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bitset error for uint64

Asked by Jason Palmer on 19 Jun 2019
Latest activity Commented on by Jason Palmer on 19 Jun 2019
There seems to be a problem with the bitset function for uint64. Here is a simple example:
>> d = 38124631952277504;
>> dec2bin(d)
ans =
'10000111011100100010011010011000101101010100000000000000'
>> dec2bin(d+1)
ans =
'10000111011100100010011010011000101101010100000000000000'
>> dec2bin(bitset(d,1,'uint64'))
ans =
'10000111011100100010011010011000101101010100000000000000'
>> dec2bin(bitset(d,2,'uint64'))
ans =
'10000111011100100010011010011000101101010100000000000000'
>> dec2bin(bitset(d,3,'uint64'))
ans =
'10000111011100100010011010011000101101010100000000000000'
>> dec2bin(bitset(d,4,'uint64'))
ans =
'10000111011100100010011010011000101101010100000000001000'
>> dec2bin(d+8)
ans =
'10000111011100100010011010011000101101010100000000001000'
So the bits only change successfully from 4th bit on, but fail to set fo the first three bits. This makes no sense that I can see and seems to be a serious error in Matlab.
Any help would be appreciated. Or advice on how to alert Matlab engineers.
Thanks,
Jason

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2 Answers

Answer by Utkarsh Belwal on 19 Jun 2019
Edited by Utkarsh Belwal on 19 Jun 2019
 Accepted Answer

Read the documentation of dec2bin, it is written that if number is greater than flintmax then it might not work properly. In your case d is greater than flintmax.

  2 Comments

Jason Palmer on 19 Jun 2019
Ok, thanks, should have read that. Just assumed it worked since it returns an answer.
Jason Palmer on 19 Jun 2019
Probably better to return error when d is greater than flintmax, since gambling on "may be correct" seems like it would never be an acceptable situation when you are dealing with bit setting.

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Steven Lord
Answer by Steven Lord
on 19 Jun 2019

>> d = 38124631952277504;
>> dec2bin(d)
ans =
'10000111011100100010011010011000101101010100000000000000'
>> dec2bin(d+1)
ans =
'10000111011100100010011010011000101101010100000000000000'
d is a double precision array, and the distance from d to the next largest representable number is greater than 1. In fact, it's 8.
>> eps(d)
ans =
8
>> isequal(d, d+1) % returns true
If you want to work with integer data that's this large, I recommend working with integer data rather than double. Make your d variable an uint64 from the beginning. This will also allow you to see the change you're making with each bitset call without needing dec2bin.
>> du = uint64(38124631952277504)
du =
uint64
38124631952277504
>> du1 = bitset(du, 1)
du1 =
uint64
38124631952277505
>> du2 = bitset(du, 2)
du2 =
uint64
38124631952277506
>> du3 = bitset(du, 3)
du3 =
uint64
38124631952277508
>> du4 = bitset(du, 4)
du4 =
uint64
38124631952277512
If you do need to check the bits explicitly, use bitget. It's vectorized so you can get all the bits at once. If you need it as a char vector, you can combine bitget with sprintf.
bitget(du, 64:-1:1)
bitget(du1, 64:-1:1)
sprintf('%d', bitget(du4, 64:-1:1))

  1 Comment

Jason Palmer on 19 Jun 2019
Ah, great, thanks for the explanation. Makes sense now. Cheers!

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