Andrew Louie wrote:
Hello,
I've run into a strange adding problem with perl:
when incrementing by 0.1 i get a strange behavior where at some
arbitrary number, it will append 0.9999999999 to the end of the
number.
test case:
------------------------------------------------------------------------------------
my $start = 0;
my $interval = 0.1;
my $end = 10;
do{ print "start: $start\n"; $start += $interval;}until($start >= $end);
------------------------------------------------------------------------------------
Results:
...
start: 4.5
start: 4.6
start: 4.7
start: 4.8
start: 4.9
start: 5
start: 5.1
start: 5.2
start: 5.3
start: 5.4
start: 5.5
start: 5.6
start: 5.7
start: 5.8
start: 5.9
start: 5.99999999999999
start: 6.09999999999999
start: 6.19999999999999
start: 6.29999999999999
start: 6.39999999999999
start: 6.49999999999999
start: 6.59999999999999
start: 6.69999999999999
start: 6.79999999999999
start: 6.89999999999999
------------------------------------------------------------------------------------
why all of suddun i get 5.999999999?
Because binary floating-point numbers can't represent decimal values
like 0.1 exactly.
Is this a perl problem?
No, it's not a Cygwin problem either. It's a floating-point problem. We
have to deal with such queries all the time in the Python community:
http://www.python.org/doc/faq/general/#why-am-i-getting-strange-results-with-simple-arithmetic-operations
[...]
The correct way out of this is to use integer variables to control the
loop and calculate the start value each time around.
regards
Steve
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