MATLAB: Onrammp exercise 14.2 stellar motion part 2, task 6 movaway

MATLABonramp 14.2

For task 6 I don't understand how matlab connects starname to a particular graph.
Also I'm not sure how this piece of code works…
"movaway = starnames(speed < 0) "
how does this line of code use a variable that is in a loop before this line of code?
Sorry in advance if this is an obvious question.
Task 1
[sHa,idx] = min(spectra);
lambdaHa = lambda(idx);
z = lambdaHa/656.28 - 1;
speed = z*299792.458
Tasks 2 – 4
for c = 1:7
    s = spectra(:,c);
    if speed(c) >= 0
        loglog(lambda,s,"--")
    else
       loglog(lambda,s,"LineWidth",3) 
    end
    hold on
end
hold off
Task 5
legend(starnames)
Task 6
movaway = starnames(speed < 0)

Best Answer

See the example, lets suppose starnames is variable having all types of numbers
starnames=[-2 3 45 7 2 0 -4 5 7 7 6 9];
speed=[2 3 -4 5 6 7 8  0 5 2 -30] ; % Just random example
## speed<0 is an conditional assignment, its return a logical array (with 0 & 1), when condition true, its 1, otherwise 0 
>> speed < 0
ans =
  1×11 logical array
   0   0   1   0   0   0   0   0   0   0   1
## starnames(speed < 0) return an array, where the 1 presence in the above logical array 
>> movaway = starnames(speed < 0)
movaway =
    45     6
This is just an example to understand the issue (This approach is good way for efficient code, conditional & array indexing)
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