有没有办法在不导入文件的情况下获取文件中的行数？
到目前为止,这就是我正在做的事情
myfiles <- list.files(pattern="*.dat")
myfilesContent <- lapply(myfiles,read.delim,header=F,quote="\"")
for (i in 1:length(myfiles)){
test[[i]] <- length(myfilesContent[[i]]$V1)
}
但由于每个文件都很大,所以太耗时了.
解决方法
如果你：
>仍然希望避免系统调用system2(“wc”…将导致
>是在BSD / Linux或OS X上(我没有在Windows上测试以下内容)
>不介意使用完整的文件名路径
>使用内联包很舒服
那么下面的内容应该尽可能快(它几乎是内联R C函数中wc的’行数’部分)：
library(inline)
wc.code <- "
uintmax_t linect = 0;
uintmax_t tlinect = 0;
int fd,len;
u_char *p;
struct statfs fsb;
static off_t buf_size = SMALL_BUF_SIZE;
static u_char small_buf[SMALL_BUF_SIZE];
static u_char *buf = small_buf;
PROTECT(f = AS_CHARACTER(f));
if ((fd = open(CHAR(STRING_ELT(f,0)),O_RDONLY,0)) >= 0) {
if (fstatfs(fd,&fsb)) {
fsb.f_iosize = SMALL_BUF_SIZE;
}
if (fsb.f_iosize != buf_size) {
if (buf != small_buf) {
free(buf);
}
if (fsb.f_iosize == SMALL_BUF_SIZE || !(buf = malloc(fsb.f_iosize))) {
buf = small_buf;
buf_size = SMALL_BUF_SIZE;
} else {
buf_size = fsb.f_iosize;
}
}
while ((len = read(fd,buf,buf_size))) {
if (len == -1) {
(void)close(fd);
break;
}
for (p = buf; len--; ++p)
if (*p == '\\n')
++linect;
}
tlinect += linect;
(void)close(fd);
}
SEXP result;
PROTECT(result = NEW_INTEGER(1));
INTEGER(result)[0] = tlinect;
UNPROTECT(2);
return(result);
";
setCMethod("wc",signature(f="character"),wc.code,includes=c("#include <stdlib.h>","#include <stdio.h>","#include <sys/param.h>","#include <sys/mount.h>","#include <sys/stat.h>","#include <ctype.h>","#include <err.h>","#include <errno.h>","#include <fcntl.h>","#include <locale.h>","#include <stdint.h>","#include <string.h>","#include <unistd.h>","#include <wchar.h>","#include <wctype.h>","#define SMALL_BUF_SIZE (1024 * 8)"),language="C",convention=".Call")
wc("FULLPATHTOFILE")
它作为一个包更好,因为它实际上必须首次编译.但是,如果你确实需要“速度”,它可以作为参考.对于我躺在的189,955行文件,我得到(来自一堆运行的平均值)：
user system elapsed 0.007 0.003 0.010
