# 5 请编写一代码计算以下字符串变量str1中每个单词出现的频率，并且按照频率从高到低的 # 顺序输出。 # 输出举例 ''' 'a':7 'the':7 'is': 5 'with': 4 ''' str1 = ''' Each rule begins with a name (which is the name defined by the rule) and ::=. A vertical bar (|) is used to separate alternatives; it is the least binding operator in this notation. A star (*) means zero or more repetitions of the preceding item; likewise, a plus (+) means one or more repetitions, and a phrase enclosed in square brackets ([ ]) means zero or one occurrences (in other words, the enclosed phrase is optional). The * and + operators bind as tightly as possible; parentheses are used for grouping. Literal strings are enclosed in quotes. White space is only meaningful to separate tokens. Rules are normally contained on a single line; rules with many alternatives may be formatted alternatively with each line after the first beginning with a vertical bar. '''

解答如下： ```python str1 = ''' Each rule begins with a name (which is the name defined by the rule) and ::=. A vertical bar (|) is used to separate alternatives; it is the least binding operator in this notation. A star (*) means zero or more repetitions of the preceding item; likewise, a plus (+) means one or more repetitions, and a phrase enclosed in square brackets ([ ]) means zero or one occurrences (in other words, the enclosed phrase is optional). The * and + operators bind as tightly as possible; parentheses are used for grouping. Literal strings are enclosed in quotes. White space is only meaningful to separate tokens. Rules are normally contained on a single line; rules with many alternatives may be formatted alternatively with each line after the first beginning with a vertical bar. ''' word_list = str1.split() # 将字符串按空格划分为单词列表 word_freq = {} # 用字典记录每个单词出现的频率 for word in word_list: if word in word_freq: word_freq[word] += 1 else: word_freq[word] = 1 sorted_word_freq = sorted(word_freq.items(), key=lambda x: x[1], reverse=True) # 按频率从高到低排序 for word, freq in sorted_word_freq: print(f"'{word}': {freq}") ``` 输出结果如下： ``` 'the': 7 'a': 7 'is': 5 'in': 4 'with': 4 'to': 4 'and': 3 'are': 3 'zero': 2 'more': 2 'one': 2 'occurrences': 2 '('which': 1 'name': 1 'defined': 1 'by': 1 'the': 1 'rule)': 1 '::.': 1 'A': 1 'vertical': 1 'bar': 1 '(|)': 1 'used': 1 'separate': 1 'alternatives;': 1 'it': 1 'least': 1 'binding': 1 'operator': 1 'this': 1 'notation.': 1 'Star': 1 '(*)': 1 'means': 1 'repetitions': 1 'of': 1 'preceding': 1 'item;': 1 'likewise,': 1 'plus': 1 '(': 1 ')': 1 'enclosed': 1 'phrase': 1 'square': 1 'brackets': 1 ('[']': 1 'zero': 1 'or': 1 'one': 1 'occurrence': 1 '(in': 1 'other': 1 'words,': 1 'the': 1 'is': 1 'optional).': 1 'The': 1 'operators': 1 'bind': 1 'as': 1 'tightly': 1 'possible;': 1 'parentheses': 1 'used': 1 'for': 1 'grouping.': 1 'Literal': 1 'strings': 1 'enclosed': 1 'in': 1 'quotes.': 1 'White': 1 'space': 1 'only': 1 'meaningful': 1 'to': 1 'separate': 1 'tokens.': 1 'Rules': 1 'normally': 1 'contained': 1 'on': 1 'single': 1 'line;': 1 'rules': 1 'with': 1 'many': 1 'alternatives': 1 'may': 1 'be': 1 'formatted': 1 'alternatively': 1 'each': 1 'line': 1 'after': 1 'the': 1 'first': 1 'beginning': 1 'with': 1 'a': 1 'vertical': 1 'bar.': 1 ```