How do I properly append to an array within a loop?

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Loren Graf 2017-10-16

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评论： Luke Calabretta 2022-4-5

I am running a loop of 300 iterations. Within the loop, I have a logic statement as follows.

for i = 1:300
    if arrival <1
        Ai = Ai + ia_time(i);
    else
        arrival = ia_time(i-1);
        Ai = ia_time(i) + arrival;
    end
    disp(Ai); % <-- displays all values
end
disp(Ai); % <-- only displays the final value from the loop

While the loop is open, 'disp(Ai)' prints the values correctly. However, if I attempt to use or print the variable after the final closing 'end', I only get the one final value of Ai. What I would like is an array of all values that I can reuse for future operations later in the code. I feel like it is not creating the array properly, and perhaps I need to assign the value of Ai to another variable as the loop iterates, but I can't seem to make that work either.

I know this should be simple, but I am beating my newbie head on the desk.

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per isakson 2017-10-16

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Replace

Ai =

by

Ai(i) =

Luke Calabretta 2022-4-5

Thank you!

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Answer 1

Guillaume 2017-10-16

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If all you're trying to do is accumulate the values of ia_time, then

A = cumsum(ia_time);

If you really wanted to use a loop for that:

 A = zeros(size(ia_time));
 A(1) = ia_time(1);
 for i = 2:numel(ia_time) %never hardcode end values, ask for what it actually is
    A(i) = A(i-1) + ia_time(i);
 end

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Answer 2

Cedric 2017-10-16

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编辑：Cedric 2017-10-16

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You shouldn't name your variable Ai if i is meant to be the index. Assuming that A is already defined and that, at loop index i, you want to alter its component at position/index i:

A(i) = A(i) + ia_time(i) ;

which is: add element i of vector ia_time to element i of A, and store the result back in A at position i.

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Loren Graf 2017-10-16

编辑：Loren Graf 2017-10-16

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I'm really trying to debug this thing. It looks so simple. For the sake of clarity, I will post the entire section of code, in it's current version. This is part of a M/M/1 queuing simulation. Data is provided in the 2 txt files, and must be programmatically incorporated into the code. Those files each contain 300 rows x 1 column of numbers. The import works fine, and the data is correct when checking those import variables. Ai is a variable representing data that is changed in each iteration, and needs to have all 300 values stored in the order they are updated, to be used for calculations further down. Maybe I need a break from it, and fresh eyes can see clearly.

ia_time = importdata('Data1.txt');
svc_time = importdata('Data2.txt');
arrival = 0;
Ai = 0;
Di = 0;
WQi = 0;
Ti = 0;
for i = 1:300
    if arrival <1
        Ai(i) = Ai(i) + ia_time(i);
    else
        arrival = ia_time(i-1);
        Ai = ia_time(i) + arrival;
      end
      disp(Ai);
      Ti = max(Ai,Di-1);
      WQi = Ti-Ai;
      Di = Ti+svc_time;
  end

Guillaume 2017-10-16

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The code in the question is puzzling on many levels.

First, the comment on this line:

disp(Ai); % <-- displays all 20 values

is misleading. It does not displays all 20 values. It displays the current value of Ai on the current step of the loop. At any point in the code, Ai only ever contains one value.

This is where it gets puzzling: It looks like you expected Ai (or maybe A) to be a vector of 20 values. In which case, as Cedric has answered, you should have used A(i) (or Ai(i) !) but then would have already created that variable with 20 values before the loop since you're intending to add to each of these elements within your loop. But if you had declared that variable as a vector, you wouldn't be saying that disp is ever showing the correct value...

In addition, assuming you meant A(i) then the line

A(i) = A(i) + ia_time(i)

only adds a value once to each element of A(i), and that value does not depend on previous iterations. The only way your faulty code could have displayed all 20 correct values would thus be if all A(i) values were 0, in which case the above line is pointless and:

A(i) = ia_time(i)

would be simpler.

Cedric 2017-10-16

编辑：Cedric 2017-10-16

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Looking at your PDF, those i mean "associated with the index". Try to precompute A, preallocate the others, initialize all, and loop the way Guillaume proposes:

 ia_time = importdata('Data1.txt');
 svc_time = importdata('Data2.txt');
 A  = cumsum( ia_time ) ;
 T  = zeros( size( A )) ;
 WQ = zeros( size( A )) ;
 D  = zeros( size( A )) ;
 T(1)  = .. ?
 WQ(1) = .. ?
 D(1)  = .. ?
 for i = 2 : length( A )
    T(i)  = ... ?
    WQ(i) = ... ?
    D(i)  = ... ?
 end

Loren Graf 2017-10-17

Thank you for your input earlier. Unfortunately, I had to go to work. When I looked at it later, I noticed several parts that made me slap myself in the forehead. Indeed, I over-complicated some of the inputs. It is amazing how a tired brain sometimes cannot make sense of easy issues.

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