Confusing problem about duration data

Hi,

I'm having a very confusing issue when I'm operating with duration data. I'm new to this data type. Here I debugged my code and simplify my question as an example. Please can someone point out the root reason of why this is not working. Basically, I first define a duration x and with this x, I want to create a new x1 with a random duration, like in the example. I describe my confusion as below. Can anyone help me out? I know it might be the problem of using rand but why?

x = duration(8,0,47);
>> x1 = x + rand*duration(0,0,10);
>> x1

x1 = 

  duration

   08:00:54

However, I realize though x1 is of type of duration, it does not equal to the duration 08:00:54.

   >> x1 == duration(8,0,54)

ans =

  logical

   0

Nevertheless, if I define a new x2 in a similar way, it works.

   >> x2 = x + duration(0,0,7);
>> x2

x2 = 

  duration

   08:00:54

>> x2 == duration(8,0,54)

ans =

  logical

   1

And x1 apparently doesn't equal to x2.

>> x1 == x2

ans =

  logical

   0

Answer 1

[Walter Roberson]

rand() produces a 53 bit random number in the range 0 to 1, effectively randi([1 2^53-1],1,1) / 2^53

When you multiply this by 10 seconds, it is very unlikely to happen to result in an exact multiple of 1 second.

This is not obvious because the default output for duration does not display fractions of a second.

x1.Format = 'hh:mm:ss.SSSSSSSSS';

[QIAO WANG]

That's great explanation. Thank you so much! The reason why I used rand was I wantted to generate random duration. Like what I showed in the question, I want a new variable of duration type that is based on an existing duration plus a random interval. What should I use? Do you have any suggestion?

[Walter Roberson]

That is what you are getting now. A random duration of 7.846832 seconds is still a random duration within 10 seconds. The default format for duration just was not showing the fraction.

Perhaps

x + seconds(randi(10))

Note that randi(10) will not produce 0 but will produce 10.

[QIAO WANG]

Thank you very much! All sorted now!