How to change the elements of a matrix based on row/column numbers?

Say I have a 6*6 matrix of ones. P=ones(6,6).

I want to change a certain number of values to 2 inside this matrix based on their row/column numbers by specifying a boundary. I have written this code:

for i=1:m
    for j=1:n
        if ((( i > left ) && (i  < (right))) && (( j> bottom ) && (j <top)))
            P(i,j)=2;
        end
    end
end

I have specified the left, right, bottom, top values beforehand. E.g. 2,5,2,5. I want the output to be P =

     1     1     1     1     1     1
     1     1     1     1     1     1
     1     1     2     2     1     1
     1     1     2     2     1     1
     1     1     1     1     1     1
     1     1     1     1     1     1 
i.e if i is between 2 and 5 (rows 3  and 4) and if j is between 2 and 5 (columns 2 and 5) then the values are changed to 2. 
It doesn't however work. 
I understand that I could just directly change the values, but I want to be able to change the values of the variables (m,n top, bottom, etc) and run the code as part of a program. 
Should I be using another for loop instead of if?

Thank you!

[the cyclist]

An example of inputs, along with the output you expect from those inputs, would help us help you.

Answer 1

[Andrei Bobrov]

l=2;r=4;b=3;t=5;
P=ones(6,6);
P(l:r,7 - (b:t)) = 2;

[Yona]

if the number of row and columns is equal you can write:

P=ones(6);

and for any n:

n=6;
P = ones(n);  
P(l:r,7 - (b:t)) = 2;

[José-Luis]

If it's for any n then it should be:

 P(l:r,size(P,2) + 1 - (b:t)) = 2;