How to count lines fast?

问题

I tried unxutils' wc -l but it crashed for 1GB files. I tried this C# code

long count = 0;
using (StreamReader r = new StreamReader(f))
{
    string line;
    while ((line = r.ReadLine()) != null)
    {
        count++;
    }
}

return count;

It reads a 500MB file in 4 seconds

var size = 256;
var bytes = new byte[size];
var count = 0;
byte query = Convert.ToByte('\n');
using (var stream = File.OpenRead(file))
{
    int many;
    do
    {
        many = stream.Read(bytes, 0, size);
        count += bytes.Where(a => a == query).Count();                    
    } while (many == size);
}

Reads in 10 seconds

var count = 0;
int query = (int)Convert.ToByte('\n');
using (var stream = File.OpenRead(file))
{
    int current;
    do
    {
        current = stream.ReadByte();
        if (current == query)
        {
            count++;
            continue;
        }
    } while (current!= -1);
}

Takes 7 seconds

Is anything faster I haven't tried yet?

回答1:

Your first approach does look like the optimal solution already. Keep in mind that you're mostly not CPU bound but limited by the HD's read speed, which at 500MB / 4sec = 125MB/s is already quite fast. The only way to get faster than that is via RAID or using SSDs, not so much via a better algorithm.

回答2:

File.ReadLines was introduced in .NET 4.0

var count = File.ReadLines(file).Count();

works in 4 seconds, the same time as the first code snippet

回答3:

Are you just looking for a tool to count lines in a file, and efficiently? If so, try MS LogParser

Something like below will give you number of lines:

LogParser "SELECT count(*) FROM file" -i:TEXTLINE

回答4:

If you really want fast, consider C code.

If this is a command-line utility, it will be faster because it won't have to initialize the CLR or .NET. And, it won't reallocate a new string for each line read from the file, which probably saves time on throughput.

I don't have any files with 1g lines, so I cannot compare. you can try, though:

/*
 * LineCount.c
 *
 * count lines...
 *
 * compile with: 
 *
 *  c:\vc10\bin\cl.exe /O2 -Ic:\vc10\Include -I\winsdk\Include 
 *          LineCount.c -link /debug /SUBSYSTEM:CONSOLE /LIBPATH:c:\vc10\Lib
 *          /LIBPATH:\winsdk\Lib /out:LineCount.exe
 */

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


void Usage(char *appname)
{
    printf("\nLineCount.exe\n");
    printf("  count lines in a text file...\n\n");
    printf("usage:\n");
    printf("  %s <filename>\n\n", appname);
}



int linecnt(char *file)
{
    int sz = 2048;
    char *buf = (char *) malloc(sz);
    FILE *fp = NULL;
    int n= 0;
    errno_t rc = fopen_s(&fp, file, "r");

    if (rc) {
        fprintf(stderr, "%s: fopen(%s) failed: ecode(%d)\n",
                __FILE__, file, rc);
        return -1;
    }

    while (fgets(buf, sz, fp)){
        int r = strlen(buf);
        if (buf[r-1] == '\n')
            n++;
        // could re-alloc here to handle larger lines
    }
    fclose(fp);
    return n;
}

int main(int argc, char **argv)
{
    if (argc==2) {
        int n = linecnt (argv[1]);
        printf("Lines: %d\n", n);
    }
    else {
        Usage(argv[0]);
        exit(1);
    }
}

回答5:

I think that your answer looks good. The only thing I would add is to play with buffer size. I feel that it may change the performance depending on your buffer size.

Please refer to buffer size at - Optimum file buffer read size?

回答6:

Have you tried flex?

%{
long num_lines = 0;
%}
%option 8bit outfile="scanner.c"
%option nounput nomain noyywrap
%option warn

%%
.+ { }
\n { ++num_lines; }
%%
int main(int argc, char **argv);

int main (argc,argv)
int argc;
char **argv;
{
yylex();
printf( "# of lines = %d\n", num_lines );
return 0;
}

Just compile with:

flex -Cf scanner.l 
gcc -O -o lineCount.exe scanner.c

It accepts input on stdin and outputs the number of lines.

来源：https://stackoverflow.com/questions/6101367/how-to-count-lines-fast

标签

streamreader