Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>Because floating point arithmetic != real number arithmetic. An illustration of the difference due to imprecision is, for some floats <code>a</code> and <code>b</code>, <code>(a+b)-b != a</code>. This applies to any language using floats. </p> <p>Since <a href="http://en.wikipedia.org/wiki/Floating_point" rel="noreferrer">floating point</a> are binary numbers with finite precision, there's a finite amount of <a href="http://en.wikipedia.org/wiki/Floating_point#Representable_numbers.2C_conversion_and_rounding" rel="noreferrer">representable numbers</a>, which leads <a href="http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems" rel="noreferrer">accuracy problems</a> and surprises like this. Here's another interesting read: <a href="http://docs.sun.com/source/806-3568/ncg_goldberg.html" rel="noreferrer">What Every Computer Scientist Should Know About Floating-Point Arithmetic</a>. </p> <hr> <p>Back to your problem, basically there is no way to accurately represent 34.99 or 0.01 in binary (just like in decimal, 1/3 = 0.3333...), so approximations are used instead. To get around the problem, you can:</p> <ol> <li><p>Use <a href="http://php.net/manual/en/function.round.php" rel="noreferrer"><code>round($result, 2)</code></a> on the result to round it to 2 decimal places.</p></li> <li><p>Use integers. If that's currency, say US dollars, then store $35.00 as 3500 and $34.99 as 3499, then divide the result by 100.</p></li> </ol> <p>It's a pity that PHP doesn't have a decimal datatype like <a href="https://en.wikipedia.org/wiki/Decimal_data_type#Language_support" rel="noreferrer">other</a> <a href="http://docs.python.org/library/decimal.html" rel="noreferrer">languages</a> do.</p>
    singulars
    1. This table or related slice is empty.
    1. POPHP - Floating Number Precision
      singulars
      1. PTQuestion
    1. PTAnswer
    1. This table or related slice is empty.
    1. USNullUserException
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POPHP - Floating Number Precision
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COi would add that 0.01 also can't be represented as is. this should be marked as correct, because it gives explanation and how to fix. but for increasing usefulness of it please explain a bit why fp != real, with all that binary stuff and precision lose
      singulars
      1. PO
      1. USAndrey
    2. CO@irc Thanks. I incorporated bits of your comment into the answer
      singulars
      1. PO
      1. USNullUserException
    3. COOne pedantic note: There is a finite set of floats `a` and `b` where `(a+b)-b == a`. They simply need to have both a prime factor of 2, and be representable in the appropriate number of bits (about 7 decimal digits for single precision, 16 for double). So `a = 0.5` and `b = 0.25` works (and will always work for systems with 32 bit single precision floats). For floats that don't fit either or both of those preconditions, then `(a+b)-b != a`. But if both `a` and `b` fit those preconditions, then `(a+b)-b == a` should be true (but it's a finite set)...
      singulars
      1. PO
      1. USircmaxell
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload