Idiomatic way of exiting from multiple nested loops?

Question

The MATLAB documentation describes the break keyword thus:

• break terminates the execution of a for or while loop. Statements in the loop after the break statement do not execute.
• In nested loops, break exits only from the loop in which it occurs. Control passes to the statement that follows the end of that loop.

(my emphasis)

What if you want to exit from multiple nested loops? Other languages, such as Java, offer labelled breaks, which allow you to specify where the flow of control is to be transferred, but MATLAB lacks such a mechanism.

Consider the following example:

% assume A to be a 2D array

% nested 'for' loops
for j = 1 : n
  for i = 1 : m
    if f(A(i, j)) % where f is a predicate
      break; % if want to break from both loops, not just the inner one
    else
      % do something interesting with A
    end
  end
  % <--- the break transfers control to here...
end
% <--- ... but I want to transfer control to here

What is an idiomatic way (in MATLAB) of exiting from both loops?

Answer 1

I would say for your original specific example, rather use linear indexing and a single loop:

%// sample-data generation
m = 4;
n = 5;
A = rand(m, n);
temp = 0;

for k = 1:numel(A)
    if A(k) > 0.8 %// note that if you had switched your inner and outer loops you would have had to transpose A first as Matlab uses column-major indexing
      break; 
    else
      temp = temp + A(k);
    end
end

Or the practically identical (but with less branching):

for k = 1:numel(A)
    if A(k) <= 0.8 %// note that if you had switched your inner and outer loops you would have had to transpose A first as Matlab uses column-major indexing
      temp = temp + A(k);
    end
end

I would think that this answer will vary from case to case and there is no general one size fits all idiomatically correct solution but I would approach it in the following way depending on your problem (note these all assumes that a vectorized solution is not practical as that is the obvious first choice)

1. Reduce the dimensions of the nesting and use either no breaks or just one single break (i.e. as shown above).
2. Don't use break at all because unless the calculation of your predicate is expensive and your loop has very many iterations, those extra iterations at the end should be practically free.
3. Failing that set a flag and break at each level.
4. Or finally wrap your loop into a function and call return instead of break.

Answer 2

As far as I know there are no such functionality built in. However, in most cases matlab does not need nested loops due to its support for vectorization. In those cases where vectorization does not work, the loops are mostly long and complicated and thus multiple breaks would not hinder readability significantly. As noted in a comment, you would not really need a nested loop here. Vectorization would do the trick,

m = 5;
n=4;
x = rand(m,n);
tmp = find(x>0.8, 1, 'first');
if (isempty(tmp))
   tmp = m*n+1;
end
tmp = tmp-1;
tot = sum(x(1:tmp));

There might of course be people claiming that for loops are not necessarily slow anymore, but the fact remains that Matlab is column heavy and using more than one loop will in most cases include looping over non optimal dimensions. Vectorized solutions does not require that since they can use smart methods avoiding such loops (which of course does not hold if the input is a row vector, so avoiding this is also good).

Answer 3

The best idiomatic way to use Python (or poison of your choice) and forget all this but that's another story. Also I don't agree with the vectorization claims of the other answers anymore. Recent matlab versions handle for loops pretty quickly. You might be surprised.

My personal preference goes to raising an exception deliberately and cradling it within a try and catch block.

% assume A to be a 2D array
A = rand(10) - 0.5;
A(3,2) = 0;

wreaker = MException('Loop:breaker','Breaking the law');

try
    for j = 1 : size(A,1)
        % forloop number 1
        for i = 1 : size(A,2)
            % forloop number 2
            for k = 1:10
                % forloop number 3
                if k == 5 && j == 3 && i == 6
                    mycurrentval = 5;
                    throw(wreaker)
                end
            end
        end
    end
catch
    return % I don't remember the do nothing keyword for matlab apparently
end

You can change the location of your try catch indentation to fall back to the loop of your choice. Also by slaying kittens, you can write your own exceptions such that they label the exception depending on the nest count and then you can listen for them. There is no end to ugliness though still prettier than having counters or custom variables with if clauses in my opinion.

Note that, this is exactly why matlab drives many people crazy. It silently throws exceptions in a pretty similar way and you get a nonsensical error for the last randomly chosen function while passing by, such as size mismatch in some differential equation solver so on. I actually learned all this stuff after reading a lot matlab toolbox source codes.