MATLAB: Loops

homeworkloops

Hello,

I have the following problem in Matlab: Consider dataset a, with the following two columns:

0 / 0

1 / 0

1 / 9

0 / 0

-1 / 0

-1 / 4

0 / 0

My problem is that the value 9 has to be spread equally during the time that column 1 is equal to 1. The same has to be done with the value 4 during the time that column 1 is equal to -1. The result I have to obtain is the following:

0 / 0

1 / 3

0 / 0

-1 / 2

0 / 0

Thanks,

Pieter

Best Answer

Guessing this is homework, but not sure, so I will give you a solid kick in the right direction. Here is part of what you need to do, with a lot of assumptions about things you were not specific about.

A = [0 0; 0 0; 1 0; 1 0; 1 9; 0 0; -1 0; -1 4; 0 0];
indexToOnes = (A(:,1)==1);
numberOfOnes = sum(indexToOnes);
valueToDistribute = max(A(indexToOnes,2));
A(indexToOnes,2) = valueToDistribute/numberOfOnes

It does not handle the -1 part, which maybe you can figure out for yourself.

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MATLAB: How to select a single, random, non-zero array from a matrix

There are almost certainly ways to make this more efficient, but here is a conceptually straightforward way to do this:

% Starting matrix
M = [ 0  1  0;
      1  0  1;
      0  1  0;
      1  0  1];
numberElements = numel(M);
% You can run the following as many times as you want,
% to change a pair of 0/1.
indexToOnes = find(M);
indexToZeros = setxor(indexToOnes,1:numberElements);
indexChangeToZero = indexToOnes(randi(numel(indexToOnes)));
indexChangeToOne  = indexToZeros(randi(numel(indexToZeros)));
M(indexChangeToZero) = 0;
M(indexChangeToOne)  = 1;

MATLAB: Help with matrix manipulation

I'm usually harder than Steve is on these things, but I am pretty sure it is not homework.

You want to form the sum of those elements, then stuff them into a new last column. So, what does this:

array(:,8) = sum(cumsum(array(:,2:7),1),2);

do? If you are not sure, then take it apart, starting at the inside. That is, assuming the matrix is called array (I'm not feeling very creative), what is this:

array(:,2:7)

Then look at this sub-expression:

cumsum(array(:,2:7),1)

Wrap one more operation around that:

sum(cumsum(array(:,2:7),1),2)

Then, stuff it into the 8'th column of array, expanding that array in the process, to get:

array(:,8) = sum(cumsum(array(:,2:7),1),2);

Anyway, learn MATLAB. Stop thinking in MATLAB as if you are using a spreadsheet. And, yes, that is what you are doing, because you are naming the columns of your arrays with letters. If you stay in that place, then you will always think of MATLAB as just another spreadsheet.

Best Answer

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MATLAB: How to select a single, random, non-zero array from a matrix

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