QByteArray comparator

phamtv

I am working on application and ran across and issue I need some confirmation on. I have a byte array of data received from a device that can have values ranging from 0x00 - 0xFF. In my program, I have a statement as follow:

@
QByteArray rcvd_buffer; // received data read from device

if (rcvd_buffer[i] > 0xC8)
{
....
}
@
The above logic is always true but my rcvd_buffer[i] value is a 5.
Why is this the case? Do I have to cast rcvd_buffer[i] as a byte to have it worked properly?

danilocesar

well. gcc complains about it, but I don't know why yet..
I'm doing some test.

phamtv

just a thought... do you think it is because rcvd_buffer[i] is a char and characters can only be from 0-0x7F?

danilocesar

Certainly is something related to the conversion.
I'm still looking for it..

Someone else?

danilocesar

outch...

0xC8 is negative if you compare it with a char...

cast it to unsigned char and it will work:
@if ((unsigned char) ba[1] > 0xC8)@

phamtv

it looks like I have to cast my data to (quint8)rcvd_buffer[i] since my data can range from 0x00-0xFF... I thought that QByteArray should be an array of unsigned char instead of signed char.... I assume I will have to double check to see if me assigning negative values to rcvd_buffer[i] would error out as well... Please post if anyone has any insights...

Asazeus

I use this method of comparation QByteArray element with literal in Qt5.2.1:
@if ( ans[startIndex+1].operator ==(0xFF) ) {/.../}
@

it is wierd, but it works :)

Chris Kawa

Asazeus - you do realize that this is the same as
@ if(ans[startIndex+1] == 0xFF)
@
just uglier and less readable? There's almost never a reason to call operators explicitly.

The problem is that == will work, but < or > will not in this case. It's a signed/unsigned problem as noted before.