Why is pre-allocating done?

In a program why is pre-allocating done?for example using arrays of zeros

ex- A=zeros(3)

Answer 1

[Yongjian Feng]

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

Answer 2

[Steven Lord]

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.

[S Priya]

Thank you @Steven Lord