How to position flexibly the start of a for cycle?

My code is something like this:

expressions_1
expressions_2
expressions_3

I need a for cycle, but the start of the cycle depends on a parameter. So I would like that my code behave like this in case of param = 1:

for i=1:N
  expressions_1
  expressions_2
  expressions_3
end

but for param = 2 it should be:

expressions_1
for i=1:N
  expressions_2
  expressions_3
end

How to do this in general?

Answer 1

[Guillaume]

The proper way of doing this would be to have each expression in its own function (if it's simple enough anonymous functions would do) and dispatch to the functions as required, e.g.:

function flexibleexpression(x, N)
   expressions = {
     @(x) x+1;
     @(x) 2*x;
     @(x) sin(x);
     @(x) expression4(x); %expression4 is a function defined separately
   };
   x = pi;
   for expr = 1 : paramater_value-1
     %execute expressions before the loop
     x = expressions{expr}(x);
   end
   %now perform the loop
   for i = 1:N
      for expr = parameter_value : numel(expressions)
          x = expressions{expr}(x);
      end
   end
end

function x = expression4(x)
   if x < 0
     x = [x 1];
   else
     x = [x -1];
   end
end

A dirty way of doing it would be to replace the anonymous functions by strings and eval those in the expr loop.

[Mr M.]

An additional thing is that I need to increment a variable in the for loop, but the variable name depends on the parameter_value. I think the only way to do this is using eval.

Answer 2

[dpb]

There is no absolutely general answer to such a question; only a specific solution to the specific condition. In this case it's fairly simple to write

for i=1:N
  if param==2 & i==1
    expressions_1
  end
  expressions_2
  expressions_3
end

Alternative constructions are, of course, possible, but you'll have to consider any future modifications wanted/needed as they come up.

[Mr M.]

I need a short and visible solution. My problem is that param can be 1,...100, and I have expression1,...,expression100

[Mr M.]

So I need a general solution. Is it not possible for example to start a for cycle inside an if statement but end it outside?

[dpb]

"Is it ... possible ... to start a for cycle inside an if statement but end it outside?"

Nope; each construct has to be complete within any other.

As noted, there is no general solution for any possible set of conditional logic; you have to write the logic that fits the situation you have.

Sometimes writing things as state variables and as transition via a "state machine" is a viable alternative.

ADDENDUM

I'll qualify the above just a little...as I think you intended the question the answer is as given, "no"; in that there is no way that clauses aren't matched and completely nested within any higher level construct.

But, you can manage to break out of a for loop and also break an if pattern but it requires having a break condition coded inside the for and more than likely some additional logical "flag" variables to use to control the location of where in a SWITCH or nested IF the execution were to branch. In languages w/ GOTO there's a little more flexibility in going from point A to point B but it comes at a price in code legibility and it's possible to write any algorithm without it. TMW has chosen the "structured" route entirely to protect users from themselves of writing "spaghetti code" by not incorporating the statement at all.

ADDENDUM 2

Even with GOTO there are limits on not being able to jump from outside into a DO...ENDDO (Fortran syntax) and the like. In early versions of FORTRAN there were some rather unique things regarding "EXTENDED DO" that were allowed, but that turned out to be so problematic the feature was removed from the language Standard going forward from F90. Even it would NOT let you do what you're proposing above, however.