MATLAB's FOR loop is static in nature; you cannot modify the loop variable between iterations, unlike the for(initialization;condition;increment) loop structure in other languages. This means that the following code always prints 1, 2, 3, 4, 5 regardless of the value of B.
A = 1:5;
for i = A
A = B;
disp(i);
end
If you want to be able to respond to changes in the data structure during iterations, a WHILE loop may be more appropriate, you'll be able to test the loop condition at every iteration, and set the value of the loop variable(s) as you wish:
n = 10;
f = n;
while n > 1
n = n-1;
f = f*n;
end
disp(['n! = ' num2str(f)])