Asked by Zahra Khodabakhshi
on 30 Aug 2019

Hello

I have a matrix and I want to add new values on the rows which are the multiples of 3. Take the below as an example

A=[A(1); A(2);...;A(n)];

% I want to create Matrix B like below

B=[A(1);A(3);NaN;A(3);A(4);NaN;......;NaN;A(n-1);A(n)];

%how to creat matrix B

Thanks alot

Answer by Andrei Bobrov
on 31 Aug 2019

Accepted Answer

A = rand(11,6);

out = addedNansRows(A,3,4)

Here addedNansRows:

function out = addedNansRows(A,m,n)

% A - array

% m - namber of rows of A in group

% n - namber of nan in nan-group

s = size(A);

ii = 1:s(1);

iii = (ceil(ii/m)-1)*n;

out = nan([iii(end),0] + s);

out(ii + iii,:) = A;

end

Jon
on 3 Sep 2019

Zahra Khodabakhshi
on 3 Sep 2019

Dear Andrei Bobrov

Thanks a lot for your code. It really worked but I need to change m and n to get the results I want.

Sign in to comment.

Answer by Jos (10584)
on 30 Aug 2019

Despite its simple appearance, this is not a trivial task, for which I created my insertrows function

A = randperm(10).'

B = insertrows(A, NaN, 2:2:numel(A)-1)

INSERTROWS is available for free via the File Exchange: https://uk.mathworks.com/matlabcentral/fileexchange/9984-insertrows

Sign in to comment.

Answer by Jon
on 30 Aug 2019

Edited by Jon
on 31 Aug 2019

Just as another possibility, this approach is much less general than @Jos but will work for your special case.

I didn't look into how @Jos implements the insertrow function, maybe uses a similar idea, for actually doing the insertion. Anyhow this example should give you some good ideas about how to manipulate the indices.

% define example A matrix, just assign random numbers for example

A = rand(15,4)

% get some dimensions for later

[numRows,numCols] = size(A)

% make indices where rows will go in new matrix

idx = ones(numRows,1);

rownum = 1:numRows;

% put an extra row counter for the rows that are multiples of three

idx(rem(rownum,3)==0) = 2;

% calculate new row numbers by counting up row counters

idx = cumsum(idx);

% make new array B

B = NaN(max(idx),numCols);

% assign rows in B

B(idx,:) = A

You will also find some other approaches if you search MATLAB answers for insert rows for example this one answered by @Guillaume https://www.mathworks.com/matlabcentral/answers/225118-insert-rows-in-a-matrix

I do wonder though why there isn't a MATLAB command to insert rows into specific locations in a matrix. Seems like an issue people would run into with some frequency.

Zahra Khodabakhshi
on 3 Sep 2019

Dear Jon

Thanks for your help,But I think something is not working well with the code you privided.

I think the probem is in this section.Since the rownumber are from Matrix A, the NaN will be after each number which is multiples of three in matrix A.

% put an extra row counter for the rows that are multiples of three

idx(rem(rownum,3)==0) = 2;

So the result is :

B =

0.8843 0.1897 0.9900 0.6609

0.5880 0.4950 0.5277 0.7298

NaN NaN NaN NaN

0.1548 0.1476 0.4795 0.8908

0.1999 0.0550 0.8013 0.9823

0.4070 0.8507 0.2278 0.7690

NaN NaN NaN NaN

0.7487 0.5606 0.4981 0.5814

0.8256 0.9296 0.9009 0.9283

0.7900 0.6967 0.5747 0.5801

NaN NaN NaN NaN

0.3185 0.5828 0.8452 0.0170

0.5341 0.8154 0.7386 0.1209

0.0900 0.8790 0.5860 0.8627

NaN NaN NaN NaN

0.1117 0.9889 0.2467 0.4843

0.1363 0.0005 0.6664 0.8449

0.6787 0.8654 0.0835 0.2094

NaN NaN NaN NaN

0.4952 0.6126 0.6260 0.5523

Jon
on 4 Sep 2019

Sorry, I thought you wanted to insert the NaN's before the rows that are multiples of three. It wasn't clear from your initial description as I think you had a typo there. If instead you want to put the NaN's after the rows that are multiples of three, you could use the same approach, just modify the indices as follows:

% put an extra row counter for the row after each multiple of three

idx(rem(rownum-1,3)==0) = 2;

% calculate new row numbers by counting up row counters

% note need to shift count down by 1 as zero is a multiple of 3 and so a

% row would have been inserted before row 1

idx = cumsum(idx) -1;

In anycase, as you can see there are many ways to do the insertion. @Andrei Bobrov's approach which you accepted is quite clean.

Sign in to comment.

Opportunities for recent engineering grads.

Apply Today
## 1 Comment

## Walter Roberson (view profile)

## Direct link to this comment

https://se.mathworks.com/matlabcentral/answers/478329-add-new-rows-to-a-matrix#comment_740805

Sign in to comment.