I want to store the value generated by the for loop into a new variable so I can use it further in my code. How do I do it ?

Question

Vaibav Reddy 2022-8-13

0
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https://ww2.mathworks.cn/matlabcentral/answers/1778685-i-want-to-store-the-value-generated-by-the-for-loop-into-a-new-variable-so-i-can-use-it-further-in-m

评论： Walter Roberson 2022-8-13

采纳的回答： Walter Roberson

My code :

for iIdx = 1:4

for jIdx = 1:4

sum = iIdx + jIdx;

if rem(sum,2) == 1

num = 1;

else

num = 0;

end

fprintf("%d ",num);

end

fprintf("\n");

end

I created this matrix and I got my expected output. I just want to know how to store this value into a new variable. Can you help me with it. Thank you.

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Stephen23 2022-8-13

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

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Answer 1

Walter Roberson 2022-8-13

在 MATLAB Online 中打开

for iIdx = 1:4
  for jIdx = 1:4
      total = iIdx + jIdx;
      num(iIdx, jIdx) = rem(sum, 2);
  end
end

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Walter Roberson 2022-8-13

With the above code since you did not assign anything to num before this point,

When i and j are both 1, the 1 by 1 variable num is created.

When j increases to 2, num(1,2) is assigned to. num is 1 by 1 at that point, so matlab creates a new 1x2 vector and copies the existing 1x1 num into it and the new value as well. num grew inside the loop.

When (1,3) and (1,4) are assigned to, each time a new vector has to be allocated that is slightly bigger, the old content copied in, and the new value as well. This takes increasing time as more data is being copied each time.

When (2,1) is assigned to, with num already being 1 x 4, matlab has to expand to two rows. It allocates space for 2x4 filled with 0, and copies the existing 1x4 in and the new element as well.

When (2,2) is assigned to it turns out that num is already big enough to hold that and the new value can be stored immediately. Matlab does not need to grow the array again until (3,1) is assigned to, and then later (4,1). Each growing operation has an increasing cost.

Now imagine if ahead of time before the i j loops you had assigned something that was at least 4 x 4 to num, then no grow operations would be needed.