javascript - 如何计算每个单词在字符串中出现的次数？

Question

我现在正在学习 JavaScript，正在开发一个函数，该函数将计算单词在字符串中出现的次数，然后将答案作为对象吐出。根据本网站上类似线程的一些建议，我决定使用 .split 和计数器函数来创建二维数组，然后用结果填充对象。当我在使用某些字符串时遇到一些问题时，我正在使用示例文本运行测试。我不明白为什么有些计数器显示为未定义。

function countWords(str) {
  var answer = {};
  if (str === '') {
    return answer;
  }
  var strArray = [];
  strArray = str.split(' ');
  //strArray is an array that holds the words as separate strings
  console.log('strArray: ' + strArray);
  var resultWords = [];
  var resultCount = [];
  var counter = 0;

  // only if the word doesnt show up in resultWords, push it in, and increase the counter. if it has shown up, disregard
  for (var i = 0; i<strArray.length; i++) {
    counter = 0;
    if (resultWords.indexOf( strArray[i] ) === -1)  {
      resultWords.push(strArray[i]);
      counter += 1;
      // if the word shows up again, increase the counter
      for (var j = i + 1; j < strArray.length; j++) {
        if (resultWords[i] === strArray[j]) {
          counter += 1;
        }
        // push to resultCount the counter for each word
        resultCount[i] = counter;
      }
    }
    // create an object where the key is the word from resultWords and the value is the number from wordCount
    for (var k = 0; k < resultWords.length; k++) {
      answer[ resultWords[k] ] = resultCount[k];            
    }
  }
  console.log('resultWords: ' + resultWords);
  console.log('resultCount: ' + resultCount);
  return answer;
}

var sample = 'how now brown cow how now';
console.log(sample);
var output = countWords( sample ); 

我发现当我在示例中使用“this”和“is”这两个词时，我经常返回“undefined”作为这些词的字数。例如，“to be or not to be”对于“that”和“is”返回“未定义”。有人可以帮助解释一下这里发生了什么吗？谢谢。

Answer 1

您的代码不可读:

• 代码块太长
• 非信息性变量名称(例如 answer )

接下来，你的算法非常慢:你可以通过只解析整个数组一次来计算单词数。

最后但并非最不重要的一点是，您应该创建 answer循环外的数组。

这是一个使用现代 JavaScript 功能的较短实现(为了学习):

function countWords(str) {
  const wordCounts = new Map()
  str.split(' ').forEach(word => {
    const currentWordCount = wordCounts.get(word) || 0
    wordCounts.set(word, currentWordCount+1)
  })

  /* Reproduce your output */
  const resultWords = [...wordCounts.keys()]
  const resultCount = [...wordCounts.values()]
  console.log('resultWords: ' + resultWords);
  console.log('resultCount: ' + resultCount);

  return wordCounts
}

在较旧的js环境中，您无法使用Map和箭头函数:

function countWords(str) {
  const wordCounts = {}
  str.split(' ').forEach(function(word) {
    const currentWordCount = wordCounts[word] || 0
    wordCounts[word] = currentWordCount+1
  })

  /* Reproduce your output */
  const resultWords = Object.keys(wordCounts)
  const resultCount = resultWords.map(function(word) { return wordCounts[word] })
  console.log('resultWords: ' + resultWords);
  console.log('resultCount: ' + resultCount);

  return wordCounts
}

回到你的代码，你会得到一些undefined因为这一行:

// push to resultCount the counter for each word
resultCount[i] = counter;

索引i是 strArray 中当前单词的索引。您可以通过删除此行来修复它，然后执行

resultCount.push(counter)

以 for (var j = i + 1; j < strArray.length; j++) 开头的循环结束后.