How to save current time in file name

Hi,

since MATLAB function datestr is now not recommended, I wanted to try saving with datetime. Unfortunately I keep getting errors.

What I used before:

filename = ['output ' datestr(now,'yyyy-mm-dd_HH_MM_SS') '.mat'];
save(filename,'-v7.3')

What I tried now:

filename = ['output ' string(datetime('now')) '.mat'];
save(filename,'-v7.3')

I wanted it in the 'yyyy-mm-dd_HH_MM_SS' format, but it's not that important.

The error I keep getting is:

"Error using save.

Argument must be a text scalar."

I also tried different things (what was shown in HELP) such as these

filename = ['output ' string(datetime('now'), 'yyyy-MM-dd hh:mm:ss') '.mat'];
filename = ['output %d .mat', string(datetime('now'), 'yyyy-MM-dd hh:mm:ss')];

or to separate it with commas, but nothing I come up with works. Can you guys help?

Answer 1

[Stephen23]

You are trying to concatenate STRINGS... and that gives you STRING array with multiple elements, not a character vector nor a scalar string (either of which is what you need). Remember that STRING arrays and CHAR arrays have very very very very very different syntaxes and behaviors when concatenating!

To get a scalar string use + on other strings (not concatenation like you tried):

"output "+string(datetime('now'))+".mat" % string scalar

ans = "output 15-Jan-2024 12:15:55.mat"

OR to get a character vector use concatenation of other character vectors (but NO strings!):

['output ',char(datetime('now')),'.mat'] % char vector

ans = 'output 15-Jan-2024 12:15:55.mat'

But do not mix them up like you were doing!

Tip: if you are not sure what you need to use, then SPRINTF will always work for any input text (and will even coerce the DATETIME into text for you). Note that the output type is determined by the format type:

sprintf('%s', "output ", datetime('now'), ".mat") % char vector

ans = 'output 15-Jan-2024 12:15:55.mat'

sprintf("%s", "output ", datetime('now'), ".mat") % string scalar

ans = "output 15-Jan-2024 12:15:55.mat"

"I wanted it in the 'yyyy-mm-dd_HH_MM_SS' format, but it's not that important."

Of course it is important, it is easy too by simply specifying the Format with DATETIME:

"output "+string(datetime('now','Format','yyyy-MM-dd_HH_mm_ss'))+".mat"

ans = "output 2024-01-15_12_15_55.mat"

or exactly as you found in the STRING help:

"output "+string(datetime('now'),'yyyy-MM-dd_HH_mm_ss')+".mat"

ans = "output 2024-01-15_12_15_55.mat"

[Simon]

Thank you for your help. The code works now.

I guess 2 things happened:

1. I forgot that string and char concatenate differently.
2. I was confused by datestr because it didn't need + or , to be concatenated.

Once again thanks for thorough eplanation.