计算文本文件中的行数

Question

我正在阅读文本文件中的行,我想知道这是否是一个好方法？我必须编写函数numberoflines来减少number_of_lines variable一个,因为在while循环中,对于它读取的每一行,它都会向number_of_lines变量添加2.

#include <iostream>
#include <fstream>
using namespace std;

int number_of_lines = 0;

void numberoflines();
int main(){
    string line;
    ifstream myfile("textexample.txt");

    if(myfile.is_open()){
        while(!myfile.eof()){
            getline(myfile,line);
            cout<< line << endl;
            number_of_lines++;
        }
        myfile.close();
    }
    numberoflines();

}

void numberoflines(){
    number_of_lines--;
    cout<<"number of lines in text file: " << number_of_lines << endl;
}

还有其他更容易更好的方法吗？

Answer 1

你在最后减少计数的黑客就是那个 - 一个黑客.

首先正确编写循环要好得多,所以它不会计算最后一行两次.

int main() { 
    int number_of_lines = 0;
    std::string line;
    std::ifstream myfile("textexample.txt");

    while (std::getline(myfile, line))
        ++number_of_lines;
    std::cout << "Number of lines in text file: " << number_of_lines;
    return 0;
}

我个人认为,在这种情况下,C风格的代码是完全可以接受的:

int main() {
    unsigned int number_of_lines = 0;
    FILE *infile = fopen("textexample.txt", "r");
    int ch;

    while (EOF != (ch=getc(infile)))
        if ('\n' == ch)
            ++number_of_lines;
    printf("%u\n", number_of_lines);
    return 0;
}

编辑:当然,C++也会让你做一些类似的事情:

int main() {
    std::ifstream myfile("textexample.txt");

    // new lines will be skipped unless we stop it from happening:    
    myfile.unsetf(std::ios_base::skipws);

    // count the newlines with an algorithm specialized for counting:
    unsigned line_count = std::count(
        std::istream_iterator<char>(myfile),
        std::istream_iterator<char>(), 
        '\n');

    std::cout << "Lines: " << line_count << "\n";
    return 0;
}

Answer 2

我认为你的问题是,"我为什么要获得比文件中更多的一行？"

想象一个文件:

line 1
line 2
line 3

该文件可以用ASCII表示,如下所示:

line 1\nline 2\nline 3\n

(\n字节在哪里0x10.)

现在让我们看看每次getline通话之前和之后会发生什么:

Before 1: line 1\nline 2\nline 3\n
  Stream: ^
After 1:  line 1\nline 2\nline 3\n
  Stream:         ^

Before 2: line 1\nline 2\nline 3\n
  Stream:         ^
After 2:  line 1\nline 2\nline 3\n
  Stream:                 ^

Before 2: line 1\nline 2\nline 3\n
  Stream:                 ^
After 2:  line 1\nline 2\nline 3\n
  Stream:                         ^

现在,您认为流将标记eof为表示文件的结尾,对吧？不!这是因为getline套eof如果达到最终的文件标记"期间,它的操作".因为getline在到达时终止,所以\n不读取文件结束标记,并且eof不标记该标记.因此,myfile.eof()返回false,循环经历另一次迭代:

Before 3: line 1\nline 2\nline 3\n
  Stream:                         ^
After 3:  line 1\nline 2\nline 3\n
  Stream:                         ^ EOF

你是如何解决这个问题的？而不是检查eof(),看看是否.peek()返回EOF:

while(myfile.peek() != EOF){
    getline ...

您还可以检查getline(隐式转换为bool)的返回值:

while(getline(myfile,line)){
    cout<< ...