Table versus Cell Array to append a disp function that logs steps in a script file
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Hello, I am new to Matlab and was hoping to get some opinions on the best approach of creating a log file that will ultimately be inserted into SQL at the very end. Throughout the script file I have been writing disp which is great when you use the matlab compiler as it adds it to a log file. But you can't append to it and you can probably use fprintf to append. But, I decided to store in SQL instead. So the question. I would ideally like to create a table in Matlab with Columns defined and append values (will no longer use disp) to that table as the script is run. At the end of the script I will use runstoredprocedure to insert the values from the Matlab table into SQL table. Any ideas? Table or cell array?
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Walter Roberson
2017-4-27
0 个投票
Cell array should be more efficient, especially if you can pre-allocate the cell array to the maximum size. (If you know the maximum number of entries, then you can allocate that many cells even if you might not use them all, as you can remove the unused ones somewhat efficiently at the end.)
Even if you have to transform into a table at the end for the purpose of inserting into the database, the cell array should be more efficient.
If you have a new enough MATLAB (R2016b or later) you could also consider using string arrays. String arrays use less memory then cell arrays of strings do.
2 个评论
Mark Mcgrath
2017-4-27
Peter Perkins
2017-4-28
"Efficient" is kind of a broad term. It's difficult to give recommendations without knowing more about the specifics of what you have, but in general terms ...
If your data are mostly numeric, and tall not wide, the memory required by a table will be much smaller than that for a cell array, because the table stores each of its variables as one MATLAB array whereas a cell array stores each of its cells as one MATLAB array:
>> t = array2table(randn(1000,10)); >> c = table2cell(t); >> whos Name Size Bytes Class Attributes
c 1000x10 1200000 cell
t 1000x10 82952 table
If possible (and it sounds like it is possible), you want to preallocate the table and overwrite existing rows, rather than writing to end+1, and that's true anywhere in MATLAB.
Walter, I think what you're referring to is that the actual assignment for a row of a cell array is less expensive than assignment to a row of a table. True, but whether or not that matters depends on what else is going on. And the syntax would be essentially the same:
>> t = table([0;0;0],["";"";""])
t =
3×2 table
Var1 Var2
____ ____
0 ""
0 ""
0 ""
>> t(1,:) = {1 "one"};
>> t(2,:) = {2 "two"};
>> t(3,:) = {3 "three"};
>> t
t =
3×2 table
Var1 Var2
____ _______
1 "one"
2 "two"
3 "three"
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