我正在使用Perl运行树，然后使用内部节点作为运算符计算树的叶节点。我希望能够以后缀方式打印这个，并且我使用基本操作数很容易实现这一点（在调用父节点之前分别调用左侧和右侧节点）但是我无法生成所需的输出平均功能。我打印计算的实际结果没有任何问题，但我希望能够以后缀表示法打印运算符和操作数。
例如，1 +平均值（3,4,5）将显示为1; 3 4 5平均值+。
这是我的代码：
use strict;
use warnings;
use Data::Dumper;
$Data::Dumper::Indent = 0;
$Data::Dumper::Terse = 1;
my $debug = 0;
# an arithmetic expression tree is a reference to a list, which can
# be of two kinds as follows:
# [ 'leaf', value ]
# [ 'internal', operation, leftarg, rightarg ]
# Evaluate($ex) takes an arithmetic expression tree and returns its
# evaluated value.
sub Evaluate {
my ($ex) = @_;
$debug and
print "evaluating: ", Dumper($ex), "\n";
# the kind of node is given in the first element of the array
my $node_type = $ex->[0];
# if a leaf, then the value is a number
if ( $node_type eq 'leaf' ) {
# base case
my $value = $ex->[1];
$debug and
print "returning leaf: $value\n";
return $value;
}
# if not a leaf, then is internal,
if ( $node_type ne 'internal' ) {
die "Eval: Strange node type '$node_type' when evaluating tree";
}
# should now have an operation and two arguments
my $operation = $ex->[1];
my $left_ex = $ex->[2];
my $right_ex = $ex->[3];
# evaluate the left and right arguments;
my $left_value = Evaluate($left_ex);
my $right_value = Evaluate($right_ex);
# if any arguments are undefined, our value is undefined.
return undef unless
defined($left_value) and defined($right_value);
my $result;
# or do it explicitly for the required operators ...
if ($operation eq 'average') {
$result = ($left_value + $right_value) / 2;
}
if ($operation eq '+') {
$result = $left_value + $right_value;
} elsif ($operation eq '-') {
$result = $left_value - $right_value;
} elsif ($operation eq '*') {
$result = $left_value * $right_value;
} elsif ($operation eq 'div') {
if ($right_value != 0 ) {
$result = int ($left_value / $right_value);
} else {
$result = undef;
}
} elsif ($operation eq 'mod') {
$result = $left_value % $right_value;
} elsif ($operation eq '/') {
if ( $right_value != 0 ) {
$result = $left_value / $right_value;
}
else {
$result = undef;
}
}
$debug and
print "returning '$operation' on $left_value and $right_value result: $result\n";
return $result;
}
# Display($ex, $style) takes an arithmetic expression tree and a style
# parameter ('infix' or 'postfix') and returns a string that represents
# printable form of the expression in the given style.
sub Display {
my ($ex, $style) = @_;
# the kind of node is given in the first element of the array
my $node_type = $ex->[0];
# if a leaf, then the value is a number
if ( $node_type eq 'leaf' ) {
# base case
my $value = $ex->[1];
return $value;
}
# if not a leaf, then is internal,
if ( $node_type ne 'internal' ) {
die "Display: Strange node type '$node_type' when evaluating tree";
}
# should now have an operation and two arguments
my $operation = $ex->[1];
my $left_ex = $ex->[2];
my $right_ex = $ex->[3];
# evaluate the left and right arguments;
my $left_value = Display($left_ex, $style);
my $right_value = Display($right_ex, $style);
my $result;
if ($operation ne 'average') {
$result = "($left_value $operation $right_value) \n $left_value $right_value $operation";
} else {
$result = "($left_value $operation $right_value) \n $left_value $right_value $operation";
}
return $result;
}
# module end;
1;
这是一个测试：
use strict;
use warnings;
use Display;
use arith;
my $ex1 = [ 'leaf', 42];
my $ex2 = [ 'internal', '+', [ 'leaf', 42], [ 'leaf', 10 ] ];
my $ex3 = [ 'internal', 'average', $ex2, [ 'leaf', 1 ] ];
print "ex1 is ", Evaluate($ex1), "\n";
print "ex1: ", Display($ex1), "\n";
print "\n";
print "ex2 is ", Evaluate($ex2), "\n";
print "ex2: ", Display($ex2), "\n";
print "\n";
print "ex3 is ", Evaluate($ex3), "\n";
print "ex3: ", Display($ex3), "\n";
print "\n";
Display::Render(\$ex3);
为了做到这一点，我意识到我将不得不改变子程序“显示”，但我不知道如何获得输出 - ＆gt;价值; ＃表示未平均的值＃值平均值操作数等。
有什么想法吗？
答案 0 :(得分：2)
我不是100％确定我理解你的问题，但这是对你的两个功能的清理/改进：
my %ops = ( # dispatch table for operations
average => sub {my $acc; $acc += $_ for @_; $acc / @_},
'+' => sub {$_[0] + $_[1]},
'-' => sub {$_[0] - $_[1]},
'*' => sub {$_[0] * $_[1]},
'mod' => sub {$_[0] % $_[1]},
(map {$_ => sub {$_[1] ? $_[0] / $_[1] : undef}} qw (/ div)),
);
sub Evaluate {
my $ex = shift;
print "evaluating: ", Dumper($ex), "\n" if $debug;
my $node_type = $ex->[0];
if ( $node_type eq 'leaf' ) {
print "returning leaf: $$ex[1]\n" if $debug;
return $$ex[1];
}
elsif ( $node_type ne 'internal' ) {
die "Eval: Strange node type '$node_type' when evaluating tree";
}
my $operation = $ex->[1];
my @values = map {Evaluate($_)} @$ex[2 .. $#$ex];
defined or return for @values;
if (my $op = $ops{$operation}) {
return $op->(@values);
} else {
print "operation $operation not found\n";
return undef;
}
}
此处，大if/elsif块被替换为调度表。这允许您将逻辑与解析器分开。我还将$left_value和$right_value变量替换为@values数组，允许您的代码扩展为n-arity操作（如average）。
以下Display函数也已更新，以处理n-arity操作：
my %is_infix = map {$_ => 1} qw( * + / - );
sub Display {
my ($ex, $style) = @_;
my $node_type = $ex->[0];
# if a leaf, then the value is a number
if ( $node_type eq 'leaf' ) {
return $$ex[1];
}
# if not a leaf, then is internal,
if ( $node_type ne 'internal' ) {
die "Display: Strange node type '$node_type' when evaluating tree";
}
# should now have an operation and n arguments
my $operation = $ex->[1];
if ($style and $style eq 'infix') {
my @values = map {Display($_, $style)} @$ex[2 .. $#$ex];
if ($is_infix{$operation}) {
return "$values[0] $operation $values[1]"
} else {
local $" = ', '; # "
return "$operation( @values )"
}
} else { # postfix by default
my @out;
for (@$ex[2 .. $#$ex]) {
if (@out and $_->[0] eq 'internal') {
push @out, ';'
}
push @out, Display($_, $style)
}
return join ' ' => @out, $operation;
}
}
您可以将Display称为Display($tree)或Display($tree, 'postfix')作为后缀表示法。并且Display($tree, 'infix')用于中缀表示法。
ex1 is 42 ex1: 42 ex1: 42 ex2 is 52 ex2: 42 10 + ex2: 42 + 10 ex3 is 26.5 ex3: 42 10 + 1 average ex3: average( 42 + 10, 1 )
我相信你正在寻找的东西。
最后，使用您的第一个示例1 + average(3, 4, 5)：
my $avg = ['internal', 'average', [leaf => 3], [leaf => 4], [leaf => 5] ];
my $ex4 = ['internal', '+', [leaf => 1], $avg ];
print "ex4 is ", Evaluate($ex4), "\n";
print "ex4: ", Display($ex4), "\n";
print "ex4: ", Display($ex4, 'infix'), "\n";
print "\n";
打印：
ex4 is 5 ex4: 1 ; 3 4 5 average + ex4: 1 + average( 3, 4, 5 )
答案 1 :(得分：0)
也许试试AlgebraicToRPN？