On Sat, 22 Aug 2015 16:57:41 +0200, hw <h...@gartencenter-vaehning.de> wrote: > > > Am 22.08.2015 um 15:43 schrieb Alan McKinnon: > > On 22/08/2015 15:26, hw wrote: > >> > >> Hi, > >> > >> I have the following in a perl script: > >> > >> > >> if ($a != $b) { > >> print "e: '$a', t: '$b'\n"; > >> } > >> > >> > >> That will print: > >> > >> e: '69.99', t: '69.99' > >> > >> > >> When I replace != with ne (if ($a ne $a) {), it doesn't print. > >> > >> > >> Is that a bug or a feature? And if it's a feature, what's the explanation? > >> > >> And how do you deal with comparisions of variables when you get randomly > >> either correct results or wrong ones? It's randomly because this > >> statement checks multiple values in the script, and 69.99 is the only > >> number showing up yet which isn't numerically equal to itself (but equal > >> to itself when compared as strings). > > > > > > > > Computer languages have a much more exact idea of what equality means > > than you do. In your head (because you are human, not silicon) you are > > completely comfortable with taking "69.99" and treat8ing it as a string, > > or a number, or a mostly-rounded-off floating point number. > > > > The computer does not do it like that. To a computer, the same must be > > exactly the same. Two things a little bit different are completely > > different (or not equal). And perl has two different operators for > > (in)equality: > > > > != does a numerical comparison. More on this below > > ne does a string comparison. When viewed as a bunch of characters, 69.99 > > and 69.99 are identical. > > When the value is numerically not 69.99 but something like 69.99001, > then printing the value should print 69.99001 rather than 69.99.
To take your perl statement: perl -e 'printf("%34.32f\n", 23.33*3)' 69.98999999999999488409230252727866 It doesnt print that strange value because it rounds it to something "more readable", as the value is known to be not as precise. > > perl -e 'print 1/3 . "\n";' prints 0.333333333333333 > > perl -e 'printf("%34.32f\n", 1/3);' prints > 0.33333333333333331482961625624739 > > perl -e 'print (((1/3 == 0.333333333333333) ? "equal" : "not equal") . > "\n");' prints "not equal" > > perl -e 'print (((1/3 == 0.0.33333333333333331482961625624739) ? "equal" > : "not equal") . "\n");' prints "Integer overflow in decimal number at > -e line 1." a couple times typo ;) 1/3 == 0.0.333[...] Here it prints "equal". > > This is random, may it be predictable or not, and what's the integer here? > > > Now, your comparisons are NOT random. They are entirely predictable, as > > long as you know what is going on; you are running into floating point > > numbers. And as it turns out, computers never represent these things > > exactly (they are NOT integers). Even though they look identical > > on-screen, in RAM they will not be (this must be so for perl to do the > > print). Maybe they actually resolve to 69.990000001 and 69.99000000. You > > see them as close-as-dammit equal, perl sees them as entirely different. > > Why can't it print the number as it is, or at least as it is compared, > like it should? If it would, one could see at once what the problem is. > > > This is such as huge IT problem that many solutions have been proposed. > > You get classes like BigFloat that represent a floating point as an > > integer so that equality works, you can round the floats off before > > comparing them, or just make the things integers. > > > > The last one is nice: don't represent money as dollars and cents, > > represent it as cents or decicents and only divide by 100 (or 1000) when > > you finally get to display it. > > That would add quite a lot of complexity, and the problem should either > be handled transparently, or the value should be printed as the > software/computer sees it. It is a recipe for disaster when you tell > your computer to print something but it prints something else instead. > > > So how to fix your problem: you are doing what you shouldn't do - trying > > equality on floats. Turn them into integers, or round them off, or use > >> =/<= instead of != > > '=/<=' is not an operator in perl? >