Matlab 中有没有办法在不遍历每一行的情况下确定文件中的行数? [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 或类似函数遍历文件并增加计数器，但是有没有一种方法可以确定文件中的行数不进行这样的循环?

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?

推荐答案

我喜欢使用下面的代码来完成这个任务

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);

如果您有足够的内存来一次读取整个文件，速度会非常快.它应该适用于 Windows 和 Linux 风格的行尾.

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.

我测量了迄今为止提供的答案的性能.这是确定包含 100 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.

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

使用 Perl 并将所有文件作为二进制数据读取的方法是最快的.如果 Perl 在内部也一次读取文件的大块而不是逐行循环，我不会感到惊讶(只是猜测，对 Perl 一无所知).

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).

使用简单的 fgetl() 循环比其他方法慢 25-75 倍.

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

编辑 2: 包括 Edric 的第二种方法，我会说，它快得多并且与 Perl 解决方案相当.

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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