For loops vs. While loops

I know the two loops are used differently, although they could be used for the same purpose. However, how would you generally know which one to use, especially in cases where one over the other would be valid?

Answer 1

[Jan]

The FOR loop is nicer and more compact, if the number of iterations is known before the loop is started. The WHILE loop is nicer, when the number of iterations is determined inside the loop.

Compare:

for k = 1:10
  disp(k);
end

with:

k = 1;
while k <= 10
  disp(k);
  k = k + 1;
end

On the other hand:

a = 1e6;
while a > 1
  a = a * rand;
end

with this (1e8 is an arbitrary large guess - a bad idea!):

a = 1e6;
for k = 1:1e8   % Ugly, although it is not created explicitly
  a = a * rand;
  if a <= 1
    break; 
  end
end

There are some overlaps, e.g. running an iteration until a certain number of loops or a specific criterion is reached. Then usually WHILE is preferred with the convergence limit as test, because this reflects the main character of the loop, while the limitation of iterations is a fallback mechanism only.

[Jos (10584)]

The infinite for loop syntax is also nice to know:

for k=0:1e5:Inf,
    if k > 1e6
        break
    end
    disp(k)
end