Can someone explain me how to make this loop efficiently?

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Santos García Rosado
Santos García Rosado le 17 Fév 2021
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

Réponse acceptée

Jon
Jon le 17 Fév 2021
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
  2 commentaires
Jon
Jon le 17 Fév 2021
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
Santos García Rosado le 18 Fév 2021
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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Plus de réponses (1)

Rik
Rik le 17 Fév 2021
Modifié(e) : Rik le 17 Fév 2021
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

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