Working with files in Qt

Question

Let's say I write some information in a file, and it writes with n cycles, for example as follows:

a,a,a,a,
b,b,b,b,
c,c,c,c,
a,a,a,a,
b,b,b,b,
c,c,c,c,
.......
a,a,a,a,
b,b,b,b,
c,c,c,c,

Now I want to open the file check the first line, find where it repeats and delete everything after that. For my example case let's say I want to wind where a,a,a,a, meets again, and to delete it, and everything after that, getting the follows instead:

a,a,a,a,
b,b,b,b,
c,c,c,c,

Q: How can I do that?

@Angew I don't have any idea how to do it directly in the file. — Mike, Sep 03 '13 at 14:25
Have a look at [this question](http://stackoverflow.com/questions/873454/how-to-truncate-a-file-in-c). — Angew is no longer proud of SO, Sep 03 '13 at 14:32
Start by skimming through `QFile` docs, and reading everything relevant you spot. Then refine the question to be a bit more specific... — hyde, Sep 03 '13 at 14:39

score 1 · Answer 1 · answered Sep 03 '13 at 14:20

1

You want to remove duplicate lines in a file. If you follow the next steps you will get what you want.

Create a vector that will store the hashes of unique lines ( QVector<QString>) Notice that using QMap would be faster.
Create an ouput file
For every line in the file calculate it's hash. Use QCryptographicHash or qHash (in this case you should have a vector of uints.
- If the calculated hash is contained in the vector skip this line
- Otherwise add the hash to the vector and print the line to the output file.
At the end the output file should contain only unique instances of the input file.

answered Sep 03 '13 at 14:20

pnezis

12,023
2
38
38

Hi, thanks for the answer. That does not actually fit me, as the number of cycles can be very big, and I do not want to keep so much information in RAM. I want to do everything directly in the file. – Mike Sep 03 '13 at 14:24
@Mike create a temp file. It's not possible to insert or delete in a middle of a file. Not sure if you want this, but sounds like it. – hyde Sep 03 '13 at 14:41

score 1 · Accepted Answer · answered Sep 04 '13 at 14:49

You can use QTextStream to stream your file (so, do not care about RAM). Then use readLine() function to read one line at a time to QString, and compare with new line. Here some code sample:

int lineCounter = 0; //count for line
QFile f(filePath);
if (!f.open(QIODevice::ReadOnly | QIODevice::Text))
    return false;
QTextStream stream(&f);
QString line;
// read the first line
line = stream.readLine();
lineCounter++;
QString hash = QString(QCryptographicHash::hash(line.toAscii(), QCryptographicHash::Md5).toHex());
do
{
    line = stream.readLine();
    if (QString(QCryptographicHash::hash(line.toAscii(), QCryptographicHash::Md5).toHex()).compare(hash) == 0)
    {
        // Save data from 1st line to "lineCounter" to new file, or do your own works;
        // and then break;
    }
    lineCounter++;
} while (!line.isNull());

Working with files in Qt

2 Answers2