Temperature convert two int digits to float - c++

Question

How can I convert two unsigned integers that represent the digit and decimal part of a float, into one float.
I know there are a few ways todo this, like converting the decimal component into a float and multiplying it to get a decimal and added it to the digit, but that does not seem optimal.
I'm looking for the optimal way todo this.
/*
* Get Current Temp in Celecius.
*/
void GetTemp(){
int8_t digit = 0; // Digit Part of Temp
uint16_t decimal = 0; // Decimal Part of Temp
// define variable that will hold temperature digit and decimal part
therm_read_temperature(&temperature, &decimal); //Gets the current temp and sets the variables to the value
}
I want to take the Digit and Decimal parts and convert them to a float type, such that it looks like digit.decimal .
It might look like this in end, but I want to find the MOST optimal solution.
/*
* Get Current Temp in Celecius.
*/
float GetTemp(){
int8_t digit = 0; // Digit Part of Temp
uint16_t decimal = 0; // Decimal Part of Temp
// define variable that will hold temperature digit and decimal part
therm_read_temperature(&temperature, &decimal); //Gets the current temp and sets the variables to the value
float temp = SomeFunction(digit, decimal); //This could be a expression also.
return temp;
}
////UPDATE/// - July 5th
I was able to get the source code instead of leveraging just the library. I posted it in this GIST DS12B20.c.
temperature[0]=therm_read_byte();
temperature[1]=therm_read_byte();
therm_reset();
//Store temperature integer digits and decimal digits
digit=temperature[0]>>4;
digit|=(temperature[1]&0x7)<<4;
//Store decimal digits
decimal=temperature[0]&0xf;
decimal*=THERM_DECIMAL_STEPS_12BIT;
*digit_part = digit;
*decimal_part = decimal;
Although the function will not force us to return separate parts as digit and decimal, reading from the temperature sensor seems to require this (unless i'm missing something and it can be retrieved as a float).
I think the original question still stands as what is the optimal way to make this into a float in C (this is for use with AVR and an 8bit microprocessor, making optimization key) using the two parts or to be able to retrieve it directly as a float.

Answer 1

What you are really running into is using fixed-point numbers. These can be represented in two ways: either as a single integer with a known magnitude or multiplier (ie. "tenths", "hundredths", "thousandths", and so on; example: value from a digital scale in ten-thousandths of a gram, held in a 32-bit integer -- you divide by 10000 to get grams), or as two integers, with one holding the "accumulated" or "integer" value, and the other holding the "fractional" value.
Take a look at the <stdfix.h> header. This declares types and functions to hold these fixed-point numbers, and perform math with them. When adding fractional parts, for example, you have to worry about rolling into the next whole value, for which you then want to increment the accumulator of the result. By using the standard functions you can take advantage of built-in processor capabilities for fixed-point math, such as those present in the AVR, PIC and MPS430 microcontrollers. Perfect for temperature sensors, GPS receivers, scales (balances), and other sensors that have rational numbers but only integer registers or arithmetic.
Here is an article about it: "Fixed Point Extensions to the C Programming Language", https://sestevenson.wordpress.com/2009/09/10/fixed-point-extensions-to-the-c-programming-language/
To quote a portion of that article:
I don’t think the extensions simplify the use of fixed types very
much. The programmer still needs to know how many bits are allocated
to integer and fractional parts, and how the number and positions of
bits may change (during multiplication for example). What the
extensions do provide is a way to access the saturation and rounding
modes of the processor without writing assembly code. With this level
of access, it is possible to write much more efficient C code to
handle these operations.
Scott G. Hall
Raleigh, NC, USA

Answer 2

Your question contains a wrong assumption.
If you're given a decimal string and want a floating-point value, the first step should generally not be to turn it into two integers.
For instance, consider the numbers 2.1 and 2.01. What's the "decimal part" in each case? 1 and 01? Both of those equal 1. That's no good.
The only case in which this approach makes any sense is where you have a fixed number of places after the decimal point -- in which case maybe 2.1 turns into (2,1000) and 2.01 turns into (2,100), or something. But unless you've got a positive reason for doing that (which I strongly doubt) you should not do it this way.
In particular, unless therm_read_temperature is a function someone else is providing you with and whose interface you can't influence, you should make that function behave differently -- e.g., just returning a float. (If it is a function someone else is providing and whose interface you can't influence, then to get a useful answer here you'll need to tell us exactly what it's defined to do.)