How to use QFile, QByteArray and QString?

victor wang

Hi, i'm using Qt5.5 in my computer.
I'm programming a code that can read what i write in the EEprom.
I just wanna read the valid ASCII out instead of all of the value in EEprom.
This is my code.

I had try to use not only readLine()、Read() but also readAll().
But none of it do what i want.
This is the picture that i cat from my eeprom here.

I just want to read the word "o" in the beginning and "6" in the end.
What can i so ?
Please help!

jsulm

@victor-wang What exactly does not work?
It is just a file - so just read and parse it as any other file.

victor wang

@jsulm
When i read it out it will read not only words but also the unknown symbol below the end of the word "6".

Like this picture shows what it shows.

how can i get avoid from the unknown words at the end?

jsulm

@victor-wang It depends on what exactly you want to get from the file. You can read line by line until you get what you want and then stop.

victor wang

@jsulm
I've used readLine() but there are some quest.
1.How can i know it is the end of the word?
2.Will it know by itself?

jsulm

@victor-wang What does "word" mean? It will not know by itself, you should know.
Can you explain EXACTLY what you want to read from the file? It is really unclear.

victor wang

@jsulm
I just wanna read this.
Not the black question marks.

jsulm

@victor-wang And what is the problem? Just read line by line the first 6 lines.

victor wang

@jsulm
That because if there is other person who want to write things into eeprom.
I will show what he write in the eeprom.
According to this addition, I can't just read for six lines.
Because not every one will just write 6 lines.

I have to know where is the last word.

jsulm

@victor-wang Again: what is "last word"? How is it defined? I still don't understand what information you want to get. Are those MAC2, Model, PN, ...?

victor wang

@jsulm

For example the picture i gave you.
The last word means the last line of the "6".
Picture here

Those words is what i write in eeprom.
For other example if i write "skdjb,smdnv6546321" into eeprom.
Then the last Alphabet is "1" and i just want to read what i write.

jsulm

@victor-wang Then read line by line and parse each line to check whether it is what you want.
And you should use correct wording, else it's hard to understand what you mean: a word is not a line.

victor wang

@jsulm
Sorry, how to parse it?
I will not know what will other people write, how can parse?

jsulm

@victor-wang OK, you don't know what others are writing, right? But you know what you want to read, right?
If so then:

QByteArray whatIWantToRead = "ABCD";
QString line = file.readLine();
if (line == whatIWantToRead)
    // do something

victor wang

@jsulm
If i want to read a~z、A~Z and some other exclamation mark and question mark and etc..
I have to write all of it into whatIWantToRead ?

jsulm

@victor-wang You can use regular expressions, see https://en.wikipedia.org/wiki/Regular_expression and http://doc.qt.io/qt-5.8/qregularexpression.html

victor wang

@jsulm
Do you have an example?
Or can i parse it by ASCII ?

jsulm

@victor-wang There are a lot of examples for regular expressions on the Internet

J.Hilk

@victor-wang

If you know, that valid lines always contain legit chars between 'A' and 'z'

You can make a simple If-statement with the first char in your bytearray:

QByteArray bArray = file.readline();
if(bArray[0] < QLatin1Char('A') || bArray[0] > QLatin1Char('z'))
     //Abort condition met

use QLatin1Char if its ASCII, QChar if its unicode.