Inconsisent(?) behaviour of str2num() with a particular usage

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The task in hand for me was to generate an empty array corresponding to the class/datatype of the input.
%Example 1
input = 'string';
output = char.empty
output = 0×0 empty char array
%Example 2
input = single(rand);
output = single.empty
output = 0×0 empty single matrix
My approach to this was to obtain the class of the input and use it with str2num -
input1 = "MATLAB";
str1 = sprintf('%s.empty', class(input1));
out1 = str2num(str1)
out1 = []
ans = 'double'
As you can see above, that the output is an empty array of double class and not of string class (as expected by me). This also occurs with char data-type
input2 = 'Batman'
input2 = 'Batman'
str2 = sprintf('%s.empty', class(input2));
out2 = str2num(str2)
out2 = []
ans = 'double'
I have checked for these datatypes - single, double, all integer classes, figure, datetime, duration, calendarDuration, timeseries, string and char. It works for all the datatyes except for the last two.
I expected the code to work for all classes.
My questions are - Did I expect wrongly?
If not, why does this give incorrect (edit - different) output for 2 data types?
Dyuman Joshi
Dyuman Joshi on 22 May 2023
Edited: Dyuman Joshi on 22 May 2023
Okay, type func returns the content of a function in text form. (I use "open func" for that, which directly opens the function file)
But yes, it can be seen from there, that for string/char/cell, it returns empty double.
Once again, thank you for your input.

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Accepted Answer

VBBV on 21 May 2023
Edited: VBBV on 21 May 2023
The function str2num can be used to convert the strings that inherently contain numbers to double class.
In your case there are no numbers in the outputs from sprintf function. Therefore, it results in empty [] which belongs to double class by default.
The reason why it worked for other data types is that they all belong to or contain numbers except for char & string
Hope this clarifies
Dyuman Joshi
Dyuman Joshi on 21 May 2023
Edited: Dyuman Joshi on 21 May 2023
@Stephen23, thanks for your input. I guess there is no point of pondering over this.
@VBBV, thanks for the info. Yes, it does clarifies.
There seems to be an internal call/reference to Number property while calling datetime() as well (although it is not explicitly mentioned in the documentation of datetime() )
ans = datetime
21-May-2023 14:56:42
Also, Thank you @DGM, for the explaination!

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More Answers (1)

Steven Lord
Steven Lord on 22 May 2023
If you want to evaluate the Static empty method of your class, don't use eval or str2num. Use feval or str2func.
classname = "string";
result = feval(classname + ".empty", 2, 0)
result = 2×0 empty string array
f = str2func(classname + ".empty");
result = f(0, 5)
result = 0×5 empty string array
  1 Comment
Dyuman Joshi
Dyuman Joshi on 22 May 2023
Unfortunately (or maybe fortunately), both the functions you have mentioned are not allowed on Cody.
Nonetheless, Thank you for the info Steven, always happy to learn something new.
I am inclined to belive feval() calls eval() internally. Would that be correct?

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