I want to name variables while writing into matfile in a for loop

I have a loop over dates and all the value(mostly cell arrays) comes out from this loop is going to be saved into matfile. But I want to name those variables by this dates I've been looping over. You can see this part of the code below.

m = matfile('myFile.mat','Writable',true);
d1=datenum(dateStart);
d2=datenum(dateEnd);
for Date = d1:d2
...
....
m.datestr(Date) = outCell;
end

but obviously that doesn't work. It says "To index into the new variable 'datestr', specify at least two dimensions. MatFile objects do not support linear indexing".

[Stephen23]

That is very bad data design. Using serial date numbers as an index:

1. will likely face numeric issues with the floating point numbers used.
2. is an inefficient waste of memory: consider that today (for example) has the serial date number 737401: this means that to store just one data value you would have to create an array with nearly one million elements in it!

Using serial date numbers as indices is tantamount to putting meta-data into variable names or fieldnames: meta-data is data, and so it should be stored as data, not as indices.

"But I want to name those variables by this dates I've been looping over"

That is very bad data design. Meta-data, such as your dates, should not be forced into variable names or fieldnames. Read this to know why:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Instead, you should keep your data in arrays and use efficient indexing. Simply storing the data in two arrays with corresponding elements would likely be the simple MATLAB solution:

dates = [D1,D2, ..., Dn]
data  = {C1,C2, ..., Cn}

Or you could use a table. Or a structure:

S(1).date = D1
S(1).data = C1
...
S(n).date = Dn
S(n).data = Cn

All of those would be easy to define in a loop, or at once using table, struct, etc.

[Omer Yasin Birey]

Thanks Stephen your answer is quite helpful. However, Im taking the dates as inputs from user. So, I only have 2 variables(about dates) which are start and end. The only thing that follows the dates, is the index of the loop. That's why it feels like it has to be dynamic.

Answer 1

[Walter Roberson]

Do the datestr(Date) and store the string into a variable . Then use dynamic field naming .

m.(date_string) = values

You do not really need to use aa temporary variable but it helps make code clearer .

[Omer Yasin Birey]

I tried this before however it gives that error

"Invalid field name: '02-Dec-2018'."

[Stephen23]

This is one reason why I wrote that "Meta-data, such as your dates, should not be forced into variable names or fieldnames": meta-data is not guaranteed to be a valid variable name or fieldname, so this approach is fragile at best, and totally buggy at worst.

Use either of the two solutions I gave in my earlier comment, using indexing.

[Walter Roberson]

use the optional second parameter to datestr to force aa format that is a valid MATLAB variable name such as requesting month as text first followed by the other parts with no delimiter or _ as the delimiter

[Jos (10584)]

I completely agree with Stephen here, and advise you not to use a variable name for the name of a variable, or for the name of a field of a struct. It is the contents of a variable that should change, not its name!

MyData(1).timestamp = '7-dec-2018'
MyData(1).value = 3 ;
MyData(2).timestamp = '12-nov-2019'
MyData(2).value = 8 ;

% or in a cell
MyData = {'7-dec-2018' 3 ; '12-nov-2019' ; 8}

is way easier to handle and understand than

MyData.dec072018 = 3
MyData.nov122019 = 8

Moreover, you retain the order in the first one. You can easily look for specific dates in the structure or cell arrays.

[Walter Roberson]

You could also consider using containers.Map for fast access .

[Walter Roberson]

Using this approach of dynamic field names is probably not the best, but it can have its uses, especially when there are external interface requirements.