File. Read Lines Метод

Определение

Считывает строки файла. Reads the lines of a file.

Перегрузки

Считывает строки файла. Reads the lines of a file.

Считывает строки файла с заданной кодировкой. Read the lines of a file that has a specified encoding.

ReadLines(String)

Считывает строки файла. Reads the lines of a file.

Параметры

Файл, который нужно прочитать. The file to read.

Возвращаемое значение

Все строки файла или строки, которые являются результатом запроса. All the lines of the file, or the lines that are the result of a query.

Исключения

Параметр path является пустой строкой, содержит только пробел либо один или несколько недопустимых символов, определенных методом GetInvalidPathChars(). path is a zero-length string, contains only white space, or contains one or more invalid characters defined by the GetInvalidPathChars() method.

path имеет значение null . path is null .

Недопустимый path (например, он ведет на несопоставленный диск). path is invalid (for example, it is on an unmapped drive).

Файл, заданный параметром path , не обнаружен. The file specified by path was not found.

При открытии файла произошла ошибка ввода-вывода. An I/O error occurred while opening the file.

Длина параметра path превышает максимальную длину, определенную в системе. path exceeds the system-defined maximum length.

У вызывающего объекта отсутствует необходимое разрешение. The caller does not have the required permission.

Параметр path указывает файл, доступный только для чтения. path specifies a file that is read-only.

-или- -or- Эта операция не поддерживается на текущей платформе. This operation is not supported on the current platform.

-или- -or- path является каталогом. path is a directory.

-или- -or- У вызывающего объекта отсутствует необходимое разрешение. The caller does not have the required permission.

Примеры

В следующем примере считываются строки файла для поиска строк, содержащих указанные строки. The following example reads the lines of a file to find lines that contain specified strings.

В следующем примере используется ReadLines метод в запросе LINQ, который перечисляет все каталоги для файлов с расширением txt, считывает каждую строку файла и отображает строку, если она содержит строку «Microsoft». The following example uses the ReadLines method in a LINQ query that enumerates all directories for files that have a .txt extension, reads each line of the file, and displays the line if it contains the string «Microsoft».

Комментарии

ReadLinesМетоды и ReadAllLines отличаются следующим образом: при использовании ReadLines можно начать перечисление коллекции строк перед возвратом всей коллекции; при использовании ReadAllLines необходимо дождаться возвращения всего массива строк, прежде чем можно будет получить доступ к массиву. The ReadLines and ReadAllLines methods differ as follows: When you use ReadLines, you can start enumerating the collection of strings before the whole collection is returned; when you use ReadAllLines, you must wait for the whole array of strings be returned before you can access the array. Поэтому при работе с очень большими файлами они ReadLines могут быть более эффективными. Therefore, when you are working with very large files, ReadLines can be more efficient.

Метод можно использовать ReadLines для следующих задач: You can use the ReadLines method to do the following:

Выполнение LINQ to Objectsных запросов к файлу для получения отфильтрованного набора строк. Perform LINQ to Objects queries on a file to obtain a filtered set of its lines.

Запишите возвращенную коллекцию строк в файл с помощью File.WriteAllLines(String, IEnumerable ) метода или добавьте их в существующий файл с помощью File.AppendAllLines(String, IEnumerable ) метода. Write the returned collection of lines to a file with the File.WriteAllLines(String, IEnumerable ) method, or append them to an existing file with the File.AppendAllLines(String, IEnumerable ) method.

Создание немедленно заполненного экземпляра коллекции, которая принимает IEnumerable коллекцию строк для конструктора, например IList или Queue . Create an immediately populated instance of a collection that takes an IEnumerable collection of strings for its constructor, such as a IList or a Queue .

Этот метод использует UTF8 для значения кодировки. This method uses UTF8 for the encoding value.

How to read a line from a text file in c/c++? [duplicate]

After exhaustive googling and visiting many forums, I am yet to find a good comprehensive answer for this question. A lot of the forums suggest using the get line istream& getline (char* s, streamsize n ) function. My question is what if I don’t know what the length of each line is and cannot predict what the size may be? Also what is it’s equivalent in C?

Is there any specific function in c /c++ to read one single line each time from a text file ?

Explanation , with Code snippets will help me a lot.

5 Answers 5

In c, you could use fopen, and getch. Usually, if you can’t be exactly sure of the length of the longest line, you could allocate a large buffer (e.g. 8kb) and almost be guaranteed of getting all lines.

If there’s a chance you may have really really long lines and you have to process line by line, you could malloc a resonable buffer, and use realloc to double it’s size each time you get close to filling it.

In C++, you can use the global function std::getline, it takes a string and a stream and an optional delimiter and reads 1 line until the delimiter specified is reached. An example:

This program reads each line from a file and echos it to the console.

For C you’re probably looking at using fgets , it has been a while since I used C, meaning I’m a bit rusty, but I believe you can use this to emulate the functionality of the above C++ program like so:

With the limitation that the line can not be longer than the maximum length of the buffer that you’re reading in to.

Windows batch command(s) to read first line from text file

How can I read the first line from a text file using a Windows batch file? Since the file is large I only want to deal with the first line.

14 Answers 14

Here’s a general-purpose batch file to print the top n lines from a file like the GNU head utility, instead of just a single line.

It does not currently count blank lines. It is also subject to the batch-file line-length restriction of 8 KB.

uh? imo this is much simpler

3% for example will skip the first three chars. That is usefully when you are reading an UTF-8 file with BOM. – KargWare Jun 3 ’20 at 8:36

You might give this a try:

edit Or, say you have four columns of data and want from the 5th row down to the bottom, try this:

Thanks to thetalkingwalnut with answer Windows batch command(s) to read first line from text file I came up with the following solution:

Slightly building upon the answers of other people. Now allowing you to specify the file you want to read from and the variable you want the result put into:

This means you can use the above like this (assuming you called it getline.bat)

One liner, useful for stdout redirect with «>»:

To cicle a file ( file1.txt , file1[1].txt , file1[2].txt , etc.):

And it’s running.

The problem with the EXIT /B solutions, when more realistically inside a batch file as just one part of it is the following. There is no subsequent processing within the said batch file after the EXIT /B . Usually there is much more to batches than just the one, limited task.

To counter that problem:

(However, the so-called poison characters will still be a problem.)

More on the subject of getting a particular line with batch commands:

[Added 28-Aug-2012] One can also have:

How do I display a text file content in CMD?

I want to display the content of a text file in a CMD window. In addition, I want to see the new lines that added to file, like tail -f command in Unix.

12 Answers 12

You can use the more command. For example:

We can use the ‘type’ command to see file contents in cmd.

More information can be found HERE.

I don’t think there is a built-in function for that

This opens the files in the default text editor in windows.

This displays the file in the current window. Maybe this has params you can use.

There is a similar question here: CMD.EXE batch script to display last 10 lines from a txt file So there is a «more» command to display a file from the given line, or you can use the GNU Utilities for Win32 what bryanph suggested in his link.

You can use the ‘more’ command to see the content of the file:

Using a single PowerShell command to retrieve the file ending:

It applies to PowerShell 3.0 and newer.

Another option is to create a file called TAIL.CMD with this code:

To do this, you can use Microsoft’s more advanced command-line shell called «Windows PowerShell.» It should come standard on the latest versions of Windows, but you can download it from Microsoft if you don’t already have it installed.

To get the last five lines in the text file simply read the file using Get-Content , then have Select-Object pick out the last five items/lines for you:

If you want it to display the content of the file live, and update when the file is altered, just use this script:

That will repeat forever until you close the cmd window.

You can do that in some methods:

One is the type command: type filename Another is the more command: more filename With more you can also do that: type filename | more

The last option is using a for for /f «usebackq delims=» %%A in (filename) do (echo.%%A) This will go for each line and display it’s content. This is an equivalent of the type command, but it’s another method of reading the content.

If you are asking what to use, use the more command as it will make a pause.

There is no built in option available with Windows. To constantly monitor logs you can use this free application BareTailPro.

You can get the TAIL utility from the Windows Server 2003 Resource Kit Tools.

If you want to display for example all .config (or .ini ) file name and file content into one doc for user reference (and by this I mean user not knowing shell command i.e. 95% of them), you can try this :

• ForFiles : loop on a directory (and child, etc) each file meeting criteria
  • able to return the current file name being process (@file)
  • able to return the full path file being process (@path)
• Type : Output the file content

What’s the fastest way to read a text file line-by-line?

I want to read a text file line by line. I wanted to know if I’m doing it as efficiently as possible within the .NET C# scope of things.

This is what I’m trying so far:

8 Answers 8

To find the fastest way to read a file line by line you will have to do some benchmarking. I have done some small tests on my computer but you cannot expect that my results apply to your environment.

Using StreamReader.ReadLine

This is basically your method. For some reason you set the buffer size to the smallest possible value (128). Increasing this will in general increase performance. The default size is 1,024 and other good choices are 512 (the sector size in Windows) or 4,096 (the cluster size in NTFS). You will have to run a benchmark to determine an optimal buffer size. A bigger buffer is — if not faster — at least not slower than a smaller buffer.

The FileStream constructor allows you to specify FileOptions. For example, if you are reading a large file sequentially from beginning to end, you may benefit from FileOptions.SequentialScan . Again, benchmarking is the best thing you can do.

Using File.ReadLines

This is very much like your own solution except that it is implemented using a StreamReader with a fixed buffer size of 1,024. On my computer this results in slightly better performance compared to your code with the buffer size of 128. However, you can get the same performance increase by using a larger buffer size. This method is implemented using an iterator block and does not consume memory for all lines.

Using File.ReadAllLines

This is very much like the previous method except that this method grows a list of strings used to create the returned array of lines so the memory requirements are higher. However, it returns String[] and not an IEnumerable allowing you to randomly access the lines.