在c ++中读取大的txt文件 [英] Read large txt file in c++

问题描述

我想在内存中读取大约5MB的文件...
文件具有这种格式（这是一个文本文件）

  ID 3：0 itemId.1 0 itemId.2 0 itemId.5 1 itemId.7 ...................... .. 20 itemId.500 
 ID 50：0 itemId.31 0 itemId.2 0 itemId.4 2 itemId.70 ..................... ... 20 itemId.2120 
 ..... 
  

我该怎么做高效地在C ++？

解决方案

阅读一行文件：

  ifstream fin（file.txt）; 
 string myStr; 
 
 while（getline（fin，myStr））//总是将读取放在while条件中。 
 {//如果有数据到
，则只进入循环//使用myStr数据//进程。否则，您需要阅读，然后
} //测试读取是否正确
 // 
 //注意：读取的最后一行将读取（但不是
 / /过去）然后结束的文件。因此，当
 //文件中没有数据时，它的状态仍然是
 // OK。直到你试图显式地
 //读过EOF标志被设置的文件末尾。 
  

为什么不显式调用close请参阅：

https://codereview.stackexchange.com/questions/540/如果效率是你的主要目标（可能不是）。如果效率是你的主要目标（可能不是）。然后将整个文件读入内存并从那里解析：请参阅下面的Thomas：用c ++读取大的txt文件

I'd like to read a file of about 5MB in memory ... the file has this format (it is a text file)

ID 3:  0 itemId.1 0 itemId.2 0 itemId.5 1 itemId.7 ........................ 20 itemId.500
ID 50:  0 itemId.31 0 itemId.2 0 itemId.4 2 itemId.70 ........................ 20 itemId.2120
.....

how can I do this efficiently in c++?

解决方案

Reading a file line by line:

ifstream fin ("file.txt");
string     myStr;

while(getline(fin, myStr))   // Always put the read in the while condition.
{                            // Then you only enter the loop if there is data to
    //use myStr data         // processes. Otherwise you need to read and then
}                            //  test if the read was OK
                             //
                             // Note: The last line read will read up to (but not
                             //        past) then end of file. Thus When there is
                             //        no data left in the file its state is still
                             //        OK. It is not until you try and explicitly
                             //        read past the end of file that EOF flag is set.

For a reason to not explicitly call close see:
https://codereview.stackexchange.com/questions/540/my-c-code-involving-an-fstream-failed-review/544#544

If efficiency is your major goal (its probably not). Then read the whole file into memory and parse from there: see Thomas below: Read large txt file in c++

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