Combining several loops in one big loop

Hello everyone,

i need help with the creation of a loop. I have to create a variable which can be described as follows. For each fiscal year, consisting of 12 months from July of one year to July of the next year, i have to put in the same value. Then i have to move one year forward and repeat it, and so on. The matrix should be created from July 1965 till December 2014 (606x16505 double matrix). I have data available for July 1963 to December 2014 (618x16505 double matrix).

For a better understanding of my problem, i have posted the first 5 loops. As you can see this is not an elegant way to deal with this.

% January 1965-June-1965 

for t=1:6

         NI(t,:) = (log(sasho(13,:)) - log(sasho(1,:)));
  end

% July 1965-June 1966 (this is one fiscal year)

for t=7:18

      NI(t,:) = (log(sasho(25,:)) - log(sasho(13,:)));
  end

% July 1966-June 1967

for t=19:30

      NI(t,:) = (log(sasho(37,:)) - log(sasho(25,:)));
  end

% July 1967-June 1968

for t=31:42

      NI(t,:) = (log(sasho(49,:)) - log(sasho(37,:)));
  end

% July 1968-June 1969

for t=43:54

      NI(t,:) = (log(sasho(61,:)) - log(sasho(49,:)));
  end

...

% July 2014-December 2014

for t=595:600

      NI(t,:) = (log(sasho(613,:)) - log(sasho(601,:)));
  end

I hope you got my problem and are able to get a better way coding this than I had.

Thanks in advance.

Answer 1

[Geoff Hayes]

Sascha - let's first size the NI array appropriately.

 NI = zeros(606,16505);

I'm not sure if this is the size you really want since you mention that you only want to collect data from July 1965 until December 2014 (which I think is more like 594 months). Also, your first for loop copies data from January 1965 until June 1965..

Anyway, let's look at your first for loop. We can reduce it to a single line of code if we use repmat as

 NI(1:6,:) = repmat((log(sasho(13,:)) - log(sasho(1,:))),6,1);

We repeat the same data six times and copy it directly into the first six rows of NI. And we can do something similar to every other for loop too if we know where to do the appropriate copy. For example,

 m = 7;
 n = m + 12 - 1;
 u = 13;
 v = u + 12;
 NI(m:n,:) = repmat((log(sasho(v,:)) - log(sasho(u,:))),n - m + 1,1);

We can then put this into a loop as

 u = 13;
 for m=7:12:595
     n = min(600, m + 12 - 1); % since we don't want to exceed 600 which is Dec. 2014
     v = u + 12; 
     NI(m:n,:) = repmat((log(sasho(v,:)) - log(sasho(u,:))),n - m + 1,1);
     u = v;
 end

I haven't tested the above but I'm hoping that you can follow it and see how we've reduced the many loops to just one by considering the common parts of each.

[Sascha Winter]

Hello Geoff

Thanks a lot. It helped me a lot also for a better understanding of the loop creation. You're right that from July 1965 to December 2014 it wouldn't be 600 months. But since I actually need the variable from the beginning of 1965 it then sums to 600, so sorry about that. Great help.