while loops are usually designed to be repeated until a condition is met.
See for loops for the alternative looping structure
The generic format is
while condition_is_true
% loop body consists of statements to be repeated
end
Simple example
x = 10;
i = 0;
while x > 0.1
x = x/2.0;
i = i + 1;
fprintf('%3d %8.4f\n',i,x)
end