How to replace half the number of specific element in an array?

Hello every one,

I have a row vector having 8236 columns, out of which 863 columns have a value 4. I want to replace 431 of the columns by a value 2. For simplicity, I am giving a vector having only 23 columns

A=[1 2 3 4 4 1 2 4 4 3 4 4 4 2 1 3 4 4 4 2 1 4 4]

Here, in this vector, there are 12 columns having a value 4. How to replace half the columns by a value 2?

Any help will be appreciated.

Answer 1

[Stephen23]

To change random columns:

>> A=[1 2 3 4 4 1 2 4 4 3 4 4 4 2 1 3 4 4 4 2 1 4 4]
A =
   1   2   3   4   4   1   2   4   4   3   4   4   4   2   1   3   4   4   4   2   1   4   4
>> nnz(A==4)
ans =  12
>> idx = find(A==4);
>> num = numel(idx);
>> A(idx(randperm(num,ceil(num/2)))) = 2
A =
   1   2   3   2   2   1   2   4   4   3   2   4   2   2   1   3   2   4   4   2   1   2   4
>> nnz(A==4)
ans =  6

Answer 2

[KALYAN ACHARJYA]

A=[1 2 3 4 4 1 2 4 4 3 4 4 4 2 1 3 4 4 4 2 1 4 4];
for i=1:length(A)/2;
 if A(i)==4
    A(i)=2;
    end
end

Answer 3

[Ameer Hamza]

Here is one possible solution

A=[1 2 3 4 4 1 2 4 4 3 4 4 4 2 1 3 4 4 4 2 1 4 4];
index = find(A==4);
randomIndex = randperm(length(index));
A(index(randomIndex(1:end/2))) = 2;

it will randomly select which columns to change.