matrix result storing problem

Question

Sayanta 2011-4-8

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Dear Experts,

Could please help me in this matrix result storing problem code

Here is one the prob I'm facing in matlab I want store all the 200 steps computation in V matrix . But when it computes it displays all 200 but only stores the last value in v matrix in matlab work space. I want store all the 200 values in V matrix.

here is the code

v = zeros(1,size(transFreq,1));

% which all the coloums with zeros and no of columns of transfreq = No of column v matrix ; transfreq is 5by5 Matrix so V matrix becomes : 0 0 0 0 0%

v(1,1)=1

 %And v = zeros(1,size(transFreq,1)) makes a matrix (1*5), v(1,5)=1 gives v = 0 0 0 0 1%

for j = 1:200 % gives number of iteration v = vP v=v*transFreq end

result v = 0.2628 0.2872 0.3028 0.1472 ( only this value is stored in matlab workspace in V )

many thanks in advance

Sayanta

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Answer 1

Friedrich 2011-4-8

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Hi Sayanta,

this is because you are overwriting the value of v in every step of the loop. If you want to store every value you have to store them explicit, e.g.

for j=1:100
v = v*P*v;
store(j) = v;
end

Please don't forget to preallocate memory for the store value first. You can also store all the values in v but you have to modify the loop a little bit

for j=1:100
v(j,:) = = v(j-1,:) * P * v(j-1,:);
end

This code will not work. You have to modify the dimensions and indexing to your own needs.

I hope I could help,

Friedrich

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Answer 2

Robert Cumming 2011-4-8

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it looks like you are overwriting your variable v in your loop. I dont really follow your code in the example above, and not knowing what your variables are, e.g. vP or trasFeq.

Anyway here is some code to show you how to populate an array in a loop:

transFreq = rand(200,1);
v = zeros(4,size(transFreq,1));% pre allocate your matrix
for j = 1:200 
  temp = rand(4,1);%vP??
  v(:,j)=transFreq(j).*temp;
end

You might be able to vectorise your depending on your variables.