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Discussion on: Daily Challenge #10 - Calculator

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Dave Jacoby • 
#!/usr/bin/env perl

use strict ;
use warnings ;
use feature qw{ say postderef signatures state } ;
no warnings qw{ experimental::postderef experimental::signatures } ;

use Carp ;

my $string = "2 / 2 + 3 * 4 - 6" ;
say evaluate( $string ) ;
exit ;

sub evaluate( $string) {
        # multiplication
        while ( $string =~ m/(\-?\d+ \* \-?\d+)/ ) {
            my $before = $1 ;
            my ( $i, $j ) = $before =~ m{(\-?\d+)}g ;
            my $k = $i * $j ;
            $string =~ s/\Q$before/$k/mx ;
            }

        # division
        while ( $string =~ m/(\-?\d+ \/ \-?\d+)/ ) {
            my $before = $1 ;
            my ( $i, $j ) = $before =~ m{(\-?\d+)}g ;
            croak 'Divide by zero' if $j == 0;
            my $k = $i / $j ;
            $string =~ s/\Q$before/$k/mx ;
            }

        # addition
        while ( $string =~ m/(\-?\d+ \+ \-?\d+)/ ) {
            my $before = $1 ;
            my ( $i, $j ) = $before =~ m{(\-?\d+)}g ;
            my $k = $i + $j ;
            $string =~ s/\Q$before/$k/mx ;
            }

        # subtraction
        while ( $string =~ m/(\-?\d+ \- \-?\d+)/ ) {
            my $before = $1 ;
            my ( $i, $j ) = $before =~ m{(\-?\d+)}g ;
            my $k = $i - $j ;
            $string =~ s/\Q$before/$k/mx ;
            }

    return $string ;
    }
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jacoby profile image
Dave Jacoby •

And, I see now that I don't handle non-whole-numbers, which the problem doesn't give me, but could come along anyway. "5 / 4", for example.

And that's entirely a regex problem.

Hrm.

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Dave Jacoby • • Edited

Regex was -?\d+, which would handle whole numbers, but with division in the mix, you have to handle the possibility of decimals.

The regex became -?\d+(?:\.\d+)?. (?: indicates it's not matching, so we're not grabbing the fractional aspect independently, \.\d_ matches one dot and however many digits, and )? closes the thing and matches only if found, so that it doesn't have to be a floating point number.

#!/usr/bin/env perl

use strict ;
use warnings ;
use feature qw{ say postderef signatures state } ;
no warnings qw{ experimental::postderef experimental::signatures } ;

use Carp ;

my $string = "2 / 2 + 3 * 4 - 6" ;
say evaluate( $string ) ;
exit ;

# Remember MDAS
sub evaluate( $string) {
        while ( $string =~ m/(-?\d+(?:\.\d+)? \* -?\d+(?:\.\d+)?)/ ) {
            my $before = $1 ;
            my ( $i, $j ) = $before =~ m{(-?\d+(?:\.\d+)?)}g ;
            my $k = $i * $j ;
            $string =~ s/\Q$before/$k/mx ;
            }

        while ( $string =~ m/(-?\d+(?:\.\d+)? \/ -?\d+(?:\.\d+)?)/ ) {
            my $before = $1 ;
            my ( $i, $j ) = $before =~ m{(-?\d+(?:\.\d+)?)}g ;
            exit if $j == 0;
            my $k = $i / $j ;
            $string =~ s/\Q$before/$k/mx ;
            }

        while ( $string =~ m/(-?\d+(?:\.\d+)? \+ -?\d+(?:\.\d+)?)/ ) {
            my $before = $1 ;
            my ( $i, $j ) = $before =~ m{(-?\d+(?:\.\d+)?)}g ;
            my $k = $i + $j ;
            $string =~ s/\Q$before/$k/mx ;
            }

        while ( $string =~ m/(-?\d+(?:\.\d+)? \- \?\d+(?:\.\d+)?)/ ) {
            my $before = $1 ;
            my ( $i, $j ) = $before =~ m{(-?\d+(?:\.\d+)?)}g ;
            my $k = $i - $j ;
            $string =~ s/\Q$before/$k/mx ;
            }

    return $string ;
    }