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

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 =

     []

class(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 =

     []

class(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?

[Stephen23]

„Did I expect wrongly?“

Yes: nowhere does the STR2NUM documentation state that it will return anything other than a numeric class output.

“If not, why does this give incorrect (edit - different) output for 2 data types?“

The code is easy to look at: as DGM correctly pointed out, it’s internal checks explain what classes it can return (either documented or not). Take a look yourself.

As DGM commented, this seems to be a complicated, obfuscated, and ultimately flawed way to call EVAL. If you want to call EVAL, why not just call evil EVAL directly yourself?

„The task in hand for me was to generate an empty array corresponding to the class/datatype of the input.“

Some basic indexing can do that, where INP is your input array:

inp([])

[Dyuman Joshi]

"Yes: nowhere does the STR2NUM documentation state that it will return anything other than a numeric class output."

You are right about this, @Stephen23, but I was unsure about the relation of numeric class output to datetime() or struct() as MATLAB documentation states "Dates and Time" and "Structures" as separate Data type categories.

"as DGM correctly pointed out, it’s internal checks explain what classes it can return (either documented or not). Take a look yourself."

I was not aware of the internal checks of str2num, until DGM pointed it out. I agree, I should have done a better job at going through the documentation.

"As DGM commented, this seems to be a complicated, obfuscated, and ultimately flawed way to call EVAL. If you want to call EVAL, why not just call evil EVAL directly yourself?"

I was just experimenting until I arrived on a better solution. And I stumbled across this behaviour, which I was confused about, so I thought of getting a clarification regarding this.

And, I did arrive at the same solution you mentioned as well.

[Stephen23]

"I should have done a better job at going through the documentation."

Both DGM and I were referring to the code itself, not the documentation. Try this:

type str2num

[Dyuman Joshi]

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.

Answer 1

[VBBV]

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]

"The reason why it worked for other data types is that they all belong to or contain numbers except for char & string"

How does figure (or datetime) contain numbers?

f = figure;
e = [groot f gca];

class(e)

ans = 'matlab.graphics.Graphics'

out = str2num(sprintf('%s.empty', class(e)))

out = 
  0×0 empty Graphics array.

class(out)

ans = 'matlab.graphics.Graphics'

[DGM]

Before str2num() exits, it only performs two checks. If the call to eval() and returns a numeric output, then it exits with that. If the result is char/string/cell, it returns an empty double. If the results are anything else, it simply exits and returns whatever it has.

It seems like a roundabout way to use eval().

[Dyuman Joshi]

"If the result is char/string/cell, it returns an empty double."

Idk how I forgot to check for cell input, it returns output double for cell as well. Any particular reason why it has different behaviour/output for char/string/cell?

"It seems like a roundabout way to use eval()."

It kinda is. Though I'm thinking of another approach to solve this problem.

input = {'Batman'}

input = 1×1 cell array
    {'Batman'}

str = sprintf('%s.empty', class(input));
out = str2num(str)

out =

     []

[Stephen23]

„Any particular reason why it has different behaviour/output for char/string/cell?“

My guess is this ancient function contains lots of baggage due to its age: bad code and design decisions that have been inherited but never updated… you won‘t find the reasons for such quirky behavior by reading the MATLAB documentation.

[VBBV]

@Dyuman Joshi, Figure and datetime inherits Number property and are considered or treated as numeric by Matlab, If you read the documentation of figure it is clearly stated.

https://in.mathworks.com/help/matlab/ref/figure.html

figure(n) finds a figure in which the Number property is equal to n, and makes it the current figure. If no figure exists with that property value, MATLAB® creates a new figure and sets its Number property to n.

Hope this clarifies

[Dyuman Joshi]

@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() )

str2num('datetime')

ans = datetime
   21-May-2023 14:56:42

Also, Thank you @DGM, for the explaination!

Answer 2

[Steven Lord]

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

[Dyuman Joshi]

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?