java - 逐字符读取文件并在其中找到输入字符串

Question

我有一个文件，需要逐个字符读取并找到输入字符串。我需要返回“输入字符串”在文件中出现的次数，但我只需要逐个字符地读取文件。

我想出了下面的代码，但在弄清楚如何通过逐个字符读取文件中的字符串时遇到问题。我首先迭代 for 循环，然后在 while 循环中进行迭代，但是如果 char 不匹配，那么我需要从 for 循环重新开始，但我不知道如何做到这一点？

  public static void main(String[] args) throws IOException {
    String input = "hello world"; // "hello";
    handleFile(new File("some_file"), input);
  }

  private static int handleFile(File file, String input) throws IOException {
    int count = 0;
    try (BufferedReader br =
        new BufferedReader(new InputStreamReader(new FileInputStream(file),
            Charset.forName("UTF-8")))) {
      char[] arr = input.toCharArray();
      int r;

      // confuse here what logic I should have here?
      for (char a : arr) {
        while ((r = br.read()) != -1) {
          char ch = (char) r;
          if (ch == a) {
            break;
          }
        }
      }
    }

    return count;
  }

Answer 1

因此，从概念上讲，您需要维护匹配字符数量的偏移量，每次发生不匹配时，您将偏移量重置回0。该偏移量用于确定输入中的给定字符是否与文件中的下一个字符匹配

一个简单的实现可能看起来像......

String value = "Thistestistestatesttest";
String input = "test";

int offset = 0;
int matches = 0;
for (char next : value.toCharArray()) {
    if (next == input.charAt(offset)) {
        offset++;
        if (offset == input.length()) {
            matches++;
            offset = 0;
        }
    } else {
        offset = 0;
    }
}
System.out.println("Found " + matches);

请注意，我特意使用了 String 作为源，以便您可以对其进行测试并更好地理解它的工作原理，并采用逻辑并实现您自己的解决方案。

现在，如果您花时间对问题进行桌面检查，它可能看起来像......

+======+========+==============+=======+x
| Next | offset | offset value | match |
+======+========+==============+=======+
| T    |      0 | t            | false |
+------+--------+--------------+-------+
| h    |      0 | t            | false |
+------+--------+--------------+-------+
| i    |      0 | t            | false |
+------+--------+--------------+-------+
| s    |      0 | t            | false |
+------+--------+--------------+-------+
| t    |      0 | t            | true  |
+------+--------+--------------+-------+
| e    |      1 | e            | true  |
+------+--------+--------------+-------+
| s    |      2 | s            | true  |
+------+--------+--------------+-------+
| t    |      3 | t            | true  |
+------+--------+--------------+-------+
| i    |      0 | t            | false |
+------+--------+--------------+-------+
| s    |      0 | t            | false |
+------+--------+--------------+-------+
| t    |      0 | t            | true  |
+------+--------+--------------+-------+
| e    |      1 | e            | true  |
+------+--------+--------------+-------+
| s    |      2 | s            | true  |
+------+--------+--------------+-------+
| t    |      3 | t            | true  |
+------+--------+--------------+-------+
| a    |      0 | t            | false |
+------+--------+--------------+-------+
| t    |      0 | t            | true  |
+------+--------+--------------+-------+
| e    |      1 | e            | true  |
+------+--------+--------------+-------+
| s    |      2 | s            | true  |
+------+--------+--------------+-------+
| t    |      3 | t            | true  |
+------+--------+--------------+-------+
| t    |      0 | t            | true  |
+------+--------+--------------+-------+
| e    |      1 | e            | true  |
+------+--------+--------------+-------+
| s    |      2 | s            | true  |
+------+--------+--------------+-------+
| t    |      3 | t            | true  |
+------+--------+--------------+-------+