create new matrix with special structure

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fatema saba
fatema saba 2015-1-11
评论： Stephen23 2015-1-11
I have a Matrix named p. the sixe of it is m*n (m=6, n=4)
I want to create new matrix based on matrix p with aid of following rule
of course my real Matrix has a large size (500*500*6) and this question is just an example.
I write some thing but it must repeat in columns with so called rule and also I must save it in special structure
and my simple code:
for j=0:2:m-1
newmatrix=[p(1+j,1) p(2+j,1)]

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Stephen23
Stephen23 2015-1-11
编辑：Stephen23 2015-1-11
Try this:
>> A = reshape(1:6*4,6,4); % define a fake data array of size 6x4
>> B = struct('data',reshape(num2cell(reshape(A(1:end-6),2,[]).',2),3,[]))
B =
3x3 struct array with fields:
data
>> B(1).data
ans =
1 2
>> B(9).data
ans =
17 18
  2 个评论
fatema saba
fatema saba 2015-1-11
Thank you, Thank you very much
It works exactly.
But is there any way that I can enter the last column od matrix A in computation?

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更多回答（2 个）

Yona
Yona 2015-1-11
编辑：Yona 2015-1-11
The simple way:
for j=0:2:m-1
for i=1:n
newmatrix(1+j,2*i-1:2*i)=p(j+[1 2],1);
end
end
  3 个评论
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fatema saba
fatema saba 2015-1-11
also I have special structure I mean when I call cell(2,1)from new matrix I get two value p(3,1) and p(4,1)
Or about cell(2,3)I get two value: p(3,3) and p(4,3)
Thank you
Yona
Yona 2015-1-11
编辑：Yona 2015-1-11
to remove the zeros you need to replace
newmatrix(1+j,2*i-1:2*i)
by
newmatrix(1+j/2,2*i-1:2*i)
but in not change the stracture. for this you have the first answer.

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Stephen23
Stephen23 2015-1-11
编辑：Stephen23 2015-1-11
Try this:
>> A = reshape(1:6*4,6,4); % define a fake data array of size 6x4
>> reshape([reshape(A(1:2:end-6),3,[]);reshape(A(2:2:end-6),3,[])],3,[])
ans =
1 2 7 8 13 14
3 4 9 10 15 16
5 6 11 12 17 18
This matches exactly with the sample output in your question.
Note that numel(output)/2 must be divisible by size(output,1) (in this example 18/2 = 9, which is divisible by 3 rows).
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fatema saba
fatema saba 2015-1-11
Excuse me, you pointed that using of loop is not good but, Is it possible to give me even some clue that I revise the way using loop?
Stephen23
Stephen23 2015-1-11
The main way to reduce the number of loops in MATLAB is to use a feature of many operations and functions that can operate on complete arrays in one go without looping. This is known as vectorization . All basic arithmetic operations can operate on complete arrays, as can many, many functions. You will need to read the documentation carefully for a function to know if this is the case, and how to use it. Luckily the documentation always has good examples too!
Often vectorized code is (sometimes quite a lot) faster then loops, and it can also look a lot more like their equivalent mathematical operations than they would in other programming languages.
For example, instead of looping over elements in an array and performing some operation, you can use one of the several different indexing options to select all array elements that match some criteria, process these simultaneously and then return the results.

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