For Loop for Non-Consecutive Values in an Array...

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Michelle De Luna 2020-2-7

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评论： Michelle De Luna 2020-2-7

Hi everyone! I had a question regarding for loops when your "i" values are not consecutive.

I have a large matrix with thousands of timesteps. From this matrix, I need to find those index values where the date includes a specific month like December. I have been able to create a variable that includes the index value of all of those specifically selected timesteps quite easily, however the next step of my code includes a for loop where I use these index value to produce an image.

So, for example, my variable "December" is an array with timestep values like: 1745, 1767, 2918, 3958, 5938, and thousands more in random order.

How do I create a for loop that uses the specific timestep value (e.g. 1745 or 2918)? Would the "length" option do the trick? Any help would be greatly appreciated.

dec = find(dates.Month == 12)
for i = 1:length(dec)
    i
end

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Answer 1

Walter Roberson 2020-2-7

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for i = dec
    disp([i,dates(i).Month])
end

This might be enough for your situation. However, it is very common to need to have one output for each non-consecutive input. In such a case, you should learn this kind of pattern:

decvals = find etc
numdec = length(decvals);
outputs = zeros(numdec, 1);
for decidx = 1 : numdec
    dec = decvals(decidx);
    do something with dec
    outputs(decidx) = whatever;
end

In this kind of pattern, decvals does not need to be consecutive or integer or unique -- it does not even need to be real-valued or numeric. This kind of pattern will loop through all of whatever values are there, extracting one at a time (here into the variable dec) and storing the output computed into the location corresponding to the offset into the array of values.