Can someone explain me how to make this loop efficiently?

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Santos García Rosado 2021-2-17

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评论： Santos García Rosado 2021-2-18

采纳的回答： Jon

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Hello Mathworks community!

I'm having a bit of trouble trying to code a for loop. This is my code so far.

randomNumber1 = 100;
randomNumber2 = 150;
out(1)= 0;
B = 4;
for idx = 2:1:numel(A)
    if idx <=(B)
        ArrayValue = SomeArray(1);
        if % first condition
            out(idx) = ((out(idx - 1)) - randomNumber1);
        elseif % second condition
            out(idx) = ((out(idx - 1)) - randomNumber2);
        elseif % third condition
            out(idx) = (out(idx - 1) + ArrayValue);
        end
    elseif idx >(B) && idx <= (2*B)
         ArrayValue = SomeArray(2);
         if % first condition
             out(idx) = ((out(idx - 1)) - randomNumber1);
         elseif % second condition
             out(idx) = ((out(idx - 1)) - randomNumber2);
         elseif % third condition
             out(idx) = (out(idx - 1) + ArrayValue);
         end
    elseif idx >(2*B) && idx <= (3*B)
          ArrayValue = SomeArray(3);
          % It keeps going until I've used every "SomeArray" value
     end
end

As you can see, I have to repeat all the statements as many times as values I have inside my vector: SomeArray

The code works as it should, but I'm well awared that this is not how you do it properlly. I know aswell that it is possible to only state the conditions one time, I'm just not sure how to do it.

Could someone help me out?

Thank's!

Santos

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Answer 1

Jon 2021-2-17

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I think the key is coming up with an expression that gives you the index for someArray from the current loop index. Something like this:

randomNumber1 = 100;
randomNumber2 = 150;
out(1)= 0;
B = 4;
for idx = 2:1:numel(A)
    k = ceil(idx/4); % index into someArray
    ArrayValue = SomeArray(k);
    if % first condition
        out(idx) = ((out(idx - 1)) - randomNumber1);
    elseif % second condition
        out(idx) = ((out(idx - 1)) - randomNumber2);
    elseif % third condition
        out(idx) = (out(idx - 1) + ArrayValue);
    end
end
    
    

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Jon 2021-2-17

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Also you should preallocate your our array to the final size it will reach, you just make it one element long.

So

out = zeros(size(2:1:numel(A)));

Santos García Rosado 2021-2-18

Thank you Jon! Your code was what i was looking for. Indeed, I had preallocated the array but I forgot to cipying it down on the answer. Thank's again.

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Answer 2

Rik 2021-2-17

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编辑：Rik 2021-2-17

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Your intuition is correct; this can indeed be simplified:

for k=1:numel(SomeArray) 
    if idx <= (k*B)
        ArrayValue = SomeArray(k);
        %your other code
        break%stop the for loop with k
    end
end