Reallocating a matrix based on a row of column numbers - works for some but not others?!

Hello all, I would really appreciate any help on a problem I'm having.

I have a matrix of data points for 0-360deg for 454 measured data points (C2plotdata3), and a matrix (C2mode1plotgeom) of x and y points giving me the location of each measurement points (2x454). I have also created a matrix called "reorder", which is a 1x425 matrix. My aim is to remap the points into a different arrangement. Reorder tells me the new location of each point value in two new matrices (C2plotgeom4 and C2mode1pointgeom2). For example, the first three values in reorder are 444,395,271, meaning that the data in column 1 of C2plotdata3 and C2mode1pointgeom should go to column 444 of the new matrices, the data in column 2 should go to column 395, 3 to 271, etc...

Variables:

C2plotdata3 (360x454)
C2plotgeom (2x454)
reorder (1x425)

NB Reorder is shorter than the others (425 rather than 454) because not all the original data points are to be remapped.

Code:

for y=1:size(reorder,2)
C2mode1plotgeom2(1,(reorder(1,y)))=C2mode1plotgeom(1,y)
C2mode1plotgeom2(2,(reorder(1,y)))=C2mode1plotgeom(2,y)
C2mode1plotdata4(:,(reorder(1,y)))=C2mode1plotdata3(:,y);
end

Problem:

This works for the point geometries (C2mode1plotgeom2 is correct), but doesn't appear to for the large matrix. C2plotgeom4 is created, but the values do not match (e.g. those in columns 444,395,and 271 do not match those in 1,2,3). I have tried removing this last line from the loop and entering a y value manually, and this does seem to work, but I can't get it to work in the loop.

Does anyone have any ideas? I suspect it will be something simple but this has cost me a few days now.

Thanks in advance, Stu

PS. I'm using 2017a

Answer 1

[Star Strider]

The problem is most likely due to using the loop.

Try something like this:

FirstMatrix_1 = randi(99, 4, 6)
SecondMatrix_1 = randi(99, 2, 6)
reorder = randperm(6)

FirstMatrix_2(:,reorder) = FirstMatrix_1
SecondMatrix_2(:,reorder) = SecondMatrix_1

FirstMatrix_1 =
    26    73    46    23    40    88
    80    49    96    49    37    91
     3    58    55    62    98    79
    92    24    52    68     4    10

SecondMatrix_1 =
    26    68    72    65    78    90
    34    14    11    49    71    89

reorder =
     4     3     1     6     2     5

FirstMatrix_2 =
    46    40    73    26    88    23
    96    37    49    80    91    49
    55    98    58     3    79    62
    52     4    24    92    10    68

SecondMatrix_2 =
    72    78    68    26    90    65
    11    71    14    34    89    49

Does this do what you want?

[Guillaume]

Stuart Walker's comment moved from Answer to here. (please use comment on this Answer rather than starting a new answer)

Hi Star Strider,

Thanks. Your method appears to be much simpler than using the loop so thanks for that, but something odd is still going on. I tried the following

testpart1=ones(360,454);
testreorder=zeros(1,454);
for b=1:455
testreorder(1,455-b)=b;
end

testpart2(:,reorder)=testpart1

So reorder is a row counting down from 454 to 1, and testpart1 is 454 columns with 360 rows of ones. So testpart2 should surely also be all ones? The columns will be in a different order, but we can't tell because they will all be 1 anyway. But it isn't: some columns are zero. A plot of the first row shows it:

I realise that using all ones doesn't allow us to see where the rearranged data is going. So I tried making testdata1 360 rows from 1 to 454 (i.e. column 1 is all 1s, column 2 all 2s, etc.), and testreorder remains a row counting down from 454 to 1. So this should give us a value of 454 for column 1 of testdata2, a value of 453 for column 2, 452 column 3, etc down to 1 in column 454. Again plotting the first row, I get:

So I'm a bit stuck again. Can you or anyone see what the heck is going on? Thanks again for your help.

Best, Stuart

[Guillaume]

It'd be helpful if you provided code that worked (the bounds of your for loop is invalid, your variables change name).

I don't have any issue reordering column:

testpart1 = repmat(1:454, 360, 1);
testreorder = 454:-1:1;   %much simpler than a for loop
testpart2 = zeros(size(testpart1));
testpart2(:, testreorder) = testpart1

[Stuart Walker]

Thanks Guillaume. That was my mistake, I wrote 455 instead of 454 when I typed the code. However, the other mistake was bigger - I was using 'reorder' (the original matrix I created yesterday) instead of 'testreorder', so my results and that rest of my post are incorrect. I would edit or delete my comment but this does not appear to be allowed.

Anyway, working in the same way you did, the rearrangement now works correctly with a test reordering matrix. It does not work with the reordering matrix I want to use, but this must now be the root of the problem so I will focus on this and try to find out what is really the problem.

Thanks to you and Star Strider for your help- apologies for wasting your time with my incorrect code.

Stuart

[Star Strider]

@Stuart — Our pleasure.

@Guillaume — Thank you very much for your help. (UTC-6 here, so asleep then.)