Then permutations of these 4 numbers are also needed here.
>From: "Jonathan E. Paton"
>To: [EMAIL PROTECTED]
>Subject: Re: make a 24(another practical problem)
>Date: Fri, 18 Jan 2002 13:50:53 +0000 (GMT)
>
> >
> > 4 numbers (all from 1-9) are given, make a formula
> > (include +-*/()) for result 24. ex:
> >
> > 4, 2, 7, 5
> > (4-2)*(7+5) = 24
> >
> > shortest/quickest solution?
>
>This general problem can be solved using RPN style algebra,
>which is a postfix algebra.
>
>(4-2)*(7+4)
>
>becomes:
>
>4 2 - 7 4 + *
>
>which can be considered the same as:
>
>4 2-7 4+*
>
>This eliminates the difficulty with parathesis of normal
>algebra, and reduces the number of combinations you need to
>check. A simple RPN evaluator is:
>
>sub evalRPN {
> my (@stack, $_) = (undef, @_);
> my @op = $1 while ( /(+|-|\/|\*|\d+|[^0-9+-/*]+)/);
>
> foreach (@op) {
> if (scalar @stack == 2 && m/+|-|+|\//) {
> die "Empty stack"
> }
>
> if ( m'+'g) { push @stack, (pop()) + pop() }
> elsif ( m'-'g) { push @stack, (pop()) - pop() }
> elsif ( m'*'g) { push @stack, (pop()) * pop() }
> elsif ( m'/'g) { push @stack, (pop()) / pop() }
> elsif (/\d+/g) { push @stack, $_ }
> elsif (/\s+/g) { # Do nothing }
> else { die "Invalid operation: $_" }
> }
>
> my $size = scalar @stack;
>
> if (my $size-- = scalar @stack == 1) {
> return @stack[0];
> } else {
> die "$size elements too many remaining on stack!";
> }
>}
>
>Hope it works! (untested)
>
>Jonathan Paton
>
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