avoid negative index in array or matrix
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Damiano Capocci
il 17 Dic 2017
Risposto: Damiano Capocci
il 17 Dic 2017
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 Commenti
David Goodmanson
il 17 Dic 2017
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).
Risposta accettata
Roger Stafford
il 17 Dic 2017
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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Più risposte (2)
Jos (10584)
il 17 Dic 2017
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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