Float vs Decimal in ActiveRecord

Question

Sometimes  Activerecord data types confuse me  Err  often  One of my eternal questions is  for a given case      Should I use  decimal or  float    I ve often come across this link  ActiveRecord   decimal vs  float   but the answers aren t quite clear enough for me to be certain      I ve seen many threads where people recommend flat out to never use   float and always use decimal  I ve also seen suggestions by some   people to use float for scientific applications only    Here are some example cases    Geolocation latitude longitude  -45 756688  120 5777777      Ratio percentage  0 9  1 25  1 333  1 4143        I have used  decimal in the past  but I found dealing with BigDecimal objects in Ruby was unnecessarily awkward as compared to a float  I also know I can use  integer to represent money cents  for example  but it doesn t quite fit for other cases  for example when quantities in which precision could change over time    What are the advantages disadvantages of using each  What would be some good rules of thumb to know which type to use

User · Answer

In Rails 4 1 0  I have faced problem with saving latitude and longitude to MySql database  It can t save large fraction number with float data type  And I change the data type to decimal and working for me       def change     change column  cities   latitude   decimal   precision    15   scale    13     change column  cities   longitude   decimal   precision    15   scale    13   end

User · Answer

In Rails 3 2 18   decimal turns into  integer when using SQLServer  but it works fine in SQLite   Switching to  float solved this issue for us   The lesson learned is  always use homogeneous development and deployment databases

User · Answer

I remember my CompSci professor saying never to use floats for currency    The reason for that is how the IEEE specification defines floats in binary format  Basically  it stores sign  fraction and exponent to represent a Float  It s like a scientific notation for binary  something like  1 43 10 2   Because of that  it is impossible to store fractions and decimals in Float exactly   That s why there is a Decimal format  If you do this    irb 001 0 gt     47f     1 0 10    gt   0 10000000000000000555111512312578270211815834045    not  0 1     whereas if you just do  irb 002 0 gt   1 0 10  to s   gt   0 1    the interprer rounds the number for you   So if you are dealing with small fractions  like compounding interests  or maybe even geolocation  I would highly recommend Decimal format  since in decimal format 1 0 10 is exactly 0 1   However  it should be noted that despite being less accurate  floats are processed faster  Here s a benchmark   require  benchmark   require  bigdecimal    d   BigDecimal new 3   f   Float 3   time decimal   Benchmark measure   1  10000000  each    i  d   d      time float   Benchmark measure   1  10000000  each    i  f   f      puts time decimal     gt  6 770960 seconds  puts time float     gt  0 988070 seconds   Answer  Use float when you don t care about precision too much  For example  some scientific simulations and calculations only need up to 3 or 4 significant digits  This is useful in trading off accuracy for speed  Since they don t need precision as much as speed  they would use float   Use decimal if you are dealing with numbers that need to be precise and sum up to correct number  like compounding interests and money-related things   Remember  if you need precision  then you should always use decimal

[ruby-on-rails] Float vs Decimal in ActiveRecord

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