in reply to Maintain accuracy of floating point numbers in simple arithmetic operation

You could avoid doing math all together and treat the problem as a string manipulation exercise: 

#! perl -slw
use strict;

while( <DATA> ) {
    chomp;
    printf "Before '%s' becomes ", $_;
    s[ ( \d+ )+ (?: \. ( \d{0,2} ) ( \d* ) )? % ]{
        local $^W;
        my $n = substr( $2 . '00', 0, 2 );
        ($1||'') . $n . ( length($3) ? ".$3" : '' );
    }xe;
    print "'$_'";
}

__DATA__
0.781063003540039%
2.25%
1.455%
4.9%
0.79%
75%
1.15%
0.45999999999999996%
0.9199999999999999%
0.22999999999999998%
1.3%

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Gives:

c:\test>junk4
Before '0.781063003540039%' becomes '78.1063003540039'
Before '2.25%' becomes '225'
Before '1.455%' becomes '145.5'
Before '4.9%' becomes '490'
Before '0.79%' becomes '79'
Before '75%' becomes '7500'
Before '1.15%' becomes '115'
Before '0.45999999999999996%' becomes '45.999999999999996'
Before '0.9199999999999999%' becomes '91.99999999999999'
Before '0.22999999999999998%' becomes '22.999999999999998'
Before '1.3%' becomes '130'

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Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
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Re^2: Maintain accuracy of floating point numbers in simple arithmetic operation
by OfficeLinebacker (Chaplain) on Jun 13, 2007 at 16:20 UTC
    ++Browser,

    I must admit your solution takes the form of a brain teaser for me. I'll limit my query to the first thing that I can't figure out off the top of my head, which is why the + after the first set of parens? Isn't it redundant, given the + after the \d within?

    In general, IMHO, I think an explanation of your solution would be helpful not only to me, but to others who may come across this thread, especially in the future, perhaps from a search.

    EDIT: I guess what I don't understand is

    • The first thing with the +
    • Why test for $1 when the assumption is that there is always a digit before the decimal (w/ your program, Before '.6789%' becomes '.678900')
      • Note, that assumption is OK, for this application.

    EDIT 2: I see now that the test is not for definedness, but for 0, because you don't want to make 0.779999% become 077.9999.

    I like computer programming because it's like Legos for the mind.
[reply]
      ... why the + after the first set of parens?

      Because I made a mistake and didn't notice because I was getting the right results. It is redundant and removing it makes no difference--'cept maybe it'll run a insy winsy bit quicker? :)

      There is a second 'artifact' in the replacement. ($1||'') can become just $1.

      I was attempting to avoid the need to suppress warnings, but it turns the code it a mess of bracketed ||s and ternaries; or a long if then else chain. Warnings are optional and turning them off, as an informed decision to avoid complicated code, is (IMO) legit. So I chose the cleaner option. You can replace local $^W; with no warnings 'uninitialised'; or the chain of defined and length tests if you prefer.

      Does this explain it? 

      #! perl -slw
use strict;

while( <DATA> ) {
    chomp;
    printf "Before '%s' becomes ", $_;
    s[
        ( \d+ )             ## Grab any digits before a decimal point
        (?:
            \.              ## If there is a decimal point
            ( \d{0,2} )     ## Try grab two digits after it
            ( \d* )         ## And any more into another capture
        )?                  ## All conditionally
        %                   ## And finally...
    ]{
        local $^W;          ## I know some of the captures will be emp
+ty.

        ## Pad (trailing zeros) the digits being moved to ensure exact
+ly 2
        my $n = substr( $2 . '00', 0, 2 );

        ## Reassemble the number from 
        ##      Any digits previously before the decimal point
        ##      The two digits being moved
        ##      A decimal point and the remaining digits if there are 
+any.
        $1 . $n . ( length($3) ? ".$3" : '' );
    }xe;
    print "'$_'";
}

__DATA__

      [download]
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]
[select]

      Updates after the fact will often not be noticed unless you /msg the intended recipient.

      I used your tests which did not include a '.nnn%' example so that possibility didn't get tested. If you were my boss, I would probably deserve admonishment.

      This, as with most posts, is intended to give you ideas for solving your problem, not a fully tested piece of code that you can drop into you codebase for free.

[reply]

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