确定16位二进制数是负数还是正数

Question

I'm creating a library for a temperature sensor that has a 16-bit value in binary that is being returned. I'm trying to find the best way to check if that value returned is negative or positive. I'm curious as to whether or not I can check if the most significant bit is a 1 or a 0 and if that would be the best way to go about it, how to successfully implement it.

I know that I can convert it to decimal and check that way but I just was curious if there was an easier way. I've seen it implemented with shifting values but I don't fully understand that method. (I'm super new to c++)

float TMP117::readTempC(void)
{
    int16_t digitalTemp;      // Temperature stored in the TMP117 register

    digitalTemp = readRegister(TEMP_RESULT); //Reads the temperature from the sensor

    // Check if the value is a negative number
    /*Insert code to check here*/

    // Returns the digital temperature value multiplied by the resolution
    // Resolution = .0078125
    return digitalTemp*0.0078125; 
}

I'm not sure how to check if the code works and I haven't been able to compile it and run it on the device because the new PCB design and sensor has not come in the mail yet.

Answer 1

I know that I can convert it to decimal and check that way

I am not sure what you mean. An integer is an integer, it is an arithmetic object you just compare it with zero:

if( digitalTemp < 0 )
{
    // negative
}
else
{
    // positive
}

You can as you suggest test the MSB, but there is no particular benefit, it lacks clarity, and will break or need modification if the type of digitalTemp changes.

if( (digitalTemp & 0x8000 )
{
    // negative
}
else
{
    // positive
}

"conversion to decimal", can only be interpreted as conversion to a decimal string representation of an integer , which does not make your task any simpler, and is entirely unnecessary.

I'm not sure how to check if the code works and I haven't been able to compile it and run it on the device because the new PCB design and sensor has not come in the mail yet.

Compile and run it on a PC in a test harness with stubs for teh hardware dependent functions. Frankly if you are new to C++, you are perhaps better off practising the fundamentals in a PC environment with generally better debug facilities and faster development/test iteration in any case.

Answer 2

In general

float TMP117::readTempC(void)
{
    int16_t digitalTemp;      // Temperature stored in the TMP117 register

    digitalTemp = readRegister(TEMP_RESULT); //Reads the temperature from the sensor

    // Check if the value is a negative number
    if (digitalTemp < 0)
    {
         printf("Dang it is cold\n");
    }


    // Returns the digital temperature value multiplied by the resolution
    // Resolution = .0078125
    return digitalTemp*0.0078125; 
}