# Can any WHILE Loop be replaced with a FOR loop?

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Dylan 2011 年 11 月 1 日
I had a test in my computer science course in which I was asked if any while loop can be replaced with a for loop. I put true. I guess the answer was false. My professor said that if you had a while loop in which you asked the user to input a certain value that had to be within a certain range and kept on iterating over the loop until the user inputted a value within the range that you wouldn't be able to do this with a for loop.
I however thought that any for loop can be written with a while loop and any while loop can be rewritten with a for loop. I think I may have even read something about this in my textbook but am unable to come up away with doing what my professor said with a for loop but believe that it's possible. Can anyone please come up with a way to do such a thing with a for loop?
for example a while loop
A=0;
while A<1
x=input('Enter a value');
if x>4 && x<10
A=1
else
end
this would force the user to enter a number between 4 and 10, not including 4 and 10, and would just keep on iterating over the loop until the user does.
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Dylan 2011 年 11 月 1 日
I even found this in my book and believe this is why I thought it was possible
"Any problem that can be solved using a while loop could also be solved using a for loop"

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### 回答 (3 件)

Fangjun Jiang 2011 年 11 月 1 日
In a twisted way:
for k=1:inf
x=input('Enter a value');
if x>4 && x<10
break;
end
end
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Alex 2011 年 11 月 1 日
Another option, so your computer does eventually crash (as k gets to large).
for k = 1:2
if(stuff)
break;
else
k = k - 1;
end
end
Keep in mind, it is not good programming to modify the loop variable within the loop, but it does work
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Walter Roberson 2011 年 11 月 1 日
In MATLAB, modifying the loop variable within a loop only affects the loop variable until the next iteration; upon starting the next iteration, the loop variable will be assigned whatever value it would normally have been as if you had not had any modification statements.

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Walter Roberson 2011 年 11 月 1 日
There are definitely while loops that cannot be done in any "for" loop with a fixed finite number of iterations. See for example http://en.wikipedia.org/wiki/Collatz_conjecture where the termination condition (N becoming 1) has not been proven to always occur.
If every loop could be executed within a fixed maximum number of steps, then the theoretically very important http://en.wikipedia.org/wiki/Halting_problem would have a positive solution, but it has been proven by way of contradiction that that cannot exist.

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