Need help with a While loop

So I'm trying to write a rainflow counting program and I'm having trouble with the repetitions

while  i <= numel(PV) ;
    i = i +1;
    N = N+ 1;
    PV(N);
  switch N
      case 3
    X = abs(PV(N) - PV(N-1))
    Y = abs(PV(N-1) - PV(N-2))
      case 4
    X = abs(PV(N) - PV(N-1))
    Y = abs(PV(N-1) - PV(N-2))
   end
end

Instead of doing cases all the way to 24 there must be an easier way but I suck at matlab and would appreciate any help. Thanks

[Nicolo Zaza]

What Im actually trying to do is to re arrange a previous code to make work with some other scripts I developed but it is in fortran and I'm having a hard time translating it.

DIMENSION E(50)
  N=0
1  N=N+1
  CALL DATA(E(N),K)
  IF(K == 1) STOP
2  IF(N < 3) GO TO 1
  X=ABS((E(N)-E(N-1))
     Y=ABS(E(N-1)-E(N-2))
3  IF(X.LT.Y) GO TO 1
4  RANGE=Y
  XMEAN=(E(N-1)+E(N-2))/2
     N=N-2
 E(N)=E(N+2)
 GO TO 2
 END

Answer 1

[Kevin Claytor]

You may want to have a look at this file to see how they implement the rainflow method; http://www.mathworks.com/matlabcentral/fileexchange/3026

Now I'm just going to take a stab at my interpretation and re-writing of your FORTRAN code (added matlab-style comments to the end);

DIMENSION E(50)                 % Not sure what this does; same as E=[1:50]?
  N=0                       % Initalize counters
1  N=N+1
  CALL DATA(E(N),K)             % K = DATA(E(N))
  IF(K == 1) STOP               % Throw an error if the first point is 1?
2  IF(N < 3) GO TO 1         % Can't do algorithm for N < 3, increment N
  X=ABS((E(N)-E(N-1))           % Compute X and Y
     Y=ABS(E(N-1)-E(N-2))
3  IF(X.LT.Y) GO TO 1        % Continue if X > Y - this is the 'flow part'
4  RANGE=Y                   % Set some other parameters
  XMEAN=(E(N-1)+E(N-2))/2
     N=N-2                  % Go back to see if this is still max
 E(N)=E(N+2)                    % Update counter with prev max at location N
 GO TO 2
 END

So to me, and I'm likely wrong, as I've never done rainflow counting before and just read the wiki page on it now in about 15 secs...

DATA = [...blargh...];
LDATA = length(DATA);
E = [1:LDATA];     % A linear index of where our flows start from?
N = 3;    % no sense starting before the algorithm can kick in
while N < LDATA
  X = abs(DATA(N) - DATA(N-1));
  Y = abs(DATA(N-1) - DATA(N-2));
  if x > Y
    RANGE = Y;
    XMEAN = (DATA(N-1) + DATA(N-2))/2;
    E(N-2) = E(N);
    N = N-2;
  else
    N = N+1;
  end
end

Maybe this helps?

[Nicolo Zaza]

This helped far more than what I was expecting.. Had to do some changes but it gave me a great idea.. Thanks a lot man.

Answer 2

[Walter Roberson]

Why have a switch at all? Why not just

if N >= 3
  X = abs(PV(N) - PV(N-1))
  Y = abs(PV(N-1) - PV(N-2))
end

[Andrei Bobrov]

if N >= 3 && N <= 4
    X = abs(PV(N) - PV(N-1))
    Y = abs(PV(N-1) - PV(N-2))
end

[Albert Yam]

Following from Nicolo's response about keeping X,Y values.

for N = 3:length(PV)
    if N >= 3 && N <= 4
        X(N-2) = abs(PV(N) - PV(N-1))
        Y(N-2) = abs(PV(N-1) - PV(N-2))
    end
end

but at this point.. Walter is right .. why loop?

X = PV(3:end);
Y = PV(3:end);
X(1) = abs(PV(3) - PV(2));
X(2) = abs(PV(2) - PV(1));
Y(1) = abs(PV(3) - PV(2));
Y(2) = abs(PV(2) - PV(1));

Answer 3

[Matt Kindig]

I reformatted the code for easier reading. A few questions:

1. 1. what are X and Y for? They don't appear to be used anywhere in the loop, and are overwritten each time.
2. 2. For N=3 or N=4, X and Y are defined in the same way.
3. 3. Are i and N initialized prior to the loop?
4. 4. What exactly is this code supposed to do? If we understand your intended output more clearly, we can probably recommend a more direct approach.

Your code:

while i <= numel(PV)
      i = i +1;
      N = N+ 1; 
      PV(N); 
      switch N 
          case 3 
            X = abs(PV(N) - PV(N-1)) 
            Y = abs(PV(N-1) - PV(N-2)) 
          case 4 
            X = abs(PV(N) - PV(N-1)) 
            Y = abs(PV(N-1) - PV(N-2)) 
       end 
  end

[Nicolo Zaza]

This is just a part of the code Im trying to write, I actually have a fortran-like version of it if it would be of any help. The thing is that case 3 and 4 are not the only ones. What Im trying to do is for the loop to continue until theres no more values

PV = [ 80 -40 60 0 60 0 80 -100 60 20 60 20 80 -100 40 -80 0 -20 ...
      0 -20 40 -80 80 -40];
SA = length(PV);
N = 0;
i = 1;
while  i <= numel(PV) ;
   i = i +1;
   N = N+ 1;
   PV(N);
switch N
   case 3
     X = abs(PV(N) - PV(N-1))
     Y = abs(PV(N-1) - PV(N-2))
   case 4
     X = abs(PV(N) - PV(N-1))
     Y = abs(PV(N-1) - PV(N-2))
   end
end
This is what i have until now, The program is suppose to array the values from max to mins in a way defined by Rainflow Counting Algorithms(not sure if you've heard of it.) I could provide the fortran code if it would be of any help.

[EDITED, code formatted, Jan] Please learn how to format the code by your own: empty line above and below the code, mark code, hit "{} code" button or insert two spaces in front of each line manually. It is not so hard.

Answer 4

[Andrei Bobrov]

switch N 
    case {3,4} 
        X = abs(PV(N) - PV(N-1)); 
        Y = abs(PV(N-1) - PV(N-2)); 
end

OR

k = abs(diff(PV(1:4)));
xy = flipud(k(hankel(1:2,2:3)))
X = xy(1,N-2);
Y = xy(2,N-2);

Answer 5

[Nicolo Zaza]

Well yeah I already checked the Rainflow Method your sending me a link to but it doesn't seem to work properly. The problem I have is that I can't get it counting. I tried your code but it kept going and only displayed 1 answer for each element. It totally looks much better than mine. I need to get a bunch of X and Y's for 24 elements i should get around 12 of those(and for every other variable, ex: xmean) that's where I get lost. How can I keep it going and outputting results until there is an index with no info. And need all the results, like a vector. Also why cant an algorithm be done for {If N<3 , N = N + 1,end} something like that? Thanks

[Albert Yam]

The reason you only get 1 set of X, Y, is because you tell Matlab to over write it. This is the first instance of you mentioning that you need to keep previous X, Y values.

for N=1:length(PV)
    if N<3
        %stuff
    end
end