# How to separate a vector into two different vectors?

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Kristin Aldridge on 2 Dec 2021
Commented: Star Strider on 2 Dec 2021
I have a vector named "age_vec" that I would like to piece into two groups, those above 37.5 and those below, and place those numbers into another vector. I keep trying to run this through a for loop but it puts all the numbers in the same vector, either all placed in agegreater or ageless, or I even tried just getting it to count those above and those below and it keeps putting everything into just one vector/variable.
Thank you.
age_vec =[ 21 18 57 52 20 22 23 21.50 38 31 30 29 58 53 21.75 86 55]
%Counting those above and those below
ageless=0;
agemore=0;
for i=1:length(age_vec)
if age_vec > 37.5
ageless= ageless + 1
else agemore= agemore + 1
end
end
%Placing into vectors
ageless=[];
agemore=[];
for i=1:length(age_vec)
if age_vec < 37.5
ageless=[ageless age_vec] + 1
else agemore=[agemore age_vec] + 1
end
end

Star Strider on 2 Dec 2021
Try this —
age_vec =[ 21 18 57 52 20 22 23 21.50 38 31 30 29 58 53 21.75 86 55];
Lv = age_vec > 37.5; % Logical Vector
age_more = age_vec(Lv)
age__more = 1×7
57 52 38 58 53 86 55
age_less = age_vec(~Lv)
age_less = 1×10
21.0000 18.0000 20.0000 22.0000 23.0000 21.5000 31.0000 30.0000 29.0000 21.7500
This uses ‘logical indexing’. See the docuemtation section on Matrix Indexing for details.
.
##### 2 CommentsShowHide 1 older comment
Star Strider on 2 Dec 2021
As always, my pleasure!
.

Image Analyst on 2 Dec 2021
Edited: Image Analyst on 2 Dec 2021
Try this:
age_vec =[ 21 18 57 52 20 22 23 21.50 38 31 30 29 58 53 21.75 86 55]
% Find indexes that are more than 37.5
moreIndexes = age_vec > 37.5
% Extract into two new vectors.
ageless=age_vec(~moreIndexes)
agemore=age_vec(moreIndexes)

Benjamin on 2 Dec 2021
Probably the easiest way to do what you want is to use logical indexing. First make a vector of logicals that say whether each element of age_vec is greater than 37.5 or not:
is_greater = age_vec > 37.5;
Then make two new vectors by separating the elements of age_vec according to the corresponding value in is_greater:
age_more = age_vec(is_greater);
age_less = age_vec(~is_greater);
If you really want or need to use a for loop, let me know and I can show you how that would work, but this way with logical indexing is much more concise.

James Tursa on 2 Dec 2021
Edited: James Tursa on 2 Dec 2021
Others have already pointed out better ways of doing this. But to answer your question as to why your current code is not working, it is because you need to use the index i in your code. E.g.,
if age_vec(i) < 37.5
ageless = [ageless age_vec(i)];
else
agemore = [agemore age_vec(i)];
end
This will incrementally build up the ageless and agemore vectors, but at each iteration you will have to deep copy one of the vectors, so the performance will be severly impacted as the size of age_vec gets large. Hence the desire to use a different method as others have suggested.