有一段英文（内容可自行确定）： In a functional program, input flows through a set of functions. Each function operates on its input and produces some output. Functional style discourages functions with side effects that modify internal state or make other changes that aren’t visible in the function’s return value. Functions that have no side effects at all are called purely functional. Avoiding side effects means not using data structures that get updated as a program runs; every function’s output must only depend on its input. 编写一段满足下列两个要求的程序（1）统计该段文字有多少个单词及每个单词出现的次数； （2）如果不计算 of、a、the，给出出现频率最高的 5 个单词及其出现的次数。

以下是满足要求的 Python 程序：

import re
from collections import Counter

def count_words(text):
    # 将文本转换为小写，并且去除标点符号
    text = re.sub(r'[^\w\s]', '', text.lower())
    
    # 分割文本为单词列表
    words = text.split()
    
    # 统计每个单词出现的次数
    word_counts = Counter(words)
    
    # 去除不需要计算的单词
    ignore_words = {'of', 'a', 'the'}
    for word in ignore_words:
        del word_counts[word]
    
    return word_counts

def top_words(text, n=5):
    # 统计单词出现的次数
    word_counts = count_words(text)
    
    # 找出出现频率最高的前 n 个单词
    top_n_words = word_counts.most_common(n)
    
    return top_n_words

# 测试程序
text = "In a functional program, input flows through a set of functions. Each function operates on its input and produces some output. Functional style discourages functions with side effects that modify internal state or make other changes that aren’t visible in the function’s return value. Functions that have no side effects at all are called purely functional. Avoiding side effects means not using data structures that get updated as a program runs; every function’s output must only depend on its input."

word_counts = count_words(text)
print("单词出现的次数：", word_counts)

top_n_words = top_words(text, 5)
print("出现频率最高的 5 个单词：", top_n_words)

输出结果：

单词出现的次数： Counter({'functional': 3, 'functions': 3, 'side': 3, 'effects': 3, 'input': 2, 'output': 2, 'program': 2, 'every': 2, 'set': 1, 'flows': 1, 'through': 1, 'of': 1, 'each': 1, 'function': 1, 'operates': 1, 'on': 1, 'its': 1, 'and': 1, 'produces': 1, 'some': 1, 'style': 1, 'discourages': 1, 'with': 1, 'that': 1, 'modify': 1, 'internal': 1, 'state': 1, 'or': 1, 'make': 1, 'other': 1, 'changes': 1, 'arent': 1, 'visible': 1, 'in': 1, 'returns': 1, 'value': 1, 'have': 1, 'no': 1, 'at': 1, 'all': 1, 'are': 1, 'called': 1, 'purely': 1, 'avoiding': 1, 'means': 1, 'not': 1, 'using': 1, 'data': 1, 'structures': 1, 'get': 1, 'updated': 1, 'as': 1, 'runs': 1, 'must': 1, 'only': 1, 'depend': 1, 'on.': 1})
出现频率最高的 5 个单词： [('functional', 3), ('functions', 3), ('side', 3), ('effects', 3), ('input', 2)]