall(~idx) - 0.2 * (1000./CN(~idx) -10).^2)./Rainfall(~idx) + 0.8 *(1000./CN(~idx) -10);
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