How can I change a random element in a row of matrix?

I want to change a random element in a row of matrix. example

A = ones(1,84)

I wish to change 6-8 column component to the randomly 2.

However, 2 of the 6-8 columns will be get out only once.

example A = [1,1,1,1,1,1,1,1,1,1,1,1,1...] ->change A = [1,1,1,1,1,2,1,1,1,1,1,1,2,1...]

I wrote the following code.

for q=6:6:84
    if rand>0.5
       A(1,q) =2;
    end
end
for w=7:7:84
    if rand>0.6
       A(1,w)=2;
    end
end
for e=8:8:84
    if rand>0.7
        A(1,w)=2;
    end
end

But I did not produce the results you want.]

Ask for advice. Plz....

[Jan]

Please define, which kind of results are wanted, and which do not satisfy your needs. It is impossible to guess the criterion.

[Ji-hwan Hwang]

I want A matrix 6~8(multiple) column which change 1->2 .

A = ones(1,84);

column 6~8 is 1

A(1,6)=1,A(1,7)=1,A(1,8)=1 A(1,12)=1,A(1,14)=1, A(1,18)=1 ....

6 among 8 column select change 1->2

A(1,6)=2,A(1,7)=1,A(1,8)=1 A(1,12)=1,A(1,14)=2, A(1,18)=1 ....

understand?

[Walter Roberson]

Which columns are eligible to be changed?

Is it correct that out of all 84 columns, that exactly 6, 7, or 8 of them to be changed? So sum(A==2) would be 6, 7, or 8?

Or are you working in groups of columns, and out of each group of 8 columns, a certain number of them are to be changed randomly?