Exponential values manipulation in perl - perl

Question

I have a select statement which return capacity as exponential value e.g.
Capacity=5.4835615662E+003
in Perl code
I am using a db2 database, and if I explicitly run a query in database it returns
5483.5615662
but when I use next select query when I use capacity value in condition it doesn't match
e.g. pseudo code is as below,
my $capacity = 'SELECT capacity FROM table';
# it returns $capacity = 5.4835615662E+003
my $result = "SELECT MEASUREMENT FROM TABLE WHERE CAPACITY = $capacity";
Here $capacity is 5.4835615662E+003, so it does not match any row in the table. It should be 5483.5615662.
How to convert exponential value to float without rounding off?

Answer 1

You are interpolating the value of $capacity into a string. Instead, you should use placeholders as in:
my $sth = $dbh->prepare(q{SELECT MEASUREMENT FROM TABLE WHERE CAPACITY=?});
$sth->execute($capacity);
It is hard to say if there are any other problems because the code snippets you provide don't really do anything.
It is likely that the number stored in the database is not exactly 5483.5615662 and that is just the displayed string when you query it.
If possible, I would recommend taking #Сухой27's advice and letting the database do the work for you:
SELECT MEASUREMENT FROM TABLE
WHERE CAPACITY = (SELECT CAPACITY FROM TABLE where ..?)
Alternatively, decide ahead of time how many digits past the decimal point really matter and use ROUND or similar functionality:
my $sth = $dbh->prepare(q{
SELECT MEASUREMENT FROM TABLE
WHERE ROUND(CAPACITY, 6)=ROUND(?, 6)
});
$sth->execute($capacity);

Answer 2

Please take a look at Why doesn't this sql query return any results comparing floating point numbers?
I'm concerned about the 5.4835615662+003 that you show in your question. That isn't a valid representation of a number, and it means just 5.4835615662 + 3. You need an E or an e before the exponent to use it as it is
There is also an issue with comparing floating-point values, whereby two numbers that are essentially equal may have a slightly different binary representation, and so will not compare as equal. If your value has been converted to a string (and that seems highly likely, as Perl will not use an exponent to display 5483.5615662 unless told to do so) and back again to floating point, then it is extremely unlikely to result in exactly the same value. Your comparisons will always fail
In Perl, and most other languages, a numeric values has no specific format. For example, if I run this
perl -E 'say 5.4835615662E+003'
I get the output
5483.5615662
showing that the two string representations are equivalent
It would help to see exactly how you got the value of $capacity from the database, because if it were a simple number then it wouldn't use the scientific representation. You would have to use sprintf to get what you have shown
SQL is the same and doesn't care about the format of the number as long as it's valid, so if you wrote
SELECT measurement FROM table WHERE capacity = 5.4835615662E+003
then you would get a result where capacity is exactly equal to that value. But since it has been trimmed to eleven significant digits, you are hugely unlikely to find the record that the value came from, unless it contains 5483.56156620000000000
Update
If I run
perl -MMath::Trig=pi -E 'for (0 .. 20) { $x = pi * 10**$_; say qq{$x}; }'
I get this result
3.14159265358979
31.4159265358979
314.159265358979
3141.59265358979
31415.9265358979
314159.265358979
3141592.65358979
31415926.5358979
314159265.358979
3141592653.58979
31415926535.8979
314159265358.979
3141592653589.79
31415926535897.9
314159265358979
3.14159265358979e+015
3.14159265358979e+016
3.14159265358979e+017
3.14159265358979e+018
3.14159265358979e+019
3.14159265358979e+020
So by default Perl won't resort to using scientific notation until the value reaches 1015. It clearly doesn't apply to 5483.5615662. Something has coerced the floating-point value in the question to a much less precise string in scientific notation. Comparing that for equality doesn't stand a chance of succeeding