Iam trying to compare two floating point numbers and perl complains that they are not equal even though they are equal.

Try this sample code 

#!/usr/local/bin/perl -w

use strict;

my ( $number, $premium, $expected );

$number   = 1.80;
$premium  = $number * ( 1 + 10/100 );  # 1.8 + 10% of 1.8
$expected = 1.98;  # As we know 1.8 + 10% of 1.8 is 1.98
 
print "Number 1 : $premium  \n";
print "Number 2 : $expected \n";
print "Not" if ( $expected != $premium );
print "Equal !! ";

[download]

The output is 

Number 1 : 1.98  
Number 2 : 1.98 
NotEqual !!

[download]

Well, by now you should be thinking perl is crazy. Let me explain what happens here.

The floating point numbers are stored in binary format in the computer and even though 10/100 = 0.1 is a finite decimal in base 10 arithmetic, when converted to binary floating point it has to be rounded off at some point. Hence when it is converted back to decimal we will get 0.999999 or .1000001 and not 0.1 as we would expect. So comparing floating point numbers for equality wont give the correct results.

But when I printed the numbers it was showing properly ?

This is because while printing the numbers they are rounded off and hence we saw the same numbers even though their internal representation varied by a small fraction. <b Solution

While comparing floating point numbers we will have to test for range and not for equality. 

;# Sub  : isEqualFloat
;# Desc : to compare two floating point numbers and find out if 
;#        they are equal
;#
;# Args : float1, float2, delta value(optinal) or 0.00001
;# 
;# Returns : True if they are apart by the delta value, false otherwis
+e
;#------------------------------------------------------------

sub isEqualFloat{
  my ( $float1, $float2, $delta ) = @_;
  
  $delta ||= 0.00001;  # default value of delta
  
  return ( abs ( $float1 - $float2 ) < $delta )  
}

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call this function as 

if (  isEqualFloat(1.98, ( 1.8 * (1 + 10/100) )) ) {
  # do something 
  ...
}

# for high precision comparison
if (  isEqualFloat(1.98, ( 1.8 * (1 + 10/100) ), 0.0000001) ) {
  # do something 
  ...
}

[download]

References

  • What Every Scientist Should Know About Floating-Point Arithmetic for more details on Floating point arithmetic and the internal representation.

    -T

    Originally posted as a Categorized Question.

    In reply to My floating point comparison does not work. Why ? by thens

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