FOR loop control structure

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richard
richard 2014-8-15
编辑： per isakson 2014-8-19
I have written this code:
for k=5:2:13
fprintf('The square of %d is %d.\n',k,k^2)
end
I have been trying to understand FOR loops, and trying to understand what the index (k in my example) is. Is k a row vector? When I run my script, and try to kind out what k is, matlab says it simply the number is 13, but I would expect it to be a row vector looking like:
k=[5 7 9 11 13]
Can someone please explain?

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回答（2 个）

Joseph Cheng
Joseph Cheng 2014-8-15
k will be exactly as you say however the for loop will iteratively go through each item within k. if you put in a break point in the for loop you'll see that it doesn't do it all at once but one at a time.
per isakson
per isakson 2014-8-19
编辑：per isakson 2014-8-19
IMO: The description of for, Execute statements specified number of times, is not particularly good. It doesn't make justices to this powerful "Loop Control Statement". If you know basic numeric for-loops from other languages it's easy to miss the Matlab's for-loop is much more powerful. (OK, there are even more clever for-loops out there.)
Forget about "specified number of times" and think: loop over all columns (or for each column)
for item = any_type_of_array
do something with item
end
I modify your example slightly
any_type_of_array = (5:2:13);
for k = any_type_of_array
fprintf('The square of %d is %d.\n',k,k^2)
end
outputs
The square of 5 is 25.
The square of 7 is 49.
The square of 9 is 81.
The square of 11 is 121.
The square of 13 is 169.
above any_type_of_array is a row vector and the columns are scalars. Now make any_type_of_array a column vector
any_type_of_array = transpose(5:2:13);
for k = any_type_of_array
fprintf('The square of %d is %d.\n',k,k.^2)
end
outputs
The square of 5 is 7.
The square of 9 is 11.
The square of 13 is 25.
The square of 49 is 81.
The square of 121 is 169.
which is not what you wanted. (Note element-wise .^). And after that
>> k
k =
5
7
9
11
13
I guess Loren wrote a blog post on for-loops, but I cannot find one.
  2 个评论
Guillaume
Guillaume 2014-8-19
编辑：Guillaume 2014-8-19
Nitpick: while matlab for loop are arguably more powerful than the basic numeric for loops that you find in many languages, it's still severely limited compared to the generic enumerator/iterator interface available in just as many languages. See for example the foreach of C# that allows you to iterate over any arbitrary sequence including infinite ones.
Other than that, yes, for loops in matlab need to be understood as:
for iter = ...
for each loop iteration, iter will be a single column of ... whatever that may be (matrix, vector, cell array)
per isakson
per isakson 2014-8-19
编辑：per isakson 2014-8-19
Indeed!
I took the liberty to
  • replace "might be" by "are" and
  • add "basic numeric"
in my answer. (There is no strike out option.)

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