xiaoyafeng has asked for the wisdom of the Perl Monks concerning the following question:

Hi respected monks!

I've written a calculated script just for fun.Below is my code: 
print "Welcome to calculator script!!\n";
print "Please choose calculator range:\n";
print "-100 -- +100 => 1 (default)\n";
print "-10 -- +10 => 2 \n";
print "your choice:";
my $choice = <>;
chomp $choice;
my @operators = qw(+ - * /);
my ($value1,$value2,$true_answer,$answer,$time_cal,$operator);
die "error !!! please check $!\n" unless $choice == 1 or $choice == 2;
if ($choice == 1) {
$value1 = int rand(100);
$value2 = int rand(100);
}
else {
$value1 = int rand(10);
$value2 = int rand(10);
}
$operator = $operators[int rand(4)];
$true_answer = $value1 + $value2 if $operator eq $operators[0];
$true_answer = $value1 - $value2 if $operator eq $operators[1];
$true_answer = $value1 * $value2 if $operator eq $operators[2];
$true_answer = int($value1 / $value2) if $operator eq $operators[3];
print "$value1$operator$value2=";
$time_cal = time;
eval{
$answer = <>;
$time_cal = time - $time_cal;
die "you are wrong!!!!\n" unless $answer == $true_answer;
};
die "please input decimal number!!!!\n" if $@ ;
print "you spend $time_cal seconds!!!!\n";

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my question is: Could I change a string to a mathematics expression directly?So I needn't change (* / + -) to arithmetic operator one by one.

Thanks in advance! UPDATE: Thank liverpole,and correct some ridiculous mistakes.


I am trying to improve my English skills, if you see a mistake please feel free to reply or /msg me a correction

Replies are listed 'Best First'.
Re: string to expression
by liverpole (Monsignor) on Jan 10, 2007 at 02:49 UTC
    Hi xiaoyafeng,

    You could use a dispatch table.

    For example: 

    my $pdispatch = {
    '+' => sub { $_[0] + $_[1] },
    '-' => sub { $_[0] - $_[1] },
    '*' => sub { $_[0] * $_[1] },
    '/' => sub { $_[0] / $_[1] },
};

# and later ...
my $operator = qw(+ - * /)[int rand 4];
my $true_answer = $pdispatch->{$operator}->($value1, $value2);

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    The dispatch table $pdispatch is a hash reference, where each key evaluates to an anonymous subroutine to perform that operation on its two arguments.

    The operator $operator is randomly selected from a list of 4 operators, and $true_answer is assigned the results of the subroutine for the random operator, performed on its 2 arguments.

    s''(q.S:$/9=(T1';s;(..)(..);$..=substr+crypt($1,$2),2,3;eg;print$..$/
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Re: string to expression
by GrandFather (Saint) on Jan 10, 2007 at 03:05 UTC

    Maybe you just want eval? Consider:

    use strict;
use warnings;

my @operators = qw(+ - * /);
my $expression = (int rand(100)) . $operators[int rand(4)] . (int rand
+(100));
my $answer = eval $expression;
print "$expression = $answer";

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    Prints (for example):

    78+63 = 141

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    DWIM is Perl's answer to Gödel
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      Great!! eval looks like a best way! Thank you! I'm going to read perlfunc.

      btw.any other tutorial for eval?
      I am trying to improve my English skills, if you see a mistake please feel free to reply or /msg me a correction
[reply]
        Tutorial? eval EXPR runs Perl code. Any Perl code. 78+63 happens to be valid Perl. (For that reason, you'd be better with liverpole's dispatch table.)
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Re: string to expression
by mkirank (Chaplain) on Jan 10, 2007 at 03:35 UTC
    check overload 
    #!perl
use strict;
use warnings;

my $num = new SomeThing 57;
    $b=5+$num;
    print "b is $b \n";
    my $minus = $num - 10;
    print "minus is $minus \n";


package SomeThing;

 sub new { my $p = shift; bless [@_], $p }
use overload
    '+' => \&myadd,
    '-' => \&mysub;

sub myadd {
    my $self = shift;
    my $var = shift;
    return ($var + $self->[0]);
}

sub mysub {
    my $self = shift;
    my $var = shift;
    return ( $self->[0] - $var);
}
1;

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