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deriving new variable from existing column vector using for-loop

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Hello Matlab community!
I am a newbie matlab user and trying to be comfortable in using for-loops to derive a new variable.
In my example code below. I designated variable 'c' as my column vector of zeros and ones. I wish to run a for loop for that variable and create a new variable 'data_new' with the following conditions:
** if the value of c is 1 then assign a random variable between 5 to 29 else 0.
**it seems to create those zeros but failed to assign a random variable for the if condition that I stated. here's the error message that I encountered: Unable to perform assignment because the left and right sides have a different number of elements.
a=ones(1,4);
b=zeros(1,4);
c=[a b]'; %column vector 8x1
existing_data=c
for i=1:length(c)
if (existing_data(i)==0)
data_new(i)=randi([5,29],8,1)' %% i wish to generate the same size column vector where the ones == randomly assigned data bet 5-29
else
data_new(i)=0
end
end
Thank you!

Accepted Answer

Harry Laing
Harry Laing on 9 Dec 2020
Simple error. Your data_new(i)=randi([5,29],8,1)' is the problem. Try putting the line randi([5,29],8,1) ' into the command window and see what happens. You're wanting to assign a single value into data_new but your code tries to assign an entire vector, hence the error.
Try this line instead (change the 8 to a 1):
data_new(i)=randi([5,29],1,1);
As side note, MATLAB will provide you with a warning saying that data_new will change size each iteration. Whilst it may not make much difference with such a small vector, with larger datasets this can cause issues with making the code run much slower. Wherever possible, it is good practice to pre-initiaalise the vector before the loop. For numeric matrices I personally like to create an 'empty' matrix of NaN values before loops like yours. For example, I would write the following into your script before the for loop, but after you create the existing_data variable:
data_new = NaN( size(existing_data) );
  3 Comments
Arturo Jr. Ongkeko
Arturo Jr. Ongkeko on 10 Dec 2020
Hi, Harry. Appreciate your reply. I saved my workspace where I created the random values and created another script for data analysis.

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