MATLAB: Simple for cycle!

forfor loopMATLABvectors

I have one vector:
massa=massa =
   -0.2000
   -0.1932
   -0.1729
   -0.1397
   -0.0946
   -0.0386
    0.0265
    0.0992
    0.1773
    0.2587
    0.3413
    0.4227
    0.5008
    0.5735
    0.6386
    0.6946
    0.7397
    0.7729
    0.7932
    0.8000
I run this cycle to have to have two separate vectors (one for negative valuse and one for positive values), one for the negative massa, and one for the positive:
for i=1:length(massa)
       if massa(i) < 0
        massa_neg(i) = massa(i); 
       else  
         massa_pos(i) = massa(i);  
       end
   end
I get the right for the massa_neg but for massa_pos it puts zeros instead of nothing:
   0	0	0	0	0	0	0,0265259209387866	0,0991522876735153	0,177257256429600	0,258710327263834	0,341289672736166	0,422742743570400	0,500847712326485	0,573474079061213	0,638640785812871	0,694570254698197	0,739736875603245	0,772908620850317	0,793180651701361	0,800000000000000

Best Answer

Think what happens for massa_pos when i=1. Nothing happens, because massa(i) is negative. But when you get to i=7, you then get massa_pos(7)=massa(7). What is matlab to think massa_pos(1:6) consists of, when you have not told anything about it? A vector will be filled with 0s as default.
In your example, you can add a running counter;
negcounter=1;poscounter=1;
for i=1:length(massa)
       if massa(i) < 0
        massa_neg(negcounter) = massa(i); 
        negcounter=negcounter+1;
       else  
         massa_pos(poscounter) = massa(i);
         poscounter=poscounter+1;
       end
end
Alternatively, a more clean approach is to simply use logical indexing:
massa_neg=massa((massa < 0)); %all values of massa which is below 0
massa_pos=massa((massa >= 0)); %and, similarly, all values of massa which is greater than or equal to 0.
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