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Why is pre-allocating done?

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S Priya
S Priya 2021-6-30
评论： S Priya 2021-6-30
In a program why is pre-allocating done?for example using arrays of zeros
ex- A=zeros(3)

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Yongjian Feng
Yongjian Feng 2021-6-30
Hello Priya,
This is more like setting default values. Just in case, they are not set in some code paths, the default values will be there.
Thanks,
Yongjian

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Steven Lord
Steven Lord 2021-6-30
See this documentation page for a brief discussion of why one may want to preallocate an array.
Imagine you were building a home. Which would be preferable?
  1. Buy a lot large enough to build the entire home.
  2. Build the bathroom.
  3. Build the bedroom.
  4. Build the kitchen.
  5. etc.
or
  1. Buy a lot large enough to build the bathroom.
  2. Build the bathroom.
  3. Buy a new lot large enough to hold the bathroom and a bedroom.
  4. Move the bathroom to the new lot.
  5. Build the bedroom.
  6. Buy a new lot large enough to hold the bathroom, a bedroom, and a kitchen.
  7. Move the bathroom and the bedroom to the new lot.
  8. Build the kitchen.
  9. etc.
Moving a building is hard. Ideally you don't do it, or you do it as few times as you can.

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