use strict;
use warnings;

my $score = 85;

if ($score >= 90) {
    print "优秀\n";
} elsif ($score >= 80) {
    print "良好\n";
} elsif ($score >= 60) {
    print "及格\n";
} else {
    print "不及格\n";
}

# unless 等价于 if (!condition)
my $password = "";

unless ($password) {
    die "密码不能为空！\n";
}

# 等价于
if (!$password) {
    die "密码不能为空！\n";
}

# Perl 没有三元运算符以外的"条件表达式"语法
# 但可以这样写：
my $max = ($a > $b) ? $a : $b;

# 或者使用 do 块
my $greeting = do {
    if ($hour < 12) { "早上好" }
    elsif ($hour < 18) { "下午好" }
    else { "晚上好" }
};

# C 风格 for 循环
for (my $i = 0; $i < 10; $i++) {
    print "$i ";
}
print "\n";   # 0 1 2 3 4 5 6 7 8 9

# 省略部分
for (my $i = 0; ; $i++) {   # 无限循环
    last if $i >= 5;         # last 相当于 break
    print "$i ";
}

# 多变量
for (my ($i, $j) = (0, 10); $i < $j; $i++, $j--) {
    print "i=$i j=$j\n";
}

# 遍历列表
my @fruits = qw(apple banana cherry);

foreach my $fruit (@fruits) {
    print "水果: $fruit\n";
}

# foreach 可以简写为 for
for my $fruit (@fruits) {
    print "水果: $fruit\n";
}

# 使用默认变量 $_
for (@fruits) {
    print "水果: $_\n";      # $_ 是默认变量
}

# 遍历哈希
my %info = (name => "Perl", year => 1987);
for my $key (sort keys %info) {
    printf "%-10s: %s\n", $key, $info{$key};
}

# 遍历范围
for my $i (1..10) {
    print "$i ";
}

# 遍历空列表不会执行循环体
for my $x () {
    print "这不会执行\n";
}

# while 循环
my $count = 0;
while ($count < 5) {
    print "$count ";
    $count++;
}
print "\n";

# until 等价于 while (!condition)
my $attempts = 0;
until ($attempts >= 3) {
    print "尝试第 $attempts 次\n";
    $attempts++;
}

# 读取文件（最常见的 while 用法）
open my $fh, '<', 'data.txt' or die $!;
while (my $line = <$fh>) {
    chomp $line;
    print "> $line\n";
}
close $fh;

# 条件修饰符
print "是成年人\n" if $age >= 18;
print "未成年\n"    if $age < 18;
die "密码为空\n"    unless $password;
warn "数值为零\n"   if $count == 0;

# 循环修饰符
print "$_\n" for @array;              # 遍历打印
print "$_\n" while <$fh>;             # 逐行打印
push @results, $_ for @input;         # 收集结果
do_something() until $done;           # 直到完成

# 语句修饰符（适合单行逻辑）
print "OK\n" if $success;

# 块语法（适合多行逻辑）
if ($success) {
    log("操作成功");
    send_notification();
    print "OK\n";
}

for my $i (1..100) {
    last if $i > 10;       # 当 $i > 10 时跳出循环
    print "$i ";
}
# 输出: 1 2 3 4 5 6 7 8 9 10

for my $i (1..10) {
    next if $i % 2 == 0;   # 跳过偶数
    print "$i ";
}
# 输出: 1 3 5 7 9

my $count = 0;
for my $i (1..5) {
    $count++;
    redo if $count < 3;    # 前两次 redo，count 继续增加
    print "i=$i count=$count\n";
}

my $i = 0;
while ($i < 5) {
    print "循环体: $i\n";
} continue {
    $i++;                   # continue 块在每次迭代后执行
}

OUTER: for my $i (1..5) {
    INNER: for my $j (1..5) {
        next OUTER if $j == 3;    # 跳到外层循环的下一次
        print "i=$i j=$j\n";
    }
}

# last 也可以用标签
OUTER: for my $i (1..5) {
    for my $j (1..5) {
        last OUTER if $i * $j > 10;  # 跳出外层循环
        print "$i*$j=", $i * $j, " ";
    }
}

# for/foreach 返回修改后的列表
my @doubled = map { $_ * 2 } 1..5;   # (2, 4, 6, 8, 10)

# while 循环的返回值
my @lines;
{
    open my $fh, '<', 'data.txt' or die $!;
    @lines = <$fh>;    # 列表上下文读取所有行
    close $fh;
}

use feature 'switch';   # Perl 5.10-5.34
# use experimental 'switch'; # Perl 5.38+

my $fruit = "apple";
given ($fruit) {
    when ("apple")  { print "苹果\n" }
    when ("banana") { print "香蕉\n" }
    default         { print "未知水果\n" }
}

my %handler = (
    apple  => sub { print "苹果\n" },
    banana => sub { print "香蕉\n" },
);

my $fruit = "apple";
if (exists $handler{$fruit}) {
    $handler{$fruit}->();
} else {
    print "未知水果\n";
}

#!/usr/bin/env perl
use strict;
use warnings;

# 过滤 CSV 数据：只保留金额 > 1000 的记录
sub filter_csv {
    my ($input_file, $output_file, $min_amount) = @_;

    open my $in,  '<', $input_file  or die "读取失败: $!\n";
    open my $out, '>', $output_file or die "写入失败: $!\n";

    my $header = <$in>;
    print $out $header;               # 写入表头

    my ($total, $filtered) = (0, 0);
    while (my $line = <$in>) {
        chomp $line;
        my @fields = split /,/, $line;

        $total++;
        if (@fields >= 3 && $fields[2] > $min_amount) {
            print $out "$line\n";
            $filtered++;
        }
    }

    close $in;
    close $out;

    return ($total, $filtered);
}

# 使用
my ($total, $filtered) = filter_csv("sales.csv", "filtered.csv", 1000);
print "共 $total 条记录，筛选出 $filtered 条\n";