Applying monthly coefficients to daily data

Sorry if the question is elementary but I am trying to do a daily soil moisture balance for a long dataset but am using monthly crop coefficients. I have a column in my main array where I have used the "month(A)" function and I have made a month column in the array storing my coefficient.

I want to apply the corresponding co-efficient to the correct month for each day of the ~100 years. The following code does the job but am new to MATLAB and hoping to find something neater.

Where Data(:,n) are the appropriate columns of the main dataset and kcT(:,n) are columns in the array containing co-efficients with the corresponding month.

for i=1:12
    for j = 1 : length(Data(:,1))
        if Data(j,4)==kcT(i,2)
            DID(j,1)=Data(j,2)*kcT(i,1);
        end
    end
end

Answer 1

[JohnGalt]

The key to a simpler and faster implementation of this is knowing that you can construct an array using an array of indices... For example

array1 = [1 3 5 7 9]
newArrayIndices = [2 2 2];
array1(newArrayIndices) % will be = [3 3 3]

So in this case, if we assume that kcT is in order ie.

[ .5 1  
  .3 2
  .4 3
 ...
 .38 12]

then we can just use the coeff column to construct an array the same size as Data which will have all the coefficients:

monthlyCoeffArray = kcT(Data(:,4,1);

And the whole piece of code becomes...

monthlyCoeffArray = kcT(Data(:,4,1);
DID = Data(j,2).*monthlyCoeffArray; % note the dot-multiply

No need for any for-loops.

[Nathan Dick]

Thanks for the quick reply John, that is exactly what I was after. Thanks also for explanation, don't like using code if I don't know how it works. Am I right in thinking that as the actual moisture balance will require a loop due to the fact that it is dependent on the previous time step, or is there a function which has been designed for that situation?

[JohnGalt]

no prob - glad to help.

I'm not sure I understand the question... you need to use a shifted version of the DID variable? if so just shift the index to realign i.e.

Vals = 1:10:
prevDayVals = Vals(1:end-1);
currentDayVals = Vals(2:end);

failing that, a for-loop isn't so bad... it's great to avoid when possible but can make the code much more readable