avoid negative index in array or matrix

Question

Damiano Capocci 2017-12-17

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回答： Damiano Capocci 2017-12-17

I'm working on 2 loops in which i have the difference of two indices like this:

for i=1:interval_1
  for j=0:interval_2
    if(i-j>0)
      X(i+1)=c(j+1)*X(i-j);
    else
      %nothing
    end
  end

In my situation interval_1 and interval_2 are more or less 1 million so i want to optimize and remove if/else. It would be useful if there exist a command which ignores operations with negative index. Tell me.

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Stephen23 2017-12-17

编辑：Stephen23 2017-12-17

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Use

if

Or change the loop index range.

David Goodmanson 2017-12-17

Hi Damiano, it looks like the j loop is overwriting X(i+1) every time, with the effect that the loop collapses down to the single j value min(interval_2,i-1).

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Answer 1

Roger Stafford 2017-12-17

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Just do this:

   for i=1:interval_1
     j = min(i-1,i2);
     X(i+1)=c(j+1)*X(i-j);
   end

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Answer 2

Jos (10584) 2017-12-17

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Why not switch the loops, and let i start from j+1

for j=0:interval_2
  for i=j+1:interval_1
  % if(i-j>0) % this is now always true
    X(i+1)=c(j+1)*X(i-j);
  % else
  %  %nothing
  % end
  end
end

And then you also see immediately that X(i+1) gets overwritten all the time (as David pointed out) and X(1) is never calculated ...

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Answer 3

Damiano Capocci 2017-12-17

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Yes, all of u are right in particular david, my code is a bit complex so i've tried to explain the question giving the idea of my code but i've made a mistake. (I think) The best solution of my problem is

for k=1:interval_1 for p=0:(k-1,interval_2)

x(k+1)=x(k+1)+c(p+1)*x(k-p)

end end

tic toc gives a little improvement but however it is always a function called every step