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How to preallocate tables using the table-function???

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Hi Guys,
according to the Matlab documentation for the 'table' function, it should be possible to preallocate tables using the following syntax:
T = table('Size',sz,'VariableTypes',varTypes)
Does anyone know which version of Matlab is required to use this syntax?
When I'm trying to use it, either in Matlab R2015a or R2016b, I'm getting the following error message.
t = table('Size', [2 2], 'VariableTypes', {'double', 'string'})
Error using table (line 281)
Invalid parameter name: Size.
Caused by:
You may have intended to create a table with one row from one or more variables that are character strings. Consider using cell arrays of strings rather
than character arrays. Alternatively, create a cell array with one row, and convert that to a table using CELL2TABLE.

Accepted Answer

Philip Borghesani
Philip Borghesani on 17 Apr 2018

That syntax requires MATLAB R2018a. In most situations there is no need to pre-allocate a table. Just construct it from efficiently created columns of data. If we knew more about where your data was coming from we might suggest alternative allocation/creation methods.

Pre-allocation of complex MATLAB structures and objects causes much confusion and is often not needed. Pre-allocate simple things (like numeric matrices or the top level of a cell array structure). Compound objects contain any number of individual contiguous simple data structures and do not need pre-allocation in general.

Read this answer discussing object pre-allocation for way to much (most of it good and correct) information on pre-allocation.

  4 Comments
Philip Borghesani
Philip Borghesani on 8 Nov 2018
You are correct that the preallocation is not particularly slow,however it is wasted effort in the second example. The assignments completely replace the initially preallocated memory voiding the preallocation. There is no advantage to preallocating over just assigning the variables.
fprintf('\n 3. No preallocation of table : \n')
tic
t3 = table;
toc
fprintf('then do assignments without looping, calling randi 3 times: \n')
tic
t3.a2=randi(10,sz,1);
t3.b2=randi(10,sz,1);
t3.c2=randi(10,sz,1);
toc
2. preallocate table:
Elapsed time is 0.002449 seconds.
then do assignments without looping, calling randi 3 times:
Elapsed time is 0.002245 seconds.
3. No preallocation of table :
Elapsed time is 0.000350 seconds.
then do assignments without looping, calling randi 3 times:
Elapsed time is 0.002473 seconds.

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