For loop with the loop variable equal to a matrix

Hello, this block of code was given on a previous exam

M = [1 3 -2; 7 -5 1];
temp = 0;
for k = M
    temp = temp + k(2)
end
temp

And we are supposed to give the final output of temp. I have no idea how one loops over a matrix, not even elements or anything. What the heck does k = M mean?

So I just do temp = 0+7 on the first iteration, but what happens on the next iteration? temp = 7 + 7?? I ran this code and got 3. Does the value of k(2) change or something?

Answer 1

[Azzi Abdelmalek]

Check what your code is doing

M = [1 3 -2; 7 -5 1]
temp = 0;
for k = M
  'k=',k   % Look at the value of k
  temp = temp + k(2)
end
temp

[Rick]

Thanks, but why does it output k like this? I mean, it just does a column wise loop for no apparent reason.

[Azzi Abdelmalek]

Each nth iteration, k takes the nth column

firdt iteration k=[1;7]

second iteration k=[3;-5]

and so on

Inside the loop, the second value k(2) is used

Answer 2

[Joseph Cheng]

So i would suggest save this as a .m file so you can use the break points. If you put a breakpoint at the for loop and you step through you can see that k iterates through the columns of the matrix M. In a normal for loop you'll have for k=1:10 which means k will loop through each iteration of 1 through 10. Similarly if you go for k=1:10:100 you'll get a k for each entry of 1:10:100.

With the break point you'll see that temp = temp +k(2). if we get rid of the (2) and just do temp = temp+k. you can see that in each iteration you'll be adding 1+3+-2 and 7+-5+1 with the result of [2;3]. So limiting the k(2) you'll only be adding using the second row.