“printf 中的冗余参数”警告答案

【问题标题】："Redundant argument in printf" warning“printf 中的冗余参数”警告
【发布时间】：2021-07-13 20:11:26
【问题描述】：

为什么我在 Perl 中收到此错误“printf 中的冗余参数”？这表示格式字符串需要的值多于所包含的值。我提供的格式字符串对吗？

输入文件：

<?xml version='1.0'?>
<playlist>
  <movie id="tt0112384">
    <title>Apollo 13</title>
    <director>Ron Howard</director>
    <release-date>1995-06-30</release-date>
    <mpaa-rating>PG</mpaa-rating>
    <running-time>140</running-time>
    <genre>adventure</genre>
    <genre>drama</genre>
    <cast>
      <person name="Tom Hanks" role="Jim Lovell" />
      <person name="Bill Paxton" role="Fred Haise" />
      <person name="Kevin Bacon" role="Jack Swigert" />
      <person name="Gary Sinise" role="Ken Mattingly" />
      <person name="Ed Harris" role="Gene Kranz" />
    </cast>
    <imdb-info url="http://www.imdb.com/title/tt0112384/">
      <synopsis>
        NASA must devise a strategy to return Apollo 13 to Earth safely
        after the spacecraft undergoes massive internal damage putting
        the lives of the three astronauts on board in jeopardy.
      </synopsis>
      <score>7.6</score>
    </imdb-info>
  </movie>
  <movie id="tt0307479">
    <title>Solaris</title>
    <director>Steven Soderbergh</director>
    <release-date>2002-11-27</release-date>
    <mpaa-rating>PG-13</mpaa-rating>
    <running-time>99</running-time>
    <genre>drama</genre>
    <genre>mystery</genre>
    <genre>romance</genre>
    <cast>
      <person name="George Clooney" role="Chris Kelvin" />
      <person name="Natascha McElhone" role="Rheya" />
      <person name="Ulrich Tukur" role="Gibarian" />
    </cast>
    <imdb-info url="http://www.imdb.com/title/tt0307479/">
      <synopsis>
        A troubled psychologist is sent to investigate the crew of an
        isolated research station orbiting a bizarre planet.
      </synopsis>
      <score>6.2</score>
    </imdb-info>
  </movie>
</playlist>

我的代码：

use warnings;
use XML::LibXML;
my $filename = 'playlist.xml';
my $dom = XML::LibXML->load_xml(location => $filename);
printf("%-20s %-35s %-10s %-10s\n", "Title: ", "Director: ", "Rating: ", "Duration: ");
foreach my $movie ($dom->findnodes('//movie')) {
    printf("%-20s %-35s %-10s %-10s\n",   # <-- This is line 7
           $movie->findvalue('./title'), 
           $movie->findvalue('./director'), 
           $movie->findvalue('./mpaa-rating'), 
           $movie->findvalue('./running-time'), 
           " minutes");  
}

预期输出：

Title:          Director:       Rating:         Duration:
Apollo 13       Ron Howard      PG              140
Solaris         Steven Soderbergh PG-13           99
Ender's Game    Gavin Hood      PG-13           114
Interstellar    Christopher Nolan PG-13           169
The Martian     Ridley Scott    PG-13           144

获取输出：

Title:                    Director:                 Rating:                   Duration:
Redundant argument in printf at movie.pl line 7.
Apollo 13                 Ron Howard                PG                        140
Redundant argument in printf at movie.pl line 7.
Solaris                   Steven Soderbergh         PG-13                     99
Redundant argument in printf at movie.pl line 7.
Ender's Game              Gavin Hood                PG-13                     114
Redundant argument in printf at movie.pl line 7.
Interstellar              Christopher Nolan         PG-13                     169
.
.
.

为什么 Perl 会这样？

【问题讨论】：

标签： perl

【解决方案1】：

这个printf：

printf("%-20s %-35s %-10s %-10s\n", 
       $movie->findvalue('./title'), 
       $movie->findvalue('./director'), 
       $movie->findvalue('./mpaa-rating'), 
       $movie->findvalue('./running-time'), 
       " minutes");

需要 4 个参数，因为它包含 4 个%<something>。但是，您提供了 5 个参数。您可以将minutes 放在格式字符串中（并将其从参数中删除）：

printf("%-20s %-35s %-10s %-10s minutes\n", 
       $movie->findvalue('./title'), 
       $movie->findvalue('./director'), 
       $movie->findvalue('./mpaa-rating'), 
       $movie->findvalue('./running-time'));

或者，如果您希望 minutes 紧邻分钟数，请将其连接到 $movie->findvalue('./running-time')：

printf("%-20s %-35s %-10s %-10s\n", 
       $movie->findvalue('./title'), 
       $movie->findvalue('./director'), 
       $movie->findvalue('./mpaa-rating'), 
       $movie->findvalue('./running-time') . " minutes");

您可能会感到困惑的是，print 采用要打印的参数列表，而 printf 采用格式字符串和要在格式字符串中替换和格式化的参数列表。这样做会很好：

print($movie->findvalue('./title'), 
      $movie->findvalue('./director'), 
      $movie->findvalue('./mpaa-rating'), 
      $movie->findvalue('./running-time'), 
      " minutes\n");

（虽然这不会像你想要的那样格式化输出）

【讨论】：