warning C4244: '=' : conversion from 'double' to 'float', possible loss of data

Hello. I'm still new in programming. (It has been about 2 days since I started reading tutorials online and practicing it). I still have a lot of time, a lot to learn, fortunately the school holiday is long in my local area (about 3 months).

Anyway, sorry for the long introduction. I just want to ask you a question regarding the warning message I got stated in the title.

I'm using an IDE, Microsoft Visual 2010 C++ Express. Please, take a look on this and tell me the part that is needed to be altered so that I solve this problem.

Code:

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//This program is an answer to the question number 1 (now question 2!) given in http://www.doc.ic.ac.uk/~wjk/C++Intro/RobMillerE2.html.

/*To convert Fahrenheit to Celcius:
    substract by 32,
    multiply by 5,
    then divide by 9.


  To convert Celcius to Kelvin;
    add 273.15.                        */




#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;


int main() {


    float LOWER, UPPER, STEP;


   


    cout << "This program can help you to make a table of conversion of temperature between Fahrenheit, Celcius, and Kelvin.\n";
    cout << "Please specify the range of values of Fahrenheit temperature (from lowest to highest)\n";
    cout << "and also the desired step (e.g: for 10 steps between range 0 and 100 is: 0,10,20,30... 100).\n";
    cout << "by answering the questions given below.\n\n";


    cout << "Enter the desired lowest value of Fahrenheit temperature and press ENTER.\n";
    cin  >> LOWER >> "\n";


    cout << "Enter the desired upper value of Fahrenheit temperature and press ENTER.\n";
    cin  >> UPPER >> "\n";


    cout << "Enter the desired steps within the range and press ENTER.\n";
    cin  >> STEP >> "\n";


    //declaring variable.
    float Fahrenheit, Celcius, Kelvin;
 
      cout << setiosflags (ios::left);
      cout.width(20);
      cout << "Fahrenheit";
      cout << "Celcius";
      cout << setiosflags (ios::right);
      cout.width(20);
      cout << "Kelvin\n\n";


     
      cout.setf(ios::fixed);
      cout.precision(2);


      for (Fahrenheit = LOWER; Fahrenheit <= UPPER; Fahrenheit = Fahrenheit + STEP) {


          Celcius = ((Fahrenheit - 32)*5)/9;


       


          Kelvin = Celcius + 273.15;


       


          cout << setiosflags (ios::left);
          cout.width(20);
          cout << Fahrenheit;
          cout << setiosflags (ios::left);
          cout.width (20);
          cout << Celcius;
          cout << setiosflags (ios::right);
          cout.width(20);
          cout << Kelvin << "\n";
      }
     
    system("pause");


    return 0;


}

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Code:

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cin  >> LOWER >> "\n";

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You can't do this. cin stores input from the user into a variable. "\n" isn't a variable.

Regarding the warning... the standard says that a double may be larger than a float, and if such is the case, then you may lose data as a double is truncated to fit into a float.

Kelvin = Celcius + 273.15;
Here, Kelvin and Celsius are floats, but 273.15 is a double, hence the warning.
To avoid it, make all variables double or change 273.15 to 273.15f to tell the compiler that it's a float, not a double.

There are two representations of floating-point numbers - double precision and single precision. A variable declared to be of type float is single precision, one of type double is double precision. Double-precision floating-point numbers can store a greater range of numbers, and store numbers to a greater accuracy, than single-precision floating-point numbers. So when you convert from double to single precision, there's a chance you could lose information.

Now, when you just write a number with a decimal point in it, like 273.15, that has double precision. In your code you do a couple of calculations involving such numbers:

Code:

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    Celcius = ((Fahrenheit - 32)*5)/9;
    Kelvin = Celcius + 273.15;

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In e.g. Celcius + 273.15 the calculation must be done with both numbers as double-precision numbers, since otherwise the compiler might force you to lose information - so the result of this calculation is a double-precision number. When assigning this to Kelvin, a conversion to single precision must be made, but the compiler cannot circumvent this in the way it did for Celcius + 273.15, since that's what you've told it to do - instead, it issues a warning to let you know that this could be an error.

Some people will tell you "you can safely ignore this warning" - my advice would be to always fix all warnings your compiler gives you. It will make your life easier.

There are two solutions - first, you could use an explicit cast so that the compiler knows that the conversion to single precision is what you want, e.g:

Code:

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    Kelvin = (float)(Celcius + 273.15);

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Secondly, you could change the types of your variables to double.

Thanks Elysia and JohnGraham!

From your reply and a good long explanation, I can safely say that the conversion from float (smaller range) to double (larger range) will not lead to any data loss, right?

And one more thing, the note from which I read from use this kind of 'explicit cast':

Code:

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Kelvin = static_cast<float>(Celcius + 273.15);

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Is this the same as

Code:

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Kelvin = (float)(Celcius + 273.15);

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??

Anyway, I'm moving on to question 3.

Thanks again! :)

Quote:

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Originally Posted by Meerul264

From your reply and a good long explanation, I can safely say that the conversion from float (smaller range) to double (larger range) will not lead to any data loss, right?

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You are correct. The compiler does that all the time. In your code, it will convert your Celcius variable to a double before doing the calculation before assigning to Kelvin. So it's okay.

Quote:

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And one more thing, the note from which I read from use this kind of 'explicit cast':

Code:

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Kelvin = static_cast<float>(Celcius + 273.15);

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Is this the same as

Code:

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Kelvin = (float)(Celcius + 273.15);

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Yes, it is the same thing. However, explicit casts are safer because they protect you from more mistakes. Hence, if you must use a cast, then you should use the explicit cast.

Quote:

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Originally Posted by Elysia

You are correct. The compiler does that all the time. In your code, it will convert your Celcius variable to a double before doing the calculation before assigning to Kelvin. So it's okay.

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The C/C++ standards also specify that the set of values that a float can represent is a strict subset of the values that a double can represent (or words to that effect).

Quote:

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Originally Posted by Elysia

Hence, if you must use a cast, then you should use the explicit cast.

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What you say is perfectly correct (assuming strong highlighting of "if you must use a cast"). In this particular case, it is safest to not use a cast at all (eg use a literal value 273.15f rather than 273.15).