Line by line reading in C and C++?

问题:

I want to read line by line from a file in C or C++, and I know how to do that when I assume some fixed size of a line, but is there a simple way to somehow calculate or get the exact size needed for a line or all lines in file? (Reading word by word until newline is also good for me if anyone can do it that way.)

回答1:

If you use a streamed reader, all this will be hidden from you. See getline. The example below is based from the code here.

// getline with strings
#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main () {
  string str;
  ifstream ifs("data.txt");
  getline (ifs,str);
  cout << "first line of the file is " << str << ".\n";
}

回答2:

In C, if you have POSIX 2008 libraries (more recent versions of Linux, for example), you can use the POSIX getline() function. If you don't have the function in your libraries, you can implement it easily enough, which is probably better than inventing your own interface to do the job.

In C++, you can use std::getline().

Even though the two functions have the same basic name, the calling conventions and semantics are quite different (because the languages C and C++ are quite different) - except that they both read a line of data from a file stream, of course.

There isn't an easy way to tell how big the longest line in a file is - except by reading the whole file to find out, which is kind of wasteful.

回答3:

I would use an IFStream and use getline to read from a file.

http://www.cplusplus.com/doc/tutorial/files/

int main () {
    string line;
    ifstream myfile ("example.txt");
    if (myfile.is_open())
    {
        while ( myfile.good() )
        {
            getline (myfile,line);
            cout << line << endl;
        }
        myfile.close();
    }
    else cout << "Unable to open file"; 

    return 0;
}

回答4:

You can't get the length of line until after you read it in. You can, however, read into a buffer repeatedly until you reach the end of line.

For programming in c, try using fgets to read in a line of code. It will read n characters or stop if it encounters a newline. You can read in a small buffer of size n until the last character in the string is the newline.

See the link above for more information.

Here is an example on how to read an display a full line of file using a small buffer:

#include <stdio.h>
#include <string.h>

int main()
{
   FILE * pFile;

   const int n = 5;
   char mystring [n];
   int lineLength = 0;

   pFile = fopen ("myfile.txt" , "r");
   if (pFile == NULL) 
   {
       perror ("Error opening file");
   }
   else 
   {

        do 
        {
            fgets (mystring , n , pFile);
            puts (mystring);    
                    lineLength += strlen(mystring); 
        } while(mystring[strlen ( mystring)-1] != '\n' && !feof(pFile));

       fclose (pFile);
   }

   printf("Line Length: %d\n", lineLength);
   return 0;
}

回答5:

In C++ you can use the std::getline function, which takes a stream and reads up to the first '\n' character. In C, I would just use fgets and keep reallocating a buffer until the last character is the '\n', then we know we have read the entire line.

C++:

std::ifstream file("myfile.txt");
std::string line;
std::getline(file, line);
std::cout << line;

C:

// I didn't test this code I just made it off the top of my head.
FILE* file = fopen("myfile.txt", "r");
size_t cap = 256;
size_t len = 0;
char* line = malloc(cap);

for (;;) {
    fgets(&line[len], cap - len, file);
    len = strlen(line);
    if (line[len-1] != '\n' && !feof(file)) {
        cap <<= 1;
        line = realloc(line, cap);
    } else {
        break;
    }
}

printf("%s", line);

回答6:

getline is only POSIX, here is an ANSI (NO max-line-size info needed!):

const char* getline(FILE *f,char **r)
{
  char t[100];
  if( feof(f) )
    return 0;
  **r=0;
  while( fgets(t,100,f) )
  {
    char *p=strchr(t,'\n');
    if( p )
    {
      *p=0;
      if( (p=strchr(t,'\r')) ) *p=0;
      *r=realloc(*r,strlen(*r)+1+strlen(t));
      strcat(*r,t);
      return *r;
    }
    else
    {
      if( (p=strchr(t,'\r')) ) *p=0;
      *r=realloc(*r,strlen(*r)+1+strlen(t));
      strcat(*r,t);
    }
  }
  return feof(f)?(**r?*r:0):*r;
}

and now it's easy and short in your main:

  char *line,*buffer = malloc(100);
  FILE *f=fopen("yourfile.txt","rb");
  if( !f ) return;
  setvbuf(f,0,_IOLBF,4096);
  while( (line=getline(f,&buffer)) )
    puts(line);
  fclose(f);
  free(buffer);

it works on windows for Windows AND Unix-textfiles, it works on Unix for Unix AND Windows-textfiles

回答7:

Here is a C++ way of reading the lines, using std algorithms and iterators:

#include <iostream>
#include <iterator>
#include <vector>
#include <algorithm>

struct getline :
  public std::iterator<std::input_iterator_tag, std::string>
{
    std::istream* in;
    std::string line;
    getline(std::istream& in) : in(&in) {
        ++*this;
    }
    getline() : in(0) {
    }
    getline& operator++() {
        if(in && !std::getline(*in, line)) in = 0;
    }
    std::string operator*() const {
        return line;
    }
    bool operator!=(const getline& rhs) const {
        return !in != !rhs.in;
    }
};

int main() {
    std::vector<std::string> v;
    std::copy(getline(std::cin), getline(), std::back_inserter(v));
}