4 bytes array into float

[sandycoolxyz]

Hello,
I am using qt 4.8.5.
I have an array of 4 elements of _u8 type.
I would like to convert it into float. But I end up in wrong value.

= {1, 0, 0, 0} (hex values are 01 00 00 00)

qint32 value;
float value2;
for(i = 0; i < 4; i++)
      {
        value = (value << 8) + (data[i] & 0xff);                // if MSB is bit number 1
      }
value2 = (float)value;

If I out the value as int, the value I get is 16777216.
If I out the value2 as float, I get 1.67772e07. But I am expecting a float value 2.35099e-38(https://gregstoll.dyndns.org/~gregstoll/floattohex/).
How can I convert the 4 byte array into float?

[jsulm]

@sandycoolxyz Shouldn't it be like:

qint32 value = 0;
for(i = 0; i < 4; i++)
{
     value |= data[i]<<(8*i);
    // or (depending on byte order in the array
    value |= data[i]<<((4-i)*8);
}

[Paul Colby]

One option, assuming your array is always big-endian, and your code has to support both big an little-endian platforms:

    quint8 a[] = {1, 0, 0, 0};
    quint32 localEndian = qFromBigEndian<quint32>(a);
    qDebug() << *reinterpret_cast<float *>(&localEndian);

Output:

2.35099e-38

Cheers.

[sandycoolxyz]

Thanks Paul Colby and jsulm for the reply.
@jsulm I believe Its the same code as mine, just using addition instead of OR.

@Paul-Colby Thanks for the solution.

Cheers.

[mvuori]

Problems like that are exactly why people like to avoid bitwise operation when possible... For example, in this case many would prefer using an union of float & array for the conversion as it is more readable and maintainable years later...

[jsulm]

@mvuori But the union would not solve the problem with endianness.