在Matlab中有没有一种方法来确定文件中的行数，而不是循环通过每一行？ [英] Is there a way in Matlab to determine the number of lines in a file without looping through each line?

问题描述

显然，可以使用fgetl或类似的函数遍历一个文件，并增加一个计数器，但是有没有一种方法可以确定文件中的行数，而不用做这样的循环？

解决方案

我完全喜欢使用下面的代码：

pre > fid = fopen（'someTextFile.txt'，'rb'）;
％＃获取文件大小。
fseek（fid，0，'eof'）;
fileSize = ftell（fid）;
frewind（fid）;
％＃阅读整个文件。
data = fread（fid，fileSize，'uint8'）;
％＃计算换行数并增加1。
numLines = sum（data == 10）+ 1;
fclose（fid）;


如果您有足够的内存一次读取整个文件，速度相当快。它应该适用于Windows和Linux风格的行结束。

编辑：我测量了迄今为止提供的答案的性能。以下是确定包含1百万个双精度值（每行一个值）的文本文件的行数的结果。平均10次尝试。

 作者平均时间+  - 标准差（s）
 ------- ----------------------------------------------- 
 Rody Oldenhuis 0.3189 +  -  0.0314 
 Edric（2）0.3282 +  -  0.0248 
 Mehrwolf 0.4075 +  -  0.0178 
 Jonas 1.0813 +  -  0.0665 
 Edric（1）26.8825 +  -  0.6790 
  

使用Perl并将所有文件读为二进制数据的方法最快。我不会感到惊讶，如果Perl内部也一次读取大块文件，而不是逐行循环（只是一个猜测，不知道有关Perl的任何内容）。

使用一个简单的 fgetl（） -loop是比其他方法慢25-75倍。

$ b $编辑2：包括Edric的第二种方法，这比Perl解决方案更快速，并且与Perl解决方案相提并论。

Obviously one could loop through a file using fgetl or similar function and increment a counter, but is there a way to determine the number of lines in a file without doing such a loop?

解决方案

I like to use the following code for exactly this task

fid = fopen('someTextFile.txt', 'rb');
%# Get file size.
fseek(fid, 0, 'eof');
fileSize = ftell(fid);
frewind(fid);
%# Read the whole file.
data = fread(fid, fileSize, 'uint8');
%# Count number of line-feeds and increase by one.
numLines = sum(data == 10) + 1;
fclose(fid);

It is pretty fast if you have enough memory to read the whole file at once. It should work for both Windows- and Linux-style line endings.

Edit: I measured the performance of the answers provided so far. Here is the result for determining the number of lines of a text file containing 1 million double values (one value per line). Average of 10 tries.

 Author           Mean time +- standard deviation (s)
------------------------------------------------------
 Rody Oldenhuis      0.3189 +- 0.0314
 Edric (2)           0.3282 +- 0.0248
 Mehrwolf            0.4075 +- 0.0178
 Jonas               1.0813 +- 0.0665
 Edric (1)          26.8825 +- 0.6790

So fastest are the approaches using Perl and reading all the file as binary data. I would not be surprised, if Perl internally also read large blocks of the file at once instead of looping through it line by line (just a guess, do not know anything about Perl).

Using a simple fgetl()-loop is by a factor of 25-75 slower than the other approaches.

Edit 2: Included Edric's 2nd approach, which is much faster and on-par with the Perl solution, I'd say.

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