Arithmetic evaluation: Difference between revisions
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ReadEvalPrintLoop
end.
=={{header|Perl}}==
<perl>sub ev
# Evaluates an arithmetic expression like "(1+3)*7" and returns
# its value.
{my $exp = shift;
# Delete all meaningless characters. (Scientific notation,
# infinity, and not-a-number aren't supported.)
$exp =~ tr {0-9.+-/*()} {}cd;
return ev_ast(astize($exp));}
{my $balanced_paren_regex;
$balanced_paren_regex = qr
{\( ( [^()]+ | (??{$balanced_paren_regex}) )+ \)}x;
# ??{ ... } interpolates lazily (only when necessary),
# permitting recursion to arbitrary depths.
sub astize
# Constructs an abstract syntax tree by recursively
# transforming textual arithmetic expressions into array
# references of the form [operator, left oprand, right oprand].
{my $exp = shift;
# If $exp is just a number, return it as-is.
$exp =~ /[^0-9.]/ or return $exp;
# If parentheses surround the entire expression, get rid of
# them.
$exp = substr($exp, 1, length($exp) - 2)
while $exp =~ /\A($balanced_paren_regex)\z/;
# Replace stuff in parentheses with placeholders.
my @paren_contents;
$exp =~ s {($balanced_paren_regex)}
{push(@paren_contents, $1);
"[p$#paren_contents]"}eg;
# Scan for operators in order of increasing precedence,
# preferring the rightmost.
$exp =~ m{(.+) ([+-]) (.+)}x or
$exp =~ m{(.+) ([*/]) (.+)}x or
# The expression must've been malformed somehow.
# (Note that unary minus isn't supported.)
die "Eh?: [$exp]\n";
my ($op, $lo, $ro) = ($2, $1, $3);
# Restore the parenthetical expressions.
s {\[p(\d+)\]} {($paren_contents[$1])}eg
foreach $lo, $ro;
# And recurse.
return [$op, astize($lo), astize($ro)];}}
{my %ops =
('+' => sub {$_[0] + $_[1]},
'-' => sub {$_[0] - $_[1]},
'*' => sub {$_[0] * $_[1]},
'/' => sub {$_[0] / $_[1]});
sub ev_ast
# Evaluates an abstract syntax tree of the form returned by
# &astize.
{my $ast = shift;
# If $ast is just a number, return it as-is.
ref $ast or return $ast;
# Otherwise, recurse.
my ($op, @operands) = @$ast;
$_ = ev_ast($_) foreach @operands;
return $ops{$op}->(@operands);}}</perl>
=={{header|Pop11}}==
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