How to change the step value of a for loop during execution

Hey,

Im a little bit hung up over a bit of code that works on datasets of the form below.

x = Nan Nan Nan Nan 1 1 1 1 Nan Nan Nan 1 1.

most of the elements are nans with 2 segments containing numbers. These segments are not uniformly located in all the datasets

my code tries to do the following:

- look through each vector and save the first and last values of each contiguos segment of numbers.

To do this I use a combination of loops. its probably not the smartest way to solve the problem but there it is. the code works by using a for loop to scroll through the vector and handing control to a nested loop when it finds a number. - the nested loop saves the first and last values of the number segment and then hands back control to the main for loop. however I cannot change the step value of the for loop to skip over to the index where the nested loop left off. any ideas?

j = 1; for i=1:j:length(x) <--- how do I change the value of j?

   while x(i) is a nan
       j=1
   continue

   else result(1) = x(i)
     for k = i:length(x)
        if x(k)  is nan
           result(2) = x(k-1)
           j = k - i

           break(pass control back to main loop which attempts to skip to where this loop left off)
        end
     end

end

end

any help is highly aprreciated.

Answer 1

[Matt Fig]

You cannot change this once the loop starts. Instead use a WHILE loop and have the increment depend on an IF statement.

[Navdeep]

Ur right Buddy!!!

Answer 2

[Paulo Silva]

Here's a code that might work!

Save this function

function b=FindFirstAndLast(v)
%v is a line vector (1xN)
%the output is also a line vector with pairs of values
%odd indexes are the first element of a group of numbers
%even indexes are the last element of a group of numbers
%If a group is composed of just a number it will appear twice
%that means the number is the first and last element
b=zeros(1,numel(v)); %pre-allocate storage
c=0;d=0; %inicialize counter and index
for a=1:numel(v) %do it for all elements of the vector
    if isnan(v(a)) % check if current value is nan
        if c>0 %if we are inside a group of numbers
            d=d+1; %increment index
            b(d)=v(a-1); %save last segment value
        end
        c=0; %end of group of numbers reached so we reset the counter
    elseif (c==0) %check if we have the first number of a group
        c=c+1; %increment the counter
        d=d+1; %increment index
        b(d)=v(a); %save first segment value
    end
end
%the vector might end with a number so the code won't save the last element
if ~isnan(v(a)) %we need to check if last value is also a number
d=d+1; %increment index
b(d)=v(a); %last value is also a number so save it
end
b=b(1:d); %discard not used indexes

And try it with the code

x=[nan nan nan nan 1 2 3 4 nan nan nan 5]; %sample vector
FindFirstAndLast(x)

[Andre]

It works. thanks a bunch!