MATLAB loop through an array of vectors

Question

I can't figure out the appropriate syntax to do this. I have 4 vectors that each have 15 elements. I want to extract a vector of length 4 containing the first element of each of my original vectors, and then do things with it. Then I want to do the same things with the second element of each vector, etc. and store all the answers in a matrix or array. Something like this:

for i = 1:15
    new_vec=zeros(4);
    n=1;
        for fc = {vec_A, vec_B, vec_C, vec_D}
            new_vec(n)=fc(i);
            n=n+1;
        end
     Final_answers{i}=functionDoThings(new_vec);
end

But I get:

> The following error occurred converting from cell to double: Error
> using double Conversion to double from cell is not possible. Error in
> my_script (line 31)
>             new_vec(n)=fc(i);

I feel like there is a simpler way to do this that I am missing.

Put the 4 vectors together into a 4x15 matrix (which is probably what they should have been in the first place) and then take each 4x1 column of the matrix. — beaker
– beaker, Commented Jan 26, 2017 at 22:09

Yakedo · Accepted Answer · 2017-01-26 22:38:29Z

1

Your inner loop is looping over a 1x4 cell:

for fc = {vec_A, vec_B, vec_C, vec_D}
    new_vec(n)=fc{1}(i)
    n=n+1;
end

This results in fc being a 1x1 cell in each iteration.

To access the actual data inside the cell you would need to use curly brackets:

for fc = {vec_A, vec_B, vec_C, vec_D}
    new_vec(n)=fc{1}(i)
    n=n+1;
end

{1} will access the first cell of fc and (i) the desired element.

However it is easier and faster to use a matrix like beaker suggested:

fc = [vec_A; vec_B; vec_C; vec_D];
for i = 1:15
    new_vec = fc(:,i);

    ...
end

edited Jan 26, 2017 at 22:38

answered Jan 26, 2017 at 22:33

Yakedo

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5 Comments

beaker Over a year ago

You should probably pull fc out of the loop. There's no need to generate the same array 15 times.

Yakedo Over a year ago

valid point! I was just unraveling the inner loop, without thinking about any further improvements.

Liz Z Over a year ago

Thanks this was just exactly what I needed. Your fc gave me a 64x1 matrix though, so I ended up with: fc = [vec_A, vec_B, vec_C, vec_D]; for i = 1:15 new_vec = fc(i,:); ... end

Liz Z Over a year ago

fc = [vec_A, vec_B, vec_C, vec_D]; for i = 1:15 new_vec = fc(i,:); ... end

Yakedo Over a year ago

ah yeah, it depends whether your vec_A, vec_B, vec_C, vec_D are 1x15 or 15x1 Arrays. In my example they would be 1x15.

Konstantinos Kamaropoulos · Accepted Answer · 2017-01-26 22:25:51Z

0

The error tells that fc(i) is returning a cell while it tries to put it in the array as a double. Obviously, you can't convert an array to a single element.

So, instead of storing it into an array it should be stored into a cell.

for i = 1:15
    new_cell=cell(4);
    n=1;
    for fc = {vec_A, vec_B, vec_C, vec_D}
        new_cell{n}=fc(i);
        n=n+1;
    end
 Final_answers{i}=functionDoThings(new_cell);
end

Hope that helps!

answered Jan 26, 2017 at 22:25

Konstantinos Kamaropoulos

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1 Comment

Liz Z Over a year ago

That would help but functionDoThings needs an array of doubles. Thanks though!

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