Confusing problem about duration data

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QIAO WANG 2019-4-12

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评论： QIAO WANG 2019-4-16

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

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Answer 1

Walter Roberson 2019-4-12

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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';

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Walter Roberson 2019-4-16

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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 2019-4-16