Help: extremely strange inequality problem

IdahoEv

I have a problem with two variables, apparently containing equal and identical values, evaluating TRUE when compared with the != operator.

The simple situation is this:
$a is pulled from mysql database A. (where it is a FLOAT column)
$b is the sum of three items pulled from mysql database B.
(where all three are DECIMAL(10,2) columns)

Then a test is performed:
if ($a != $b)
{
// output reporting that the two entries are inconsistent
}

Now, both databases have 600+ rows. All of them have matching values (I have checked... the values are consistent. However, ONE of the rows - only one - is generating the output line. I've put in tons of debugging lines, tested variable types with is_numeric, etc., and only for this one entry does anything go wrong.

I'll start without posting the whole shebang of how the numbers are calculated, etc. But here's a bit of debugging code and it's output. In the example below, $Subtotal is $a as described above. $TotalExpected is $b as described above, and $BookNum is a key/row identifier common to both databases.

If I run this line for the entire set of data:

echo("$BookNum $TotalExpected $Subtotal "
	.(is_numeric($TotalExpected)?"TRUE":"FALSE")
	." "
	.(is_numeric($Subtotal)?"TRUE":"FALSE")
	." "
	.(($TotalExpected==$Subtotal)?"TRUE":"FALSE")
	."<br />");

I get this output (section from the middle of 600+) rows:

... snip ...
SB06A-013 6151.2 6151.2 TRUE TRUE TRUE
SB06A-014 6151.2 6151.2 TRUE TRUE TRUE
SB06A-015 6151.2 6151.2 TRUE TRUE TRUE
SB06A-016 6151.2 6151.2 TRUE TRUE TRUE
SB06A-017 6151.2 6151.2 TRUE TRUE TRUE
SB06A-018 6990 6990 TRUE TRUE TRUE
SB06A-019 6990 6990 TRUE TRUE TRUE
SB06B-001 7823 7823 TRUE TRUE FALSE
SB06B-002 6429.3 6429.3 TRUE TRUE TRUE
SB06B-003 6503.2 6503.2 TRUE TRUE TRUE
SB06B-004 6503.2 6503.2 TRUE TRUE TRUE
SB06B-005 6503.2 6503.2 TRUE TRUE TRUE
SB06B-006 6503.2 6503.2 TRUE TRUE TRUE
SB06B-007 6503.2 6503.2 TRUE TRUE TRUE
... snip ...

Note that SB06A-001 reports the inequality to be FALSE, but that the numbers output in string interpolation are the same, and is_numeric() returns true for both of them. All other entries in these databases report TRUE.

In a separate program I've compared all permutations of the four variables
$a=7823
$b="7823"
$c=7823.00

and all those variables resolve TRUE when compared with ==. Yet, my two variables print the same and are both numeric but fail ==. Only in that one case. Bizarre.

So, I'm at a loss! Any thoughts about what I should check next would be greatly appreciated. I can post more of the actual code if necessary. Thanks!

madwormer2

When comparing them, use (float)$Subtotal == (float)$TotalExpected

This will ensure that PHP does not do on-the-fly type conversion to something that you don't want, instead forces it to use the FLOAT type.

IdahoEv

Thanks for the tip, I didn't actually know you could do typecasts in PHP like that.

Sadly, it doesn't change the problem. Now the debug line looks like this:

echo("$BookNum  $TotalExpected  $Subtotal "
	.(is_numeric($TotalExpected)?"TRUE":"FALSE")
	." "
	.(is_numeric($Subtotal)?"TRUE":"FALSE")
	." "
	.((   ((float)$TotalExpected) == ((float)$Subtotal))?"TRUE":"FALSE")
	."<br />");

But the output is the same:

...
SB06A-018 6990 6990 TRUE TRUE TRUE
SB06A-019 6990 6990 TRUE TRUE TRUE
SB06B-001 7823 7823 TRUE TRUE FALSE
SB06B-002 6429.3 6429.3 TRUE TRUE TRUE
SB06B-003 6503.2 6503.2 TRUE TRUE TRUE
...

IdahoEv

Found the problem. One of the floats has a residual quantity that is too small to trigger display. I'm used to this kind of thing from javascript, but this is the first time I've seen it in PHP.

Changing the debug line to this:

echo("$BookNum  $TotalExpected  $Subtotal "
	.(is_numeric($TotalExpected)?"TRUE":"FALSE")
	." "
	.(is_numeric($Subtotal)?"TRUE":"FALSE")
	." "
	.((   ((float)$TotalExpected) == ((float)$Subtotal))?"TRUE":"FALSE")
	." "
	.($TotalExpected-$Subtotal));

gave me this output:

SB06A-018 6990 6990 TRUE TRUE TRUE 0
SB06A-019 6990 6990 TRUE TRUE TRUE 0
SB06B-001 7823 7823 TRUE TRUE FALSE -9.09494701773E-13
SB06B-002 6429.3 6429.3 TRUE TRUE TRUE 0
SB06B-003 6503.2 6503.2 TRUE TRUE TRUE 0

Roger_Ramjet

That is EXACTLY why I NEVER use approximate numeric data types. They are only needed by engineers and scientists who deal with extremely large or small numbers. The rest of us live in a decimal world where numbers have EXACT values, not approximate. Try adding 40 + 60 as doubles and then compare it to int 100, wont work cos 40+60 as doubles will return 99.999987 😕 In 30 years of programming I've only HAD to use floating point numbers once - for the rest of it I'm dealing with decimals (money) or integers.

Best thing you can do in php is to use the bcmath functions for your arithmetic.