How should I alter this sum?

Hey everybody, I would really appreciate help with the following:

for j=1:30

 G=-N*mat(1,20-j)+N*mat(1,25-j)+N*(1/2)*mat(1,30-j)

end

Does somebody know how I can make sure the first two summands are only counted if 20-j=>0 and/or 25-j=>0? So for example if j=27 only the third summand is counted?

Thank you very much in advance!

James

[EDITED, copied from Answers section, Jan] Thank you very much already guys. What I actually meant was that for j up till 20 i would like

G=-N*mat(1,20-j)+N*mat(1,25-j)+N*(1/2)*mat(1,30-j)

then for 20<j<25 i would like

G=N*mat(1,25-j)+N*(1/2)*mat(1,30-j)

and for 25<j i would like

N*(1/2)*mat(1,30-j)

Also I posted a small simplified example, does anyone know a more concise and simpler way to implement this? I tried to work with something like -if- statements and min(0,20-j) but it didn't work out :)

thanks already very much for the help offered!

James

[Jan]

[Copied from Comment to copied answer, Jan]:

Matt J has written:

This just seems to be a restatement of your original question, with no obvious new information. The solutions we've already given you are still applicable.

[Jan]

[Copied from Comment to Comment to copied answer, Jan]

James van Viersen has written:

haha true sorry just looking for a solution with a -for- loop incorporated

[Jan]

Please, guys, stop posting comments to comments of comments in the section of comments to comments of answers, which are actually comments, which should appear as additions in the original question. Trying to clean this up makes me really nervous. ;-)

Answer 1

[Matt J]

I assume you mean 20-j=>1 etc...

     Z=zeros(1,30);

      term1=Z; 
      term1(1:19)=-N*mat(1,19:-1:1); 

      term2=Z; 
      term2(1:24)=+N*mat(1,24:-1:1);

      term3=Z; 
      term3(1:29)=+N/2*mat(1,29:-1:1); 

     G=term1+term2+term3;

Answer 2

[José-Luis]

Note the in mat(1,25-j) the index needs to be superior to 0, otherwise an error will be returned. Considering that:

 nVals = 30;
 term1 = zeros(1,nVals);
 term2 = term1;
 term3 = term1;

 term1(end-20+1:end) = -N*mat(1,20:-1:1);
 term2(end-25+1:end) = N*mat(1,25:-1:1);
 term3(end-30+1:end) = (N/2)*mat(1,30:-1:1);

 your_vals = term1 + term2 + term3;

That you could write as an anonymous function

 term1 = zeros(1,nVals);
 your_fill_fun = @(x,nFill) x(end-nFill+1:end) = mat(1,nFill:-1:1);
 term1 = -N*your_fill_fun(term1,20); &etc

Answer 3

[Jan]

Your code example looks confusing, because the variable G is not used.

What about the trivial solution:

for j = 1:20
  G = -N*mat(1,20-j)+N*mat(1,25-j)+N*(1/2)*mat(1,30-j);
end

for j = 21:24
  G = N*mat(1,25-j)+N*(1/2)*mat(1,30-j);
end

for j = 26:30
  G = N*(1/2)*mat(1,30-j);
end

This has three [EDITED from two] problems - I assume you have overseen them in your explanation:

1. The first loop will try to access the index 20-j for j=20, but indicies must be greater than zero.
2. The case j==25 is not considered.
3. The case j==30 fails also due to zero indexing. [EDITED]

More compact soltutions:

for j = 1:30
  if j < 20  % Or 21, please fix this by your own
    G = -N*mat(1,20-j)+N*mat(1,25-j)+N*0.5*mat(1,30-j);
  elseif j < 25
    G = N*mat(1,25-j)+N*0.5*mat(1,30-j);
  else
    G = N*0.5*mat(1,30-j);
  end
end

Or:

for j = 1:30
  G = N*0.5*mat(1,30-j);    
  if j < 20  % Or 21, please fix this by your own
    G = G - N*mat(1,20-j);
  end
  if j < 25
    G = G + N*mat(1,25-j);
  end
end

[James van Viersen]

wow thanks Jan I think I'll come a long way with all your input. Everybody thanks alot already!