我使用巨大的数据文件,有时我只需要知道这些文件中的行数,通常我打开它们,一行一行地读取它们,直到我到达文件的末尾
我在想有没有更聪明的办法
我使用巨大的数据文件,有时我只需要知道这些文件中的行数,通常我打开它们,一行一行地读取它们,直到我到达文件的末尾
我在想有没有更聪明的办法
当前回答
在基于unix的系统上,在命令行上使用wc命令。
其他回答
上面的count()方法给出的答案是,如果文件末尾没有换行符,就会出现行数错误——它无法计算文件中的最后一行。
这个方法更适合我:
public int countLines(String filename) throws IOException {
LineNumberReader reader = new LineNumberReader(new FileReader(filename));
int cnt = 0;
String lineRead = "";
while ((lineRead = reader.readLine()) != null) {}
cnt = reader.getLineNumber();
reader.close();
return cnt;
}
我知道这是一个老问题,但公认的解决方案并不完全符合我所需要的。因此,我将其改进为接受各种行结束符(而不仅仅是换行)并使用指定的字符编码(而不是ISO-8859-n)。所有在一个方法(适当重构):
public static long getLinesCount(String fileName, String encodingName) throws IOException {
long linesCount = 0;
File file = new File(fileName);
FileInputStream fileIn = new FileInputStream(file);
try {
Charset encoding = Charset.forName(encodingName);
Reader fileReader = new InputStreamReader(fileIn, encoding);
int bufferSize = 4096;
Reader reader = new BufferedReader(fileReader, bufferSize);
char[] buffer = new char[bufferSize];
int prevChar = -1;
int readCount = reader.read(buffer);
while (readCount != -1) {
for (int i = 0; i < readCount; i++) {
int nextChar = buffer[i];
switch (nextChar) {
case '\r': {
// The current line is terminated by a carriage return or by a carriage return immediately followed by a line feed.
linesCount++;
break;
}
case '\n': {
if (prevChar == '\r') {
// The current line is terminated by a carriage return immediately followed by a line feed.
// The line has already been counted.
} else {
// The current line is terminated by a line feed.
linesCount++;
}
break;
}
}
prevChar = nextChar;
}
readCount = reader.read(buffer);
}
if (prevCh != -1) {
switch (prevCh) {
case '\r':
case '\n': {
// The last line is terminated by a line terminator.
// The last line has already been counted.
break;
}
default: {
// The last line is terminated by end-of-file.
linesCount++;
}
}
}
} finally {
fileIn.close();
}
return linesCount;
}
这个解决方案在速度上与公认的解决方案相当,在我的测试中大约慢了4%(尽管Java中的计时测试是出了名的不可靠)。
一个直接的方式使用扫描器
static void lineCounter (String path) throws IOException {
int lineCount = 0, commentsCount = 0;
Scanner input = new Scanner(new File(path));
while (input.hasNextLine()) {
String data = input.nextLine();
if (data.startsWith("//")) commentsCount++;
lineCount++;
}
System.out.println("Line Count: " + lineCount + "\t Comments Count: " + commentsCount);
}
我已经实现了这个问题的另一个解决方案,我发现它在计算行数时更有效:
try
(
FileReader input = new FileReader("input.txt");
LineNumberReader count = new LineNumberReader(input);
)
{
while (count.skip(Long.MAX_VALUE) > 0)
{
// Loop just in case the file is > Long.MAX_VALUE or skip() decides to not read the entire file
}
result = count.getLineNumber() + 1; // +1 because line index starts at 0
}
似乎有几种不同的方法可以使用LineNumberReader。
我是这样做的:
int lines = 0;
FileReader input = new FileReader(fileLocation);
LineNumberReader count = new LineNumberReader(input);
String line = count.readLine();
if(count.ready())
{
while(line != null) {
lines = count.getLineNumber();
line = count.readLine();
}
lines+=1;
}
count.close();
System.out.println(lines);
更简单的是,可以使用Java BufferedReader lines()方法返回元素流,然后使用stream count()方法对所有元素进行计数。然后只需在输出中添加1,就可以得到文本文件中的行数。
为例:
FileReader input = new FileReader(fileLocation);
LineNumberReader count = new LineNumberReader(input);
int lines = (int)count.lines().count() + 1;
count.close();
System.out.println(lines);