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[#]: subject: (Use awk to calculate letter frequency)
[#]: via: (https://opensource.com/article/21/4/gawk-letter-game)
[#]: author: (Jim Hall https://opensource.com/users/jim-hall)
[#]: collector: (lujun9972)
[#]: translator: (lkxed)
[#]: reviewer: ( )
[#]: publisher: ( )
[#]: url: ( )
Use awk to calculate letter frequency
======
Write an awk script to determine the most (and least) common letters in
a set of words.
![Typewriter keys in multicolor][1]
I recently started writing a game where you build words using letter tiles. To create the game, I needed to know the frequency of letters across regular words in the English language, so I could present a useful set of letter tiles. Letter frequency is discussed in various places, including [on Wikipedia][2], but I wanted to calculate the letter frequency myself.
Linux provides a list of words in the `/usr/share/dict/words` file, so I already have a list of likely words to use. The `words` file contains lots of words that I want, but a few that I don't. I wanted a list of all words that weren't compound words (no hyphens or spaces) or proper nouns (no uppercase letters). To get that list, I can run the `grep` command to pull out only the lines that consist solely of lowercase letters:
```
`$ grep  '^[a-z]*$' /usr/share/dict/words`
```
This regular expression asks `grep` to match patterns that are only lowercase letters. The characters `^` and `$` in the pattern represent the start and end of the line, respectively. The `[a-z]` grouping will match only the lowercase letters **a** to **z**.
Here's a quick sample of the output:
```
$ grep  '^[a-z]*$' /usr/share/dict/words | head
a
aa
aaa
aah
aahed
aahing
aahs
aal
aalii
aaliis
```
And yes, those are all valid words. For example, "aahed" is the past tense exclamation of "aah," as in relaxation. And an "aalii" is a bushy tropical shrub.
Now I just need to write a `gawk` script to do the work of counting the letters in each word, and then print the relative frequency of each letter it finds.
### Counting letters
One way to count letters in `gawk` is to iterate through each character in each input line and count occurrences of each letter **a** to **z**. The `substr` function will return a substring of a given length, such as a single letter, from a larger string. For example, this code example will evaluate each character `c` from the input:
```
{
    len = length($0); for (i = 1; i <= len; i++) {
        c = substr($0, i, 1);
    }
}
```
If I start with a global string `LETTERS` that contains the alphabet, I can use the `index` function to find the location of a single letter in the alphabet. I'll expand the `gawk` code example to evaluate only the letters **a** to **z** in the input:
```
BEGIN { LETTERS = "abcdefghijklmnopqrstuvwxyz" }
 
{
    len = length($0); for (i = 1; i <= len; i++) {
        c = substr($0, i, 1);
        ltr = index(LETTERS, c);
    }
}
```
Note that the index function returns the first occurrence of the letter from the `LETTERS` string, starting with 1 at the first letter, or zero if not found. If I have an array that is 26 elements long, I can use the array to count the occurrences of each letter. I'll add this to my code example to increment (using `++`) the count for each letter as it appears in the input:
```
BEGIN { LETTERS = "abcdefghijklmnopqrstuvwxyz" }
 
{
    len = length($0); for (i = 1; i <= len; i++) {
        c = substr($0, i, 1);
        ltr = index(LETTERS, c);
 
        if (ltr > 0) {
            ++count[ltr];
        }
    }
}
```
### Printing relative frequency
After the `gawk` script counts all the letters, I want to print the frequency of each letter it finds. I am not interested in the total number of each letter from the input, but rather the _relative frequency_ of each letter. The relative frequency scales the counts so that the letter with the fewest occurrences (such as the letter **q**) is set to 1, and other letters are relative to that.
I'll start with the count for the letter **a**, then compare that value to the counts for each of the other letters **b** to **z**:
```
END {
    min = count[1]; for (ltr = 2; ltr <= 26; ltr++) {
        if (count[ltr] < min) {
            min = count[ltr];
        }
    }
}
```
At the end of that loop, the variable `min` contains the minimum count for any letter. I can use that to provide a scale for the counts to print the relative frequency of each letter. For example, if the letter with the lowest occurrence is **q**, then `min` will be equal to the **q** count.
Then I loop through each letter and print it with its relative frequency. I divide each count by `min` to print the relative frequency, which means the letter with the lowest count will be printed with a relative frequency of 1. If another letter appears twice as often as the lowest count, that letter will have a relative frequency of 2. I'm only interested in integer values here, so 2.1 and 2.9 are the same as 2 for my purposes:
```
END {
    min = count[1]; for (ltr = 2; ltr <= 26; ltr++) {
        if (count[ltr] < min) {
            min = count[ltr];
        }
    }
 
    for (ltr = 1; ltr <= 26; ltr++) {
        print substr(LETTERS, ltr, 1), int(count[ltr] / min);
    }
}
```
### Putting it all together
Now I have a `gawk` script that can count the relative frequency of letters in its input:
```
#!/usr/bin/gawk -f
 
# only count a-z, ignore A-Z and any other characters
 
BEGIN { LETTERS = "abcdefghijklmnopqrstuvwxyz" }
 
{
    len = length($0); for (i = 1; i <= len; i++) {
        c = substr($0, i, 1);
        ltr = index(LETTERS, c);
 
        if (ltr > 0) {
            ++count[ltr];
        }
    }
}
 
# print relative frequency of each letter
   
END {
    min = count[1]; for (ltr = 2; ltr <= 26; ltr++) {
        if (count[ltr] < min) {
            min = count[ltr];
        }
    }
 
    for (ltr = 1; ltr <= 26; ltr++) {
        print substr(LETTERS, ltr, 1), int(count[ltr] / min);
    }
}
```
I'll save that to a file called `letter-freq.awk` so that I can use it more easily from the command line.
If you prefer, you can also use `chmod +x` to make the file executable on its own. The `#!/usr/bin/gawk -f` on the first line means Linux will run it as a script using the `/usr/bin/gawk` program. And because the `gawk` command line uses `-f` to indicate which file it should use as a script, you need that hanging `-f` so that executing `letter-freq.awk` at the shell will be properly interpreted as running `/usr/bin/gawk -f letter-freq.awk` instead.
I can test the script with a few simple inputs. For example, if I feed the alphabet into my `gawk` script, each letter should have a relative frequency of 1:
```
$ echo abcdefghijklmnopqrstuvwxyz | gawk -f letter-freq.awk
a 1
b 1
c 1
d 1
e 1
f 1
g 1
h 1
i 1
j 1
k 1
l 1
m 1
n 1
o 1
p 1
q 1
r 1
s 1
t 1
u 1
v 1
w 1
x 1
y 1
z 1
```
Repeating that example but adding an extra instance of the letter **e** will print the letter **e** with a relative frequency of 2 and every other letter as 1:
```
$ echo abcdeefghijklmnopqrstuvwxyz | gawk -f letter-freq.awk
a 1
b 1
c 1
d 1
e 2
f 1
g 1
h 1
i 1
j 1
k 1
l 1
m 1
n 1
o 1
p 1
q 1
r 1
s 1
t 1
u 1
v 1
w 1
x 1
y 1
z 1
```
And now I can take the big step! I'll use the `grep` command with the `/usr/share/dict/words` file and identify the letter frequency for all words spelled entirely with lowercase letters:
```
$ grep  '^[a-z]*$' /usr/share/dict/words | gawk -f letter-freq.awk
a 53
b 12
c 28
d 21
e 72
f 7
g 15
h 17
i 58
j 1
k 5
l 36
m 19
n 47
o 47
p 21
q 1
r 46
s 48
t 44
u 25
v 6
w 4
x 1
y 13
z 2
```
Of all the lowercase words in the `/usr/share/dict/words` file, the letters **j**, **q**, and **x** occur least frequently. The letter **z** is also pretty rare. Not surprisingly, the letter **e** is the most frequently used.
--------------------------------------------------------------------------------
via: https://opensource.com/article/21/4/gawk-letter-game
作者：[Jim Hall][a]
选题：[lujun9972][b]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]: https://opensource.com/users/jim-hall
[b]: https://github.com/lujun9972
[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/osdc-docdish-typewriterkeys-3.png?itok=NyBwMdK_ (Typewriter keys in multicolor)
[2]: https://en.wikipedia.org/wiki/Letter_frequency

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@ -0,0 +1,284 @@
[#]: subject: (Use awk to calculate letter frequency)
[#]: via: (https://opensource.com/article/21/4/gawk-letter-game)
[#]: author: (Jim Hall https://opensource.com/users/jim-hall)
[#]: collector: (lujun9972)
[#]: translator: (lkxed)
[#]: reviewer: ( )
[#]: publisher: ( )
[#]: url: ( )
使用 awk 统计字母频率
======
编写一个 awk 脚本来找到一组单词中出现次数最多（和最少）的单词。
![Typewriter keys in multicolor][1]
近一段时间，我开始编写一个小游戏，在这个小游戏里，玩家使用一个个字母块来组成单词。编写这个游戏之前，我需要先知道常见英文单词中每个字母的使用频率，这样一来，我就可以找到一组更有用的字母块。字母频次统计在很多地方都有相关讨论，包括在 [维基百科][2] 上，但我还是想要自己来实现。
Linux 系统在 `/usr/share/dict/words` 文件中提供了一个单词列表，所以我已经有了一个现成的单词列表。然而，尽管这个 `words` 文件包含了很多我想要的单词，却也包含了一些我不想要的。我想要的单词首先不能是复合词（即不包含连接符和空格的单词），也不能是专有名词（即不包含大写字母单词）。为了得到这个结果，我可以运行 `grep` 命令来取出只由小写字母组成的行：
```
`$ grep  '^[a-z]*$' /usr/share/dict/words`
```
这个正则表达式的作用是让 `grep` 去匹配仅包含小写字母的行。表达式中的字符 `^` 和 `$` 分别代表了这一行的开始和结束。`[a-z]` 分组仅匹配小写字母 **a** 到 **z**。
下面是一个输出示例：
```
$ grep  '^[a-z]*$' /usr/share/dict/words | head
a
aa
aaa
aah
aahed
aahing
aahs
aal
aalii
aaliis
```
没错，这些都是合法的单词。比如，"aahed" 是 "aah," 的过去式，表示在放松时的感叹，而 "aalii" 是一种浓密的热带灌木。
现在我只需要编写一个 `gawk` 脚本来统计出单词中各个字母出现的次数，然后打印出每个字母的相对频率。
### 字母计数
一种使用 `gawk` 来统计字母个数的方式是，遍历每行输入中的每一个字符，然后对 **a** 到 **z** 之间的每个字母进行计数。`substr` 函数会返回一个给定长度的子串，它可以只包含一个字符，也可以是更长的字符串。比如，下面的示例代码能够取到输入中的每一个字符 `c`：
```
{
    len = length($0); for (i = 1; i <= len; i++) {
        c = substr($0, i, 1);
    }
}
```
如果使用一个全局字符串变量 `LETTERS` 来存储字母表，我就可以借助 `index` 函数来找到某个字符在字母表中的位置。我将扩展 `gawk` 代码示例，让它在输入数据中只取范围在 **a** 到 **z** 的字母：
```
BEGIN { LETTERS = "abcdefghijklmnopqrstuvwxyz" }
 
{
    len = length($0); for (i = 1; i <= len; i++) {
        c = substr($0, i, 1);
        ltr = index(LETTERS, c);
    }
}
```
需要注意的是，`index` 函数将返回字母在 `LETTERS` 字符串中首次出现的位置，第一个位置返回 1，如果没有找到则返回 0。如果我有一个大小为 26 的数组，我就可以利用这个数组来统计每个字母出现的次数。我将在下面的示例代码中添加这个功能，每当一个字母出现在输入中，我就让它对应的数组元素值增加 1（使用 `++`）：
```
BEGIN { LETTERS = "abcdefghijklmnopqrstuvwxyz" }
 
{
    len = length($0); for (i = 1; i <= len; i++) {
        c = substr($0, i, 1);
        ltr = index(LETTERS, c);
 
        if (ltr > 0) {
            ++count[ltr];
        }
    }
}
```
### 打印相对频率
当 `gawk` 脚本统计完所有的字母后，我希望它能输出每个字母的频率。毕竟，我对输入中各个字母的个数没有兴趣，我更关心它们的 _相对频率_。
我将先统计字母 **a** 的个数，然后把它和剩余 **b** 到 **z** 字母的个数比较：
```
END {
    min = count[1]; for (ltr = 2; ltr <= 26; ltr++) {
        if (count[ltr] < min) {
            min = count[ltr];
        }
    }
}
```
在循环的最后，变量 `min` 会等于最少的出现次数，我可以把它为基准，为字母的个数设定一个参照值，然后计算打印出每个字母的相对频率。比如，如果出现次数最少的字母是 **q**，那么 `min` 就会等于 **q** 的出现次数。
接下来，我会遍历每个字母，打印出它和它的相对频率。我通过把每个字母的个数都除以 `min` 的方式来计算出它的相对频率，这意味着出现次数最少的字母的相对频率是 1。如果另一个字母出现的次数恰好是最少次数的两倍，那么这个字母的相对频率就是 2。我只关心整数，所以 2.1 和 2.9 对我来说是一样的（都是 2）。
```
END {
    min = count[1]; for (ltr = 2; ltr <= 26; ltr++) {
        if (count[ltr] < min) {
            min = count[ltr];
        }
    }
 
    for (ltr = 1; ltr <= 26; ltr++) {
        print substr(LETTERS, ltr, 1), int(count[ltr] / min);
    }
}
```
### 最后的完整程序
现在，我已经有了一个能够统计输入中各个字母的相对频率的 `gawk` 脚本：
```
#!/usr/bin/gawk -f
 
# 只统计 a-z 的字符，忽略 A-Z 和其他的字符
 
BEGIN { LETTERS = "abcdefghijklmnopqrstuvwxyz" }
 
{
    len = length($0); for (i = 1; i <= len; i++) {
        c = substr($0, i, 1);
        ltr = index(LETTERS, c);
 
        if (ltr > 0) {
            ++count[ltr];
        }
    }
}
 
# 打印每个字符的相对频率
   
END {
    min = count[1]; for (ltr = 2; ltr <= 26; ltr++) {
        if (count[ltr] < min) {
            min = count[ltr];
        }
    }
 
    for (ltr = 1; ltr <= 26; ltr++) {
        print substr(LETTERS, ltr, 1), int(count[ltr] / min);
    }
}
```
我将把这段程序保存到名为 `letter-freq.awk` 的文件中，这样一来，我就可以在命令行中更方便地使用它。
如果你愿意的话，你也可以使用 `chmod +x` 命令把这个文件设为可独立执行。第一行中的 `#!/usr/bin/gawk -f` 表示 Linux 会使用 `/usr/bin/gawk` 把这个文件当作一个脚本来运行。由于 `gawk` 命令行使用 `-f` 来指定它要运行的脚本文件名，你需要在末尾加上 `-f`。如此一来，当你在 shell 中执行 `letter-freq.awk`，它会被解释为 `/usr/bin/gawk -f letter-freq.awk`。
接下来我将用几个简单的输入来测试这个脚本。比如，如果我给我的 `gawk` 脚本输入整个字母表，每个字母的相对频率都应该是 1：
```
$ echo abcdefghijklmnopqrstuvwxyz | gawk -f letter-freq.awk
a 1
b 1
c 1
d 1
e 1
f 1
g 1
h 1
i 1
j 1
k 1
l 1
m 1
n 1
o 1
p 1
q 1
r 1
s 1
t 1
u 1
v 1
w 1
x 1
y 1
z 1
```
还是使用上述例子，只不过这次我在输入中添加了一个字母 **e**，此时的输出结果中，**e** 的相对频率会是 2，而其他字母的相对频率仍然会是 1：
```
$ echo abcdeefghijklmnopqrstuvwxyz | gawk -f letter-freq.awk
a 1
b 1
c 1
d 1
e 2
f 1
g 1
h 1
i 1
j 1
k 1
l 1
m 1
n 1
o 1
p 1
q 1
r 1
s 1
t 1
u 1
v 1
w 1
x 1
y 1
z 1
```
现在我可以跨出最大的一步了！我将使用 `grep` 命令和 `/usr/share/dict/words` 文件，统计所有仅由小写字母组成的单词中，各个字母的相对使用频率：
```
$ grep  '^[a-z]*$' /usr/share/dict/words | gawk -f letter-freq.awk
a 53
b 12
c 28
d 21
e 72
f 7
g 15
h 17
i 58
j 1
k 5
l 36
m 19
n 47
o 47
p 21
q 1
r 46
s 48
t 44
u 25
v 6
w 4
x 1
y 13
z 2
```
在 `/usr/share/dict/words` 文件的所有小写单词中，字母 **j**、**q** 和 **x** 出现的相对频率最低，字母 **z** 也使用得很少。不出意料，字母 **e** 是使用频率最高的。
--------------------------------------------------------------------------------
via: https://opensource.com/article/21/4/gawk-letter-game
作者：[Jim Hall][a]
选题：[lujun9972][b]
译者：[lkxed](https://github.com/lkxed)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]: https://opensource.com/users/jim-hall
[b]: https://github.com/lujun9972
[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/osdc-docdish-typewriterkeys-3.png?itok=NyBwMdK_ (Typewriter keys in multicolor)
[2]: https://en.wikipedia.org/wiki/Letter_frequency