Saving values in a recursive function

Hi! I am given a nested structure (root) that I need to analyze.

The structure has either a branch ( b1, b2, .., bn), which have other structures in it or a/p/or l which has a number.

I need to sum up the whole numbers of a, p and l in the stricture respectively.

While I was working on my code, I found out whenever the recursion occurred, the sum I found was reset to 0, so I could not sum the numbers at all. I tried to give an index but it could not solve the problem as well (I think I did something wrong)

I have tried several times, and this is currently what I have in my m file. I understand the while loop is wrong, but I want to get a gimps of the idea of how I can keep my values when the recursion occurs. Thank you :)

function [ branch ] = tree2( root );

Cn=fieldnames(root);

k=0;

if exist('f')==0

 f=1
end

while f>0

for i=1:length(Cn); eval(['branch=root.' Cn{i}]);

   if Cn{i}(1)=='b'

       k(f+1)=k(f)+1

   elseif Cn{i}=='a'

       a(f+1)=a(f)+branch

    elseif Cn{i}=='p'

             p(f+1)=p(f)+branch

    elseif Cn{i}=='l'

              l(f+1)=l(f)+branch
   end

   f=f+1

        s=isstruct(branch);
         if s==1;    
         tree2(branch)    
         end   

    end
end

end

Answer 1

[Guillaume]

The main problem is with the way you've implemented the recursion.

All variables declared in a function are local to the particular invocation of the function (unless declared global or persistent). Therefore, each time you call you recurse into tree2, a new set of k, a, p, and l is calculated. Since you don't return them at the end of tree2, their value is lost.

Thus, you need to change your function signature to:

function [a, b, p, l] = tree2(root); %I've changed k to b. Why was it called k anyway?

Then every time you invoke the recursion you add the return values to your local a, b, ...:

[branch_a, branch_b, branch_p, branch_l] = tree2(branch);
a = a + branch_a;
b = b + branch_b;
p = p + branch_p;
l = l + branch_l;

This also greatly simplifies your if elseif. You just add 1 to the relevant variable whenever you encounter a leaf/branch.

if Cn{i}(1) == 'b'
   b = b+1;
elseif ...

This should answer your question. Now, there are a few odd things about your code:

1. I find it much easier to read

if ~exist('f')

than

if exist('f') == 0

they're equivalent, but the former I read if not exist. That carries the intent better.

In any case, f is a local variable, so it never exists at that point of the code, so you might as well get rid of the if.

I don't see the point of the while loop anyway. Particularly as it'll never end since f is always > 0 as it starts at 1 and increases from there.

Finally, don't use eval to access the fields of a structure, use dynamic field names:

branch = root.(Cn{i});