Is it possible to put an if statement into a for loops counter?

Question

Travis Yeager 2016-6-22

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https://ww2.mathworks.cn/matlabcentral/answers/291751-is-it-possible-to-put-an-if-statement-into-a-for-loops-counter

评论： Travis Yeager 2016-6-23

采纳的回答： dpb

在 MATLAB Online 中打开

I need a counter that stops at one of two values, which ever is reached first.

Example

for r=1: (if statement here) 
  for c=1: (if statement here)
    do things
  end
end

I've tried achieving what I need with two nested while loops:

while r < r1 && r < r2
  r=r+1
  while r < r1 && r < r2
    c=+c+1
    if condition
    else
      stuff
    end
  end
end

but the first while loop just counts through all it's iterations and then moves onto the 2nd while loop, rather than counting once then moving into the 2nd while loop until that condition is met.

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James Tursa 2016-6-22

It isn't clear to me yet how you want the counters to behave. Do you want r to count until a condition is met, and then for that r value count c until a different condition is met? Or do you want r and c to count together somehow until a condition is met? Once the condition(s) are met, do you continue counting both counters or does one of them start over? etc.

dpb 2016-6-22

"first while loop just counts through all its iterations and then moves onto the 2nd while loop rather than counting once then moving into the 2nd while..."

That's simply not so...the first loop iterates once, enters the second which is infinite since it never increments the test variable r.

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Answer 1

dpb 2016-6-23

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/291751-is-it-possible-to-put-an-if-statement-into-a-for-loops-counter#answer_226404

编辑：dpb 2016-6-23

在 MATLAB Online 中打开

"need a counter that stops at one of two values, which ever is reached first."

while r < r1 && r < r2
  r=r+1
  while r < r1 && r < r2
    ...

This doesn't need a nested loop or any other exotic IF cases, simply

for r=initialCount:min(r1,r2)
  ...

If r1,r2 can change inside the loop, then you need a while construct instead because the for iteration limits are set on entry to the construct and not altered even if the variables are.

>> r1=3;
>> for r=1:r1
     disp([r r1])
     if r==1, r1=5; end
   end
   1     3
   2     5
   3     5
>>

As you can see, the loop only ran 3 iterations, not 5.

Also see

doc break % to terminate a loop

It can, of course, be included in an if construct that is a complex as needed on any variables needed.

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Travis Yeager 2016-6-23

Thank you very much! Was not aware of the min function. My variables are not changing in the loop but some other conditions are that depend on if it should go to r1 or r2. Thank you!

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