Qt Quickly Read the Last Line of Contents of Large Files Solution

0. Preface

Sometimes, depending on the requirement, you need to read the last line of the text.

If it is a small text, read it in order and then take the last line and then Ok; but if it is a large text, even if it is placed in a thread, there will be a delay in reading the last line, which is relatively time-consuming.

1. Solution

CheckQFileHelp manual, which has

bool seek(qint64 pos);

It means finding the location in the device. Returns true when successful; otherwise returns false.

Therefore, you can check from the back to the previous position. If you include a newline, it means that the last line has been obtained.

The code is as follows:

    QFile file("C:\\Users\\Desktop\\");
    if((QIODevice::ReadOnly | QIODevice::Text))
    {
        QTextStream in(&amp;file);
        (QTextCodec::codecForName("UTF-8"));
        int nSize = ();
        // When size == 0, return        if(nSize &lt; 1)
        {
            qDebug().noquote() &lt;&lt; "No data.";
            return;
        }
        // When size == 1, jump out        int nTmp = 0;
        while(!()){
            ();
            ++nTmp;
            if(nTmp &gt; 1)
            {
                break;
            }
        }
        // Get 1 line of content        if(nTmp &lt; 2)
        {
            (0); // Move the cursor to the text start position            qDebug().noquote() &lt;&lt; "Read the first line: " &lt;&lt; ();
            return;
        }
        // When size > 1, read from behind to front. When reading "\r\n", add the length of "\r\n", so that the position is adjusted to the last line and read a single line        // Different systems may have different lines splitting, and the Windows platform is "\r\n"        int nIndex = 1;
        (nSize - nIndex);
        while(!().contains("\r\n"))
        {
            ++nIndex;
            (nSize - nIndex);
        }
        (nSize - nIndex + 2); // Move the cursor to the last line to start        qDebug().noquote() &lt;&lt; "Read the last line: " &lt;&lt; ();
    }

It is divided into three situations:

When the text is empty, prompt or return
When there is only one line of text, the first line is output
When text > one line, perform an operation

2. Results

May you have a month of overjoyment.

Supplement: The problem of endless empty lines at the end of qt file

   QString line;
        QTextStream in(&amp;file1);  //Construct streams with files        
        while(1)
        {
            line=();//Loop reading the lower line            if(!())//There is content of the string, and the newline character is also content            {
                (line);
            }
            else
                break;
        }

This will cause the last line of the file to be a blank line and cannot be read because it is an empty string (and there is no newline afterwards), and cannot be read. If it is an empty line in the middle of the file, although it is also an empty string, there is a newline at the end of this line (it is not an empty string). It can be continued to read, which is caused by this reason.

Solution:

        QTextStream in(&amp;file1);  //Construct streams with files        lis = ().split("\n");

Summarize

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