# Can someone explain me how to make this loop efficiently?

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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

Jon on 17 Feb 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 CommentsShowHide 1 older comment
Santos García Rosado on 18 Feb 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.

Rik on 17 Feb 2021
Edited: Rik on 17 Feb 2021
Your intuition is correct; this can indeed be simplified:
for k=1:numel(SomeArray)
if idx <= (k*B)
ArrayValue = SomeArray(k);
break%stop the for loop with k
end
end
Santos García Rosado on 18 Feb 2021
Thank's Rik! Exactly what I was looking for.

R2020b

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