avoid negative index in array or matrix

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Damiano Capocci
Damiano Capocci 2017-12-17
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.
  2 个评论
David Goodmanson
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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采纳的回答

Roger Stafford
Roger Stafford 2017-12-17
Just do this:
for i=1:interval_1
j = min(i-1,i2);
X(i+1)=c(j+1)*X(i-j);
end

更多回答（2 个）

Jos (10584)
Jos (10584) 2017-12-17
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 ...
Damiano Capocci
Damiano Capocci 2017-12-17
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

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