problem with if cycle to determine the datetime

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Luca Tognetti 2023-2-3

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评论： Luca Tognetti 2023-2-8

hi, i'm using a function to retrieve weather data with API. I divided the function in two section, one where the outputs is the current weather and one for "past" weather. To decide which output the function has to print I made a simpole if cycle:

if fullDate == now
    ... %outputs current weather
else 
    ... %outputs historic weather
end

if I try to run the single part everything works perfecty, but adding this if cylce I get this error message using fullDate = now;

Numeric input data must be a matrix with three or six columns, or else three, six, or seven separate numeric arrays. You can also create datetimes from a single numeric array using the 'ConvertFrom' parameter.

the wierd thing is that if I continue to run the code (without touching it) after a few errors sometimes it runs without any problem.

I don't get where is the problem... thanks

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Luca Tognetti 2023-2-3

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reading the help I changed the time declaration with:

fullDate = datetime('now');

and consequently the if cycle becomes:

if fullDate == datetime('now')
    ... %outputs current weather
else 
    ... %outputs historic weather
end

now I'm not receiving that error anymore, but it seems like sometimes the if cycle macthes the time with the time input (which is "now") and most of the times it skips to the history part giving me wrong outputs. this means that the time input is not equal to "now" even if I set it...

Voss 2023-2-3

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datetime('now') returns the current date and time, so in general it returns a different thing each time you call it, because of the passage of time.

fullDate = datetime('now')
fullDate = datetime
   03-Feb-2023 17:42:50
pause(0.5)
fullDate = datetime('now')
fullDate = datetime
   03-Feb-2023 17:42:51

In other words, what 'now' means changes with time, as it should.

I don't have enough information about your code or what you're trying to do to advise you about what you should do instead.

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Answer 1

Les Beckham 2023-2-3

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编辑：Les Beckham 2023-2-3

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The time returned by datetime('now') will be changing as your code runs, so it seems almost impossible that your if condition will ever be met. Perhaps you should apply a tolerance or use a different condition.

Example:

t1 = datetime('now'),t2 = datetime('now')
t1 = datetime
   03-Feb-2023 17:43:49
t2 = datetime
   03-Feb-2023 17:43:49
isequal(t1, t2)
ans = logical
   0
t1.Second
ans = 49.7308
t2.Second
ans = 49.7767

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Stephen23 2023-2-8

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"idk if there's another method, but i had to convert the time in strings and compare them with 'cmpstr'."

The MATLAB approach would be to compare the difference against a tolerance, e.g.:

t1 = datetime('now');
t2 = datetime('now');
tol = minutes(1);
(t2-t1)<tol
ans = logical
   1

Luca Tognetti 2023-2-8

thanks! that's much clear

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