FOR loop control structure

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?

Answer 1

[Joseph Cheng]

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.

Answer 2

[per isakson]

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.

[Guillaume]

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]

Indeed!

I took the liberty to

• replace "might be" by "are" and
• add "basic numeric"

in my answer. (There is no strike out option.)