source of calculation error?

This is a discussion on source of calculation error? within the C++ Programming forums, part of the General Programming Boards category; After compiling the source code and executing the .exe for debugging, I've found a calculation error when I try to ...

  1. #1
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    source of calculation error?

    After compiling the source code and executing the .exe for debugging, I've found a calculation error when I try to convert celsius into fahrenheit, but I can't seem to figure out the source of it or how to fix it. Really appreciate your help, ppl.

    Code:
    //***************************************************************
    // Temperature Conversion Program
    // This program converts a Celsius temperature to its Fahrenheit
    // equivalent or vice versa
    //***************************************************************
    
    #include <iostream>
    #include <iomanip>
    #include <math.h>
    #include <stdlib.h>
    using namespace std;
    
    int main()
    {
    	float f;		// degrees farenheit
    	float c;		// degrees celsius
    	char option;		// type of conversion
    	char exit;		// flag for exit
    
    	cout.setf(ios::fixed, ios::floatfield);	// Set up floating
    	cout.setf(ios::showpoint);		// pt output format
    
    	do
    	{
    		// Introduction and Prompt type of conversion
    	
    		cout << "This program converts a Celsius temperature to"
    		     << endl << "its Fahrenheit equivalent or vice versa."
    		     << endl;
    		cout << "What type of conversion would you like to"
    		     << "perform? " << endl;
    		cout << "Enter \"c\" if you want degrees celsius" << endl;
    		cout << "and \"f\" if you want degrees farenheit. ";
    		cin >> option;
    
    		// Calculation and output
    
    		if (option == 'c')
    		{
    			cout << "Please enter the temperature in degrees";
    			cout << " farenheit: ";
    			cin >> f;
    
    			c = (5 / 9) * (f - 32);
    			
    			cout << setprecision(1) << f;
    			cout << " farenheits equals to " << setprecision(1);
    			cout << c << " celsius." << endl;
    		} // if (option == 'c')
    		else
    		{
    			cout << "Please enter the temperature in degrees";
    			cout << " celsius: ";
    			cin >> c;
    
    			f = (9 / 5) * c + 32;
    			
    			cout << f << " farenheits equals to " << c;
    			cout << " celsius." << endl;
    		} // else
    
    		// Prompt exit
    		
    		cout << endl;
    		cout << "Would you like to exit the program?('y' for yes) ";
    		cin >> exit;
    
    	} // do
    	while ( exit != 'y');
    	
    	system("pause");
    
    	return 0;
    
    } // int main()

  2. #2
    End Of Line Hammer's Avatar
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    c = (5.0 / 9.0) * (f - 32);
    f = (9.0 / 5.0) * c + 32;
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  3. #3
    C++ Developer XSquared's Avatar
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    c = (5.0f / 9.0f) * (f - 32.0f);
    f = (9.0f / 5.0f) * c + 32.0f;

    You need to use floats when dividing. If you leave off the .0, it treats them as integers, and rounds it down.

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  4. #4
    & the hat of GPL slaying Thantos's Avatar
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    You need to use floats when dividing. If you leave off the .0, it treats them as integers, and rounds it down.
    More correctly it truncates not rounds

  5. #5
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    >More correctly it truncates not rounds
    I don't get it. Why is saying it truncates more correct than saying it rounds?

  6. #6
    and the hat of wrongness Salem's Avatar
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    Because 2.9 truncates to 2, but rounds to 3
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  7. #7
    & the hat of GPL slaying Thantos's Avatar
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    another example:
    -4.6 would round to -5 but truncates to -4. Thus rounding down doesn't work in all cases.

  8. #8
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    Thank you all for your help!
    I guess I haven't been very careful all along when I was writing my source code or scanning the code for debugging purposes.

  9. #9
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    Ok, now that makes sense, thank you.

  10. #10
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    I have another problem.
    I don't understand something about the conditions of the terms inside pow().
    This is one of the examples where compiler gives me an error message because of a problem with pow().

    Code:
    //***********************************************************
    // Lease payment program
    // This program calculates the remaining balance after
    // each payment
    //***********************************************************
    
    #include <iostream>
    #include <iomanip>
    #include <stdlib.h>
    #include <math.h>
    using namespace std;
    
    const float PAYMENT = 165.25;	// amount of monthly payment
    const float I = 0.09;		// annual interest rate
    const int N = 36;		// total number of payments to be made
    
    int main()
    {
    	float balance;		// Remaining balance
    	float k;		// payment number
    
    	for ( k = 0; k <= 2; k++)
    	{
    		cout << "Payment #" << k << endl;
    		balance = PAYMENT * (1.0 - pow((1.0 + I), (k - N)) / I;
    		cout << "Balance remaining: $" << setprecision(2)
    		     << balance << endl;
    	} // for (k = 0; k <= 2; k++)
    
    	system("pause");
    
    	return 0;
    } // int main
    Please provide me a clearer explanation of the pow() function.

  11. #11
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    pow() takes two parameters of type double. The first parameter is the base and the second parameter is the power to raise the base to. pow() returns a double as the result of the calculation.

  12. #12
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    To solve the problem with the second code, should I double float every identifier that has to do with the power function (b/c it requires both of its parameters as double) and use a different flag variable in the for loop because a integer variable is required?

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