Pipes实现LeetCode(192.单词频率)

[LeetCode] 192.Word Frequency 单词频率

Write a bash script to calculate the frequency of each word in a text file words.txt.

For simplicity sake, you may assume:

  • words.txt contains only lowercase characters and space ' ' characters.
  • Each word must consist of lowercase characters only.
  • Words are separated by one or more whitespace characters.

For example, assume that words.txt has the following content:

the day is sunny the the
the sunny is is

Your script should output the following, sorted by descending frequency:

the 4
is 3
sunny 2
day 1

Note:
Don't worry about handling ties, it is guaranteed that each word's frequency count is unique.

[show hint]

Hint:
Could you write it in one-line using Unix pipes?

这道题给了我们一个文本文件，让我们统计里面单词出现的个数，提示中让我们用管道Pipes来做，在之前那道Tenth Line中，我们使用过管道命令。提示中让我们用管道连接各种命令，然后一行搞定，那么我们先来看第一种解法，乍一看啥都不明白，咋办？没关系，容我慢慢来讲解。首先用的关键字是grep命令，该命令一种强大的文本搜索工具，它能使用正则表达式搜索文本，并把匹配的行打印出来。后面紧跟的-oE '[a-z]+'参数表示原文本内容变成一个单词一行的存储方式，于是此时文本的内容就变成了：

the
day
is
sunny
the
the
the
sunny
is

下面的sort命令就是用来排序的。排完序的结果为：

day
is
is
is
sunny
sunny
the
the
the
the

后面的uniq命令是表示去除重复行命令，后面的参数-c表示在每行前加上表示相应行目出现次数的前缀编号，得到结果如下：

   1 day
3 is
2 sunny
4 the

然后我们再sort一下，后面的参数-nr表示按数值进行降序排列，得到结果：

   4 the
3 is
2 sunny
1 day 

而最后的awk命令就是将结果输出，两列颠倒位置即可：

解法一：

grep -oE '[a-z]+' words.txt | sort | uniq -c | sort -nr | awk '{print $2" "$1}'

下面这种方法用的关键字是tr命令，该命令主要用来进行字符替换或者大小写替换。后面紧跟的-s参数表示如果发现连续的字符，就把它们缩减为1个，而后面的‘ '和‘\n'就是空格和回车，意思是把所有的空格都换成回车，那么第一段命令tr -s ' ' '\n' < words.txt 就好理解了，将单词之间的空格都换成回车，跟上面的第一段实现的作用相同，后面就完全一样了，参见上面的讲解：

解法二：

tr -s ' ' '\n' < words.txt | sort | uniq -c | sort -nr | awk '{print $2, $1}'

下面这种方法就没有用管道命令进行一行写法了，而是使用了强大的文本分析工具awk进行写类C的代码来实现，这种写法在之前的那道Transpose File已经讲解过了，这里就不多说了，最后要注意的是sort命令的参数-nr -k 2表示按第二列的降序数值排列：

解法三：

awk '{
    for (i = 1; i <= NF; ++i) ++s[$i];
} END {
    for (i in s) print i, s[i];
}' words.txt | sort -nr -k 2

参考资料：

https://leetcode.com/discuss/33353/my-accepted-answer-using-tr-sort-uniq-and-awk

https://leetcode.com/discuss/46976/my-ac-solution-one-pipe-command-but-cost-20ms

https://leetcode.com/discuss/29001/solution-using-awk-and-pipes-with-explaination

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