How can I do polynomial fitting when X-axis is time

Hi ! I have a set of data that I want to do polynomial fitting, but my X-axis is duration type and the precision is up to milliseconds. I try to use seconds() to convert duration into numeric value but it ends up has many same value. And if I try to fit the curve directly it will give the following error "CLASSNAME input must be a valid numeric or logical class name". I wonder if there is any way to solve this problem

Any advise will be helpful, thanks!

data:

time value

'08:34:50.000' 20.11

'08:34:50.050' 20.12

'08:34:50.100' 20.11

'08:34:50.150' 20.00

[the cyclist]

It would be helpful if you uploaded your data in a mat file (using the paper clip icon), or at least a small representative sample.

What function are you using to do the fit?

Answer 1

[the cyclist]

Hm. Plotting the data you posted, using the seconds function, results in exactly what I would expect -- including capturing the milliseconds -- and suggests that the fit should work just fine.

time = duration(8,34,50,0:50:150)';
value = [20.11; 20.12; 20.11; 20.00];

tbl = table(time,value);

figure
plot(seconds(tbl.time),tbl.value,'.-')

[Lee YuChang]

Thank you sir, I find that I use the polyfit() function in a wrong way. And another question I want to ask is that when want to turn the seconds into duration again millisecond seems disappear. Could you give me some advice?

 duration(0,0,seconds(tbl.time)) 

the above method seems to lost the information about millisecond.

[the cyclist]

I think you may be confusing what is stored with what is displayed. To see this, use the modulus operation to remove multiples of 60 seconds from the time. Here are the original data.

>> s1 = seconds(tbl.time);
>> mod(s1,60)

ans =

   50.0000
   50.0500
   50.1000
   50.1500

Now, convert back to duration. (You claim that the milliseconds are lost.)

d2 =  duration(0,0,seconds(tbl.time));

But let's get the seconds again from the duration, and apply the modulus function again:

s2 = seconds(d2);
mod(s2,60)

ans =

   50.0000
   50.0500
   50.1000
   50.1500

The milliseconds are still there.

[Steven Lord]

The default display Format for duration arrays doesn't include fractional second information but you can specify a Format that does.

A = duration(0, 0, 0:0.5:2)
A.Format = 'hh:mm:ss.SS'