Most Compact Way to Count Number of Lines in a File in C++
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22-07-2019 - |
Question
What's the most compact way to compute the number of lines of a file? I need this information to create/initialize a matrix data structure.
Later I have to go through the file again and store the information inside a matrix.
Update: Based on Dave Gamble's. But why this doesn't compile? Note that the file could be very large. So I try to avoid using container to save memory.
#include <iostream>
#include <vector>
#include <fstream>
#include <sstream>
using namespace std;
int main ( int arg_count, char *arg_vec[] ) {
if (arg_count !=2 ) {
cerr << "expected one argument" << endl;
return EXIT_FAILURE;
}
string line;
ifstream myfile (arg_vec[1]);
FILE *f=fopen(myfile,"rb");
int c=0,b;
while ((b=fgetc(f))!=EOF) c+=(b==10)?1:0;
fseek(f,0,SEEK_SET);
return 0;
}
Solution
FILE *f=fopen(filename,"rb");
int c=0,b;while ((b=fgetc(f))!=EOF) c+=(b==10)?1:0;fseek(f,0,SEEK_SET);
Answer in c. That kind of compact?
OTHER TIPS
I think this might do it...
std::ifstream file(f);
int n = std::count(std::istreambuf_iterator<char>(file), std::istreambuf_iterator<char>(), '\n') + 1;
If the reason you need to "go back again" is because you cannot continue without the size, try re-ordering your setup.
That is, read through the file, storing each line in a std::vector<string>
or something. Then you have the size, along with the lines in the file:
#include <fstream>
#include <iostream>
#include <string>
#include <vector>
int main(void)
{
std::fstream file("main.cpp");
std::vector<std::string> fileData;
// read in each line
std::string dummy;
while (getline(file, dummy))
{
fileData.push_back(dummy);
}
// and size is available, along with the file
// being in memory (faster than hard drive)
size_t fileLines = fileData.size();
std::cout << "Number of lines: " << fileLines << std::endl;
}
Here is a solution without the container:
#include <fstream>
#include <iostream>
#include <string>
#include <vector>
int main(void)
{
std::fstream file("main.cpp");
size_t fileLines = 0;
// read in each line
std::string dummy;
while (getline(file, dummy))
{
++fileLines;
}
std::cout << "Number of lines: " << fileLines << std::endl;
}
Though I doubt that's the most efficient way. The benefit of this method was the ability to store the lines in memory as you went.
#include <stdlib.h>
int main(void) { system("wc -l plainfile.txt"); }
Count the number of instances of '\n'
. This works for *nix (\n) and DOS/Windows (\r\n) line endings, but not for old-skool Mac (System 9 or maybe before that), which used just \r. I've never seen a case come up with just \r as line endings, so I wouldn't worry about it unless you know it's going to be an issue.
Edit: If your input is not ASCII, then you could run into encoding problems as well. What's your input look like?