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@B73W56H84

存在しないキーの値をインクリメントすると

Last updated at Posted at 2016-10-25

こんばんは:whale2:
各言語で辞書[キー]+=1のような処理を書くサンプルと、その処理で「そのようなキーはありません」といった旨のメッセージが出る言語についてどうすればよいのか。(伝わりづらい)

環境

実行環境は以下のとおりです。

環境
# コンピュータ
$ sw_vers
ProductName:	Mac OS X
ProductVersion:	10.12.1
BuildVersion:	16B2555

# Ruby
$ ruby -v
ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-darwin16]

# Perl
$ perl -v | grep v5
This is perl 5, version 24, subversion 0 (v5.24.0) built for darwin-thread-multi-2level

# Perl6
$ perl6 -v
This is Rakudo version 2016.07.1 built on MoarVM version 2016.07
implementing Perl 6.c.

# Python
$ python --version
Python 3.5.1 :: Anaconda 4.1.0 (x86_64)

# PHP5
$ php -v
PHP 5.6.25 (cli) (built: Sep  6 2016 16:37:16)
Copyright (c) 1997-2016 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2016 Zend Technologies

Ruby

エラーになるパターン
エラー
target = %w[a a b b c 0 1 1 1]
count = {}

target.each{|var|
  count[ var ] += 1
}

puts count
=begin
# => エラー
inc.rb:5:in `block in <main>': undefined method `+' for nil:NilClass (NoMethodError)
	from inc.rb:4:in `each'
	from inc.rb:4:in `<main>'
=end
回避策

Rubyきれい。

inc.rb
target = %w[a a b b c 0 1 1 1]
count = Hash.new(0) # 初期値を設定

target.each{|var|
  count[ var ] += 1
}

puts count #=> {"a"=>2, "b"=>2, "c"=>1, "0"=>1, "1"=>3}
inc2.rb
target = %w[a a b b c 0 1 1 1]
count = {}
count.default = 0 # 初期値を設定

target.each{|var|
  count[ var ] += 1
}

puts count #=> {"a"=>2, "b"=>2, "c"=>1, "0"=>1, "1"=>3}

Python

エラーになるパターン
エラー
target = 'a a b b c 0 1 1 1'.split(' ')
count = {}

for var in target:
    count[ var ] += 1

print( count )
'''
# => エラー
Traceback (most recent call last):
  File "inc.py", line 6, in <module>
    count[ var ] += 1
KeyError: 'a'
'''
回避策1

Pythonはもっと良い方法がありそう。

inc.py
target = 'a a b b c 0 1 1 1'.split(' ')
count = {}

for var in target:
    count.setdefault( var, 0 )
    count[ var ] += 1

print( count ) #=> {'a': 2, '1': 3, '0': 1, 'c': 1, 'b': 2}
回避策2

defaultdict() を用いる方法。
コメント欄で@mpywさんに教えていただきました。

:inc2.py
from collections import defaultdict

target = 'a a b b c 0 1 1 1'.split(' ')
count = defaultdict(int)

for var in target:
    count[ var ] += 1

print( count ) #=> defaultdict(<class 'int'>, {'b': 2, '1': 3, '0': 1, 'a': 2, 'c': 1})
回避策3

例外処理を書く方法。どの言語にも適用できそう。素敵。

inc3.py
target = 'a a b b c 0 1 1 1'.split(' ')
count = {}

for var in target:
    try:
        count[ var ] += 1
    except KeyError:
        count[ var ] = 1

print( count )

PHP

エラーになるパターン

エラーではないもののUndefined indexなNoticeメッセージが出る。
error_reporting( -1 );を書かなければ出ない

Notice
<?php
error_reporting( -1 );

$target = explode(' ', 'a a b b c 0 1 1 1');
$count = [];

foreach( $target as $var ){
  $count[ $var ] += 1;
}

echo json_encode( $count ) . PHP_EOL ;
/*
# =>
Notice: Undefined index: a in /Users/yuhei/inc.php on line 8

Notice: Undefined index: b in /Users/yuhei/inc.php on line 8

Notice: Undefined index: c in /Users/yuhei/inc.php on line 8

Notice: Undefined offset: 0 in /Users/yuhei/inc.php on line 8

Notice: Undefined offset: 1 in /Users/yuhei/inc.php on line 8
{"a":2,"b":2,"c":1,"0":1,"1":3}
*/
回避策1(非推奨)

エラーを抑止してはいけない(戒め)

非推奨
<?php
error_reporting( -1 );

$target = explode(' ', 'a a b b c 0 1 1 1');
$count = [];

foreach( $target as $var ){
  @$count[ $var ] += 1; # エラー抑止
}

echo json_encode( $count ) . PHP_EOL ; #=> {"a":2,"b":2,"c":1,"0":1,"1":3}
回避策2

キーがない場合1を代入。という条件分岐を追加。

isset()
<?php
error_reporting( -1 );

$target = explode(' ', 'a a b b c 0 1 1 1');
$count = [];

foreach( $target as $var ){
  if( isset( $count[ $var ] ) ){
    $count[ $var ] += 1;
  }
  else{
    $count[ $var ] = 1;
  }
}

echo json_encode( $count ) . PHP_EOL ; #=> {"a":2,"b":2,"c":1,"0":1,"1":3}
回避策3

この例題については組み込み関数 array_count_values() のが良いと思う。

array_count_values()
<?php
error_reporting( -1 );

$target = explode(' ', 'a a b b c 0 1 1 1');

$count = array_count_values( $target );

echo json_encode( $count ) . PHP_EOL ; #=> {"a":2,"b":2,"c":1,"0":1,"1":3}
備考

array_count_values() の中でもキーがなければ1を代入。キーがあればインクリメント。という処理が行われている様子。
( キーが数値か文字列であることもチェックしていて素敵滅法 )

php-5.6.27/ext/standard/array.c
/* {{{ proto array array_count_values(array input)
   Return the value as key and the frequency of that value in input as value */
PHP_FUNCTION(array_count_values)
{
        zval    *input,                 /* Input array */
                        **entry,                /* An entry in the input array */
                        **tmp;
        HashTable *myht;
        HashPosition pos;

        if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "a", &input) == FAILURE) {
                return;
        }

        /* Initialize return array */
        array_init(return_value);

        /* Go through input array and add values to the return array */
        myht = Z_ARRVAL_P(input);
        zend_hash_internal_pointer_reset_ex(myht, &pos);
        while (zend_hash_get_current_data_ex(myht, (void **)&entry, &pos) == SUCCESS) {
                if (Z_TYPE_PP(entry) == IS_LONG) {
                        if (zend_hash_index_find(Z_ARRVAL_P(return_value), Z_LVAL_PP(entry), (void **)&tmp) == FAILURE) {
                                zval *data;
                                MAKE_STD_ZVAL(data);
                                ZVAL_LONG(data, 1);
                                zend_hash_index_update(Z_ARRVAL_P(return_value), Z_LVAL_PP(entry), &data, sizeof(data), NULL);
                        } else {
                                Z_LVAL_PP(tmp)++;
                        }
                } else if (Z_TYPE_PP(entry) == IS_STRING) {
                        if (zend_symtable_find(Z_ARRVAL_P(return_value), Z_STRVAL_PP(entry), Z_STRLEN_PP(entry) + 1, (void**)&tmp) == FAILURE) {
                                zval *data;
                                MAKE_STD_ZVAL(data);
                                ZVAL_LONG(data, 1);
                                zend_symtable_update(Z_ARRVAL_P(return_value), Z_STRVAL_PP(entry), Z_STRLEN_PP(entry) + 1, &data, sizeof(data), NULL);
                        } else {
                                Z_LVAL_PP(tmp)++;
                        }
                } else {
                        php_error_docref(NULL TSRMLS_CC, E_WARNING, "Can only count STRING and INTEGER values!");
                }

                zend_hash_move_forward_ex(myht, &pos);
        }
}
/* }}} */

Perl

エラーにならないPerl最高。

inc.pl
use v5.24;
use warnings;
use Data::Dumper;

my @target = qw[a a b b c 0 1 1 1];
my %count;

for my $var ( @target ){
  $count{ $var } += 1;
}

print Dumper \%count;
__DATA__
# =>
$VAR1 = {
          'c' => 1,
          '1' => 3,
          'b' => 2,
          'a' => 2,
          '0' => 1
        };

Perl6

エラーにならないPerl6最(ry

inc.pl6
use v6;

my @target = <a a b b c 0 1 1 1>;
my %count;

for @target -> Str $var {
  %count{ $var } += 1;
}

%count.say; #=> {0 => 1, 1 => 3, a => 2, b => 2, c => 1}

おわり

各言語の人たちから刺されそう:whale2:

参考と注釈

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