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C++ homework

  1. Jan 24, 2007 #1
    Hi, could some one please verify that I successfully completed the objective of my programming homework? This is my first assignment and I'm just being extra cautious that I did everything right.

    So the user provides measurements in inches but the program must then convert them into centimeters. Here is my code:

    Code (Text):
    #include <iostream>

    using namespace std;

    int main()
        double radius;
        double height;
        double cost;
        int nContainers;
        double SA;
        double costContainer;
        double costAll;
        float pi = 3.14159265;
        cout << "Enter the radius of the base (in inches): ";
        cin >> radius;
        cout << "Enter the height of the container (in inches): ";
        cin >> height;
        cout << "Enter the cost of each container per square centimeter: $";
        cin >> cost;
        cout << "Enter the number of containers to be produced: ";
        cin >> nContainers;
        double Radius = radius * 2.54;
        double Height = height * 2.54;
        SA = pi * Radius * Radius + 2 * pi * Radius * Height;
        costContainer = SA * cost;
        costAll = costContainer * nContainers;
        cout << nContainers << " container(s) at $" << cost << " per square centimeter will cost $" << costContainer << " each." << endl;
        cout << "The grand total for " << nContainers << " containers is $" << costAll << "." << endl;
        return 0;
  2. jcsd
  3. Jan 25, 2007 #2


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    Homework Helper

    Seems fine. I would probably have tried to make it more structured but because it is such a simple example, I don't think it matters.
  4. Jan 25, 2007 #3
    I have one question and one "warning".

    Why does the user input values in inches, but then cost in $/cm? Was that your choice, or might that be a mistake? Seems quite awkward.

    You define pi as 3.14159265, but on a standard compiler, float stores the first 6 digits. 3.14159265 will become 3.141592, which is wrong because the 6 should round up to make the 2 a 3. Try "float pi = 3.141593;" instead.

    Other than those slight things, this program is very solid for being your first assignment. Good job! Now all you need is comments.
    Last edited: Jan 25, 2007
  5. Jan 25, 2007 #4
    What difference does it make, neither is accurate.
  6. Jan 25, 2007 #5
    There's no sense in being less accurate than you have to. :confused:
  7. Jan 25, 2007 #6
    Well, for the sake of an argument, let's see: error is (3.141593 - 3.141592) (Radius * Radius + 2 * Radius * Height); let's have Radius = Height for the sake of simplicity, so the above expression turns into 0.000003 * Radius * Radius, right?

    Now, let's take some real world price example, like $10.77 for 30 sq. ft., or 2.7870912 sq meters. From here, we want to know how big our cylinder has to be so that the above error would be of any significance, the equation is: 0.000003 * Radius * Radius = ($0.01 / $10.77) * 2.7870912 sq. m., which yields Radius = sqrt( ((0.01 / 10.77) * 2.7870912) / 0.000003 ) ~ 29.37 meters.

    In other words, you will have less than 1 cent of error, trying to bubblewrap anything less than 60 meters in diameter.
  8. Jan 25, 2007 #7
    The user is supposed to enter dimensions in inches (per my instructor) but the program description says I need to calculate the cost per square centimeter. So unless my instructor wants us to change everything to inches and ignore that part of the description, it's just going to be awkward.

    So that is why I converted from IN to CM using 2.54 cm per inch.

    And thanks for the float tip.
  9. Jan 25, 2007 #8


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    Science Advisor
    Homework Helper

    Use the constant M_PI from the <math> include file.

    Then you get the best accuracy possible on the hardware you are running on, no chance of making a typo, and no futile discussions about the best way round decimal numbers before the compiler converts them to binary, and whether accuracy matters or not.

    Re the "typos" thing, I once was debugging a program and I used the value of pi as one of the input quantities in the test data, because it made the answer come to a simple number to check.

    Problem was, the number in my input file was 3.41459 not 3.14159. It took me three days solid work to find out there was nothing wrong with the program after all. :mad:

    BTW M_PI is a double constant, not a float, so assign it to a const double variable if you don't want to use the name M_PI everywhere in your code. There's no sense in truncating it to a float, without a good reason.
    Last edited: Jan 25, 2007
  10. Jan 25, 2007 #9

    D H

    Staff: Mentor

    This is wrong on at least two counts.

    First, almost all computers these days use the IEEE floating point standard. Floats and doubles are represented in base 2, not base 10. The computer does not store 3.141592653598793 in a float as "3.141592", or even "3.141593". It stores it in a binary representation.

    Second, the rules of C and C++ dictate that constant expression first be interpreted as a double and then demoted to a float. The demotion process involves rounding the double-precision number to the closest possible single-precision representation.

    The best advice is simply

  11. Jan 25, 2007 #10


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    Homework Helper

    That's a nice example of faulty error analysis.

    Suppose you output costs to the nearest cent. If the correct price (using a high-accuracy value of pi) was $0.494999 and the inaccurate price was $0.495001, then the price that was output would be $0.49 not $0.50 - i.e. a 1 cent error compared with the correct output.

    Imagine I'm your boss, and because if this mistake the company sold 1,000,000 of these gizmos at the wrong price.

    You say the difference doesn't matter? OK, I'll make up the difference by cutting your salary by $10,000 next year. Deal? :rolleyes:
  12. Jan 25, 2007 #11
    Edit :
    You know you're too old... when? When you treat everything as it was back in the (good old) days, when we used C. I forgot we're talking about C++ standards here. Apolagies D H.

    Another thing that I didn't realise is that C will automatically round for you anyways.​

    ISO 9899 and IEC 60559 are both still standard according to our favourite publisher, O'REILLY.

    Code (Text):

    Type           Storage size     Precision
    float          4 bytes          [color=red]6 digits[/color]
    double         8 bytes          15 digits
    long double    10 bytes         19 digits
    Last edited: Jan 25, 2007
  13. Jan 25, 2007 #12

    D H

    Staff: Mentor

    There is no difference between C and C++ regarding the representation of floating point numbers. Both mandate the IEEE floating point standard, IEEE 754, aka IEC 60559.

    Tthose precision numbers are wrong. First, they imply a decimal representation. IEC 60559 / IEE 754 is most definitely binary. See this wikipedia article for details.

    Second, they are just wrong.

    Floats provide 23 bits mantissas plus one implied bit for normalized numbers. This equates to about 6.9 decimal digits of precision. Doubles provide 52+1 bits, or about 15.7 decimal digits.
  14. Jan 25, 2007 #13
    Are you sure you aren't overcrediting your sources? I've got these facts from O'REILLY, which is a highly credible source for programming. Plus if you print out the constant FLT_DIG from float.h you get 6.

    Code (Text):
    #include <stdio.h>
    #include <float.h>

    int main(  ) {
        float pi_1 = 3.1415926535;
        float pi_2 = 3.141593;
        printf( "%f\n", pi_1 );
        printf( "%f\n", pi_2 );
        printf( "Digits: %i\n", FLT_DIG );

        return 0;
    Code (Text):
    Digits: 6
    Furthermore, here's some reading material from O'REILLY:

    Last edited: Jan 25, 2007
  15. Jan 25, 2007 #14

    D H

    Staff: Mentor

    Did you read your source?

    Here are some more on-line references.
    C standard, draft (2005) http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf
    Appendix F covers on IEC 60559.

    "What Every Computer Scientist Should Know About Floating-Point Arithmetic" http://docs.sun.com/app/docs/doc/800-7895/6hos0aou4?a=view

    Re, your code.
    • Print the difference your two representations of [itex]\pi[/itex].
    • Print FLT_RADIX, #defined in <float.h>. This is the basis of the floating point representation: [itex] n = \text{mantissa}*b^{\text{exponent}}[/itex]. A neat trick unique to base 2 is that any non-zero number can be written [itex](1+\text{fraction})*2^{\text{exponent}}[/itex]. The one is always there. You can get an extra bit of precision in base 2. (These are what IEEE 754 calls 'normalized numbers'.)
    • Print FLT_EPSILON, also #defined in <float.h>. This is the smallest positive number that, when added to 1.0, yields a number different from 1.0.

    Code (Text):
    #include <stdio.h>
    #include <float.h>

    int main() {
       float pi_1 = 3.1415926535F;
       float pi_2 = 3.141593F;
       float pi_diff  = pi_2 - pi_1;

       printf ("%f - %f = %g\n", pi_1, pi_2, pi_diff);
       printf ("FLT_RADIX   = %d\n", FLT_RADIX);
       printf ("FLT_EPSILON = %g\n", FLT_EPSILON);

       return 0;
    Here is what I get:

    Code (Text):

    3.141593 - 3.141593 = 2.38419e-07
    FLT_RADIX   = 2
    FLT_EPSILON = 1.19209e-07
    Last edited: Jan 26, 2007
  16. Jan 26, 2007 #15
    Okay. It was the "real number of six decimal digits" that threw me off. Thanks for the information D H.
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