Thread: when is 4.2 = 4.20000000000.9

ihave written a program that will give the root to a given power of a given number ie cube root of 125 is 5.

that all works. i then decided that i didnt want a lot of trailing zeros so wrote the following

Code:

int main()
{
    //declare variables
    double iBaseNum = 311.1696, iPowerNum = 0.25, iResult;

    iResult = Power( iBaseNum, iPowerNum );

    printf("%f to the power %f = ", iBaseNum, iPowerNum);

    PrinttoScreen( iResult );


    return 0;
}

double Power( double x, double y )
{
    static int itmpPower = 0, i = 0;
    static double iBase = 1, presision = 0.000001;
    if ( y == 0 )
    {
        //anything to the power zero is 1 so return 1
        return 1;
    }//*
    else if ( y < 1 && y > 0 )
    {
        itmpPower = 1 / y;
        while ( i <= x )
        {
            iBase += 1;
            i = Power( iBase, itmpPower );
        }
        iBase -= 1;

        if ( x != Power( iBase, itmpPower ) )
        {
            printf("this run!\n");
            while ( Power( iBase, itmpPower ) < x )
            {
                iBase += presision;
            }
        }

        return iBase;
    }//*/
    else if ( y == 1 )
    {
        //end of line so return base
        return x;
    }

    return x * Power( x, --y );
}
void PrinttoScreen ( double iResult )
{
    int x, iCount = 0;
    double itmpResult;

    x = iResult;
    itmpResult = iResult;

    while ( x < itmpResult )
    {
        if ( iCount == 9 )
        {
            break;
        }
        itmpResult *= 10;
        x = itmpResult;
        iCount += 1;
    }

    switch ( iCount )
    {
        case 0:
            printf("%d\n", x);
            break;

        case 1:
            printf("%0.1f\n", iResult);
            break;

        case 2:
            printf("%0.2f\n", iResult);
            break;

        case 3:
            printf("%0.3f\n", iResult);
            break;

        case 4:
            printf("%0.4f\n", iResult);
            break;

        case 5:
            printf("%f0.5\n", iResult);
            break;

        case 6:
            printf("%f0.6\n", iResult);
            break;

        case 7:
            printf("%f0.7\n", iResult);
            break;

        case 8:
            printf("%f0.8\n", iResult);
            break;

        case 9:
            printf("%f0.9\n", iResult);
            break;
    }
}

if i enter 311.1696 to the power 0.25 (4th root) i get 4.2000000.9 not 4.2 i get there is prob some rounding errors in the registers etc however why is there 2 decimal places?