Elitism in Matlab?

Question

How do i implement elitism in matlab? For example, i run a program and after each run, a value is save in a variable say a and after completing all the runs, say 6 runs i have the a as follows

a = [7, 5, 4, 3, 6, 8];

how can i apply elitism on a to end of having the content of a as a = [7, 5, 4, 3, 3, 3];

That is when i scan through a, i will replace the bigger number with the small one i encounter. From the example, after scanning through a, 5<7, so i keep 5, 4<5, so i keep 4, 3<4, so i keep 3, 3<6, so i replace 6 with 3, and again 3< 8, so i replace 8 with 3 to end of having the a as a = [7, 5, 4, 3, 3, 3];

how do u do this in Matlab.

Attempt

I said,

if a(i)< a(i+1)
a(i+1) = a(i);
end

plot(a);

so that i can have a graph that decents smoothly.

but i keep having the following error:

'Subscript indices must either be real positive integers or logicals.'

Any idea how i can do this correctly.

Also see [this question](http://stackoverflow.com/questions/20054047/subscript-indices-must-either-be-real-positive-integers-or-logicals-generic-sol) for a [generic approach](http://stackoverflow.com/a/20054048/983722) to deal with this error. — Dennis Jaheruddin, Mar 10 '14 at 10:40

Stewie Griffin · Accepted Answer · 2014-01-22T21:20:08.293

4

I believe this should work for all cases:

b = [a(1), arrayfun(@(n) min(a(1:n)), 2:length(a))]

a =
     7     4     3     6     5     2     5
b =
     7     4     3     3     3     2     2

For info:

Your initial thought was correct, but you forgot to put the if inside a loop. You could have done:

for ii = 1:length(a)-1
   if a(ii)< a(ii+1)
      a(ii+1) = a(ii);
   end
end

The reason why you got the error was because you probably hadn't defined i, thus MATLAB interpreted it as the imaginary unit (sqrt(-1)). That's also the reason why I used ii instead of i in the loop, to avoid such errors.

edited Jan 22 '14 at 21:20

answered Jan 22 '14 at 20:56

Stewie Griffin

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Thanks. You are right. My initial thoughts were right. I just put the if statement in a loop as you suggested and it worked. – David Wang Jan 22 '14 at 22:42

Cape Code · Answer 2 · 2014-01-22T21:10:21.363

2

Edited to work with local minima:

a = [7, 5, 4, 3, 6, 8];
[y, i] = findpeaks(-a, 'npeaks', 1) ;
a(i:end)=-y;
plot(a)

The plot:

a plotted

edited Jan 22 '14 at 21:10

answered Jan 22 '14 at 20:43

Cape Code

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2

If the vector is `[7 4 3 6 2 8]` this wouldn't work. – Stewie Griffin Jan 22 '14 at 20:52
1

doesn't work for all cases. `a = [ 7 4 5 3 6 8 ]` as an example. – KitsuneYMG Jan 22 '14 at 20:53

score 2 · Answer 3 · edited May 23 '17 at 12:11

2

An alternative to Robert P.'s answer without arrayfun:

min(triu(repmat(a(:),1,numel(a))) + tril(NaN(numel(a)),-1))

edited May 23 '17 at 12:11

Community

1
1

answered Jan 22 '14 at 21:05

Luis Mendo

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+1, clever... I actually had to test it in MATLAB to see the logic =P Note that this will take up a lot of memory if the vector is long. – Stewie Griffin Jan 22 '14 at 21:11
@RobertP. Thanks. Yes, it uses a lot of memory to build he matrix – Luis Mendo Jan 22 '14 at 22:10

Elitism in Matlab?

3 Answers3