float/int issue if converting C to F

This is a discussion on float/int issue if converting C to F within the C++ Programming forums, part of the General Programming Boards category; Hello, I created a program to convert Celsius to Fahrenheit. Code: #include <iostream> using namespace std; float Fahrenheit, Celsius; int ...

  1. #1
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    float/int issue if converting C to F

    Hello,

    I created a program to convert Celsius to Fahrenheit.

    Code:
    #include <iostream>
    using namespace std;
    float Fahrenheit, Celsius;
    int main()
    {
    float Fahrenheit;
    float Celsius;
    
    cout<<"Enter the temperature in Fahrenheit: ";
    cin>>Fahrenheit;
    
    Celsius=(Fahrenheit-32)*5/9;
    
    cout<<"The temperature in Celsius is: "<<Celsius;
    cout << (int)Celsius << endl;
    system ("pause");
    
    
    return 0;
    }
    But it needs to round up or down. Meaning 55 F need to be 13 Celsius and not 12.7... I read now that I need to change it to int..which I did right here

    Code:
    cout<<"The temperature in Celsius is: "<<Celsius;
    cout << (int)Celsius << endl;
    system ("pause");
    Also I need to do it by adding 0.5 to the calculation expression. Well that's it

    Code:
    Celsius=(Fahrenheit-32)*5/9;
    but of course it wont work, if I do add + 0.5 to it. Can someone help me with this?

  2. #2
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    Casting always rounds down.

    Code:
    cout << (int) (Celsius + 0.5) << endl;

  3. #3
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    You could find the the value of the first decimal place by multiplying by ten, and finding the remainder when you divide by ten:

    Code:
    int d = (Celsius * 10) % 10;
    If that number is greater than 5, cast it to an int and add one. If not, just cast it to an int.

  4. #4
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    Quote Originally Posted by cyberfish View Post
    Casting always rounds down.

    Code:
    cout << (int) (Celsius + 0.5) << endl;

    Dosen't work... 55F is still 12.7 C if I do this.
    Last edited by XodoX; 02-08-2009 at 06:13 PM.

  5. #5
    CSharpener vart's Avatar
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    Isn't this C++?

    Dosen't work... 55F is still 12.7 C if I do this.
    I do not beleive that you can output integer as 12.7
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by vart
    Isn't this C++?
    Certainly looks like it, so this thread has been moved to the C++ programming forum.
    C + C++ Compiler: MinGW port of GCC
    Version Control System: Bazaar

    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  7. #7
    Registered User QuestionKing's Avatar
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    The reason it is not working is simple....
    casting does not "round" either way.
    casting a decimal to an int simply truncates everything after the '.' leaving only the int value in the final int variable

    you would have to add 0.5 post F-C conversion and pre cast to int...
    Code:
    Celsius=((Fahrenheit-32)*5/9)+0.5;
    cout << (int)Celsius << endl;
    ...wow, edited for multiple typos lol
    Last edited by QuestionKing; 02-09-2009 at 01:47 AM.

  8. #8
    Registered User Sshakey6791's Avatar
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    Why do you declare your variable globe and then local???



    Code:
    float Fahrenheit, Celsius;
    
    int main()
    {
    
    	float Fahrenheit;
    	float Celsius;

    jw....
    "Blood you have thirsted for -- now, drink your own!"
    (Dante)

  9. #9
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    Hey!

    Thanks for the input. It helped a lot! Basically the floating-point datay type is used for those things?

    Like this following code


    Code:
    #include <iostream>  
    using namespace std;
    
    // Function Prototypes
    
    void pause(void);
    
    // Variables
    
    float      pennies;
    float      nickels;
    float      dimes;
    float      quarters;
    double       total_value;
    
    //******************************************************
    // main
    //******************************************************
    
    int main(void)
      {
      // Input	
      cout << "\nHow many pennies do you have? --->: ";
      cin >> pennies;
      cout << "\nHow many nickles do you have? -->: ";
      cin >> nickels;
      cout << "\nHow many dimes do you have -->: ";
      cin >> dimes;
      cout << "\nHow many quarters do you have -->: ";
      cin >> quarters;
      
      // Process
      total_value = (pennies * 0.01) + (nickels * 0.05) + (dimes * 0.1) + (quarters * 0.25) ;
    
      // Output
      cout << "\nThe total value of your coins is: ";
      cout << total_value;
    
      pause();
      return 0;
      }
    
    //******************************************************
    // pause
    //******************************************************
    
    void pause(void)
      {
      cout << "\n\n";
      system("PAUSE");
      cout << "\n\n";
      return;
      }
    Each coins value is a floating-point data type, a fraction of a dollar. So I use the floating-point like this, right? But int would have done it too in this case. So why use floating-point? Cause of the larger values I might get?

  10. #10
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    A good way to do this is:

    Code:
    #include <iostream>
    #include<cmath>
    using namespace std;
    float Fahrenheit, Celsius;
    int main()
    {
    float Fahrenheit;
    float Celsius;
    
    cout<<"Enter the temperature in Fahrenheit: ";
    cin>>Fahrenheit;
    
    Celsius=(Fahrenheit-32)*5/9;
    
    float decimal = Celsius - floorf(Celsius);
    if(decimal < 0.5)
    {
    Celsius = floorf(Celsius);
    }
    else
    {
    Celsius = ceilf(Celsius);
    }
    
    cout << "The temperature in Celsius is: " << Celsius << endl;
    system ("pause");
    
    
    return 0;
    }
    the new code is in bold


    and for the last question, an int would have done it since you are using floating points with coin values...
    Last edited by Cherry65; 02-15-2009 at 04:55 PM.

  11. #11
    Captain Crash brewbuck's Avatar
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    When writing code that deals with fractional values, get in the habit of appending ".0" to all integer values to force the compiler to generate floating point code.

    Instead of x*5/9, write x*5.0/9.0, and the math will automatically happen in the floating point domain.

    As far as rounding, the trick:

    Code:
    int rounded = (int)(val + 0.5);
    Only works if val is positive. If val is negative this will round the wrong way. The correct strategy is:

    Code:
    int rounded = (int)(val + (val >= 0) ? 0.5 : -0.5)
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  12. #12
    Master Apprentice phantomotap's Avatar
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    Code:
    int rounded = (int)(val + (val >= 0) ? 0.5 : -0.5)
    Code:
    int rounded = (int)(val + ((val >= 0) ? 0.5 : -0.5));
    Soma

  13. #13
    Registered User whiteflags's Avatar
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    Forgive me but the only difference I see is the parens. Addition is commutative, though.

  14. #14
    Master Apprentice phantomotap's Avatar
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    Forgive me but the only difference I see is the parens.
    Well, that's pretty much the only difference. ^_^

    Addition is commutative, though.
    This is an issue of precedence; not an issue of associativity or mathematics.

    Soma

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