Convert double to qbytearray

Daniel Williams

I have a variable defined as a double which I want to write that as binary to a blob field in sqlite. The sqlite api expects a QByteArray of the data to write to the field.

double xValue = 70.976;
QByteArray blobData;

/*
hex value of xValue is: 0x428df3b6
The resulting blob should look something like:

blobData[0] is 0x42 (01000010)
blobData[1] is 0x8d (10001101)
blobData[2] is 0xf3 (11110011)
blobData[3] is 0xb6 (10110110)

*/

How do I copy the xValue onto the end of the blobData QByteArray in binary form?

SGaist

Hi,

QByteArray::number comes to mind.

Christian Ehrlicher

A double value is not only 4 bytes long...

Daniel Williams

@Christian-Ehrlicher True that. But, they would have 0's for the other 4 bytes.

Daniel Williams

@SGaist That's what I started with:

blobData.append(QByteArray::number(xValue));

What that does is write text to the field like a text field "70.976." That's not quite what I was hoping for.

Christian Ehrlicher

@Daniel-Williams said in Convert double to qbytearray:

True that. But, they would have 0's for the other 4 bytes.

No, you're wrong. The byte representation you're showing in your first post is the float value of 70.976.

float f = 70.976;
const unsigned char *ptr = (const unsigned char*)(&f);
for (int i = 0; i < 4; ++i)
    fprintf(stderr, "%02x\n", ptr[i]);
fprintf(stderr, "\n");
double d = 70.976;
const unsigned char *ptr2 = (const unsigned char*)(&d);
for (int i = 0; i < 8; ++i)
    fprintf(stderr, "%02x\n", ptr2[i]);

-->

Daniel Williams

@Christian-Ehrlicher Oh, I see what you're saying. And, in the process you showed me a way I may be able to do what I want. Once I have a const usigned char* to the xValue, I should be able to add 8 bytes to the blobData and then use memcpy to copy the bytes directly.

Daniel Williams

I'm working from home right now and can't get to the office to turn on the laser to test it fully, but this is what I'm thinking:

const unsigned char* dPtr = nullptr;

dataSize = blobData.length();
blobData.append(16, ' ');
dPtr = reinterpret_cast<const unsigned char*>(&xValue);
memcpy(outData.data()+dataSize, dPtr, 8);

Thank you again for the help.

SGaist

Out of curiosity, why are you storing a double value as a blob in your database ?

Daniel Williams

@SGaist I get about 1040 x and a z values from the sdk that pulls the data from the laser device. Each tuple represents a point. x is the position on the laser line and z is the height at that point. I store each tuple consecutively in the blob. Then, the program that renders the image pulls the data from the blob and does the processing and display of the image.

Each 1040 points represents 1 line. I get thousands of lines in a matter of seconds. Each line is a row in the database. Each row in the database thus represents the y axis of a 3-dimensiaonal image.

fcarney

Isnt this what QDataStream is for?