Re: [boost] Quantity library: any interest?
From: [EMAIL PROTECTED] Even if angle is added as a dimension, to an implementation using an integer vector, it still wouldn't accommodate any other dimensions added later, without rewriting the library and unit definitions. how about having the basic SI dimensions and a couple of extra to-be-defined-by-the-developer dimensions. In that case, full SI compliance and for those who want something extra they can have it. I saw one of the discussed unit libraries provided something like a dimension to be defined by the user/developer. I think I know a way this may be done, now: Default parameters. There's several ways of specifying a unit; one is to use a typelist, such as typedef mpl::vector_c1,0,0,0,0,0,0 kg (Dave Abrahams also showed this approach at the recent ACCU conference), or use template parameters for the dimensions. In both ways, defaults may be used (it's used internally for mpl::vector, anyway). In fact, all could be defaulted (like mpl::vector), giving in this case a dimensionless unit. Named template parameters could be a possibility, as well, so you could do typedef unitkg, m, s-2 newton. Regards, Terje ___ Unsubscribe other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
Re: [boost] Quantity library: any interest?
From: David Abrahams [EMAIL PROTECTED] These are my (only slightly informed) opinions. I've heard Walter Brown talk about angle in this context, which was a big influence. Terje Slettebø [EMAIL PROTECTED] writes: Regarding this angle dimension, should it be treated like the other SI-dimensions? That is, say that you represent an SI quantity/unit with an integer vector giving the exponents: templateint kg,int m,int s,int A,int K,int mol,int cd,int angle class quantity; If you multiply two quantities, you multiply the value and add the exponents, so quantity0,1,0,0,0,0,0,0(10) * quantity0,1,0,0,0,0,0,0(10) = quantity0,2,0,0,0,0,0,0(100) (m * m = m^2) Would this hold for angle, as well? Yes. Angle is a dimensionless scalar (length/length). All its exponents are zero. Yes, Renej (in the posting I replied to) also pointed out that angle is a dimensionless quantity. However, in their real-life experience, they found it useful not to have it dimensionless, but instead having it as an additional dimension, to be able to distinguish things like velocity and angular velocity. It was in this context that I wondered how angle - as a dimension - should be treated. That is, does it make sense to say angle * angle = angle^2? Probably not, but only because angle * angle doesn't make much sense. Does that ever come up in real life? If it's a dimensionless quantity, it wouldn't matter (all exponents would still be zero). However, as a dimension, it would give angle^2. That's what I wondered about. I understand that e.g. angle/s (angular velocity) makes sense, but should a library allow any combination with angle and the other dimensions? Not arbitrarily: I meant - should it allow the same combinations as the other dimensions are allowed. That is (simplified, using only one dimension): quantityd1 * quantityd2 = quantityd1+d2 quantityd1 / quantityd2 = quantityd1-d2 quantityd + quantityd = quantityd quantityd - quantityd = quantityd angle(pi/2) / mass(40); // OK angle(pi/2) + mass(40); // error Let's again cast it in the quantity example template I gave in the last posting, to examine it. Let's say that the angle is expressed in radians. We then have: templateint kg,int m,int s,int A,int K,int mol,int cd,int rad class quantity; typedef quantity1,0,0,0,0,0,0,0 mass; typedef quantity0,0,0,0,0,0,0,1 angle; angle(pi/2) * mass(40) = quantity1,0,0,0,0,0,0,1(pi/2*40) (rad * kg) angle(pi/2) + mass(40) The last one will be an error, as you said. I'm guessing that the answer to my question is, yes, angle, when expressed as a dimension, may be treated like the other dimensions. There's also a question of what then if someone would like to add a money dimension, to avoid having money be interchangeable with dimensionless quantities. However, in this case, multiplying currencies with each other really doesn't make any sense (e.g. USD(10)*USD(10) = USD^2?), which _would_ be allowed, if it used the same framework as above. Therefore, maybe money is better treated in a different way, than with an exponent. Regards, Terje ___ Unsubscribe other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
Re: [boost] Quantity library: any interest?
From: [EMAIL PROTECTED] we use the int-based template approach for a couple of years now in our AGV controller software. We actually sometimes reach the stage that it works when succesfully compiled and linked. Since our control software is physics throughout (field of robotics), the type safety is very important. However, besides the basic SI units we also have 'angle' as a dimension which allows us to distinguish 'velocity' and 'angular velocity' for example. Hence, from out 'real user' experience (engineering point of view) it would be a necessity to add 'angle' as a dimension without breaking already defined quantities. Most (all?) units libraries already define 'angle' to be dimensionless, which is true in scientifically spoken, but pragmatically (engineering ;-) less handy. There's another question. If we add angle as a dimension, then what kind of angle is it? There are several kinds of angles, such as radians (plane angle) and steradians (solid angle). If both were represented by the same angle dimension, then it probably wouldn't make much sense to add radians and steradians (and what would be the resulting quantity?), yet, the library would allow it. Regards, Terje ___ Unsubscribe other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
Re: [boost] Quantity library: any interest?
Terje Slettebø [EMAIL PROTECTED] writes: From: David Abrahams [EMAIL PROTECTED] These are my (only slightly informed) opinions. I've heard Walter Brown talk about angle in this context, which was a big influence. Terje Slettebø [EMAIL PROTECTED] writes: Regarding this angle dimension, should it be treated like the other SI-dimensions? That is, say that you represent an SI quantity/unit with an integer vector giving the exponents: templateint kg,int m,int s,int A,int K,int mol,int cd,int angle class quantity; If you multiply two quantities, you multiply the value and add the exponents, so quantity0,1,0,0,0,0,0,0(10) * quantity0,1,0,0,0,0,0,0(10) = quantity0,2,0,0,0,0,0,0(100) (m * m = m^2) Would this hold for angle, as well? Yes. Angle is a dimensionless scalar (length/length). All its exponents are zero. Yes, Renej (in the posting I replied to) also pointed out that angle is a dimensionless quantity. However, in their real-life experience, they found it useful not to have it dimensionless, but instead having it as an additional dimension, to be able to distinguish things like velocity and angular velocity. They are distinguished! Velocity is l/t and angular velocity is 1/t - the same as frequency. Makes perfect sense to me. It was in this context that I wondered how angle - as a dimension - should be treated. That is, does it make sense to say angle * angle = angle^2? Probably not, but only because angle * angle doesn't make much sense. Does that ever come up in real life? If it's a dimensionless quantity, it wouldn't matter (all exponents would still be zero). However, as a dimension, it would give angle^2. That's what I wondered about. I understand that e.g. angle/s (angular velocity) makes sense, but should a library allow any combination with angle and the other dimensions? Not arbitrarily: I meant - should it allow the same combinations as the other dimensions are allowed. That is (simplified, using only one dimension): quantityd1 * quantityd2 = quantityd1+d2 quantityd1 / quantityd2 = quantityd1-d2 quantityd + quantityd = quantityd quantityd - quantityd = quantityd angle(pi/2) / mass(40); // OK angle(pi/2) + mass(40); // error Let's again cast it in the quantity example template I gave in the last posting, to examine it. Let's say that the angle is expressed in radians. We then have: templateint kg,int m,int s,int A,int K,int mol,int cd,int rad class quantity; typedef quantity1,0,0,0,0,0,0,0 mass; typedef quantity0,0,0,0,0,0,0,1 angle; angle(pi/2) * mass(40) = quantity1,0,0,0,0,0,0,1(pi/2*40) (rad * kg) angle(pi/2) + mass(40) The last one will be an error, as you said. I'm guessing that the answer to my question is, yes, angle, when expressed as a dimension, may be treated like the other dimensions. I guess I can't play in that mind-space, because I can't get away from what seems to me like a clear truth: angle is dimensionless. If you give it dimension, you'll get confusing results (like no relationship between angular velocity and frequency). I wonder what happens to physics calculations when frequency is expressed as rad/t? -- Dave Abrahams Boost Consulting www.boost-consulting.com ___ Unsubscribe other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
Re: [boost] Quantity library: any interest?
From: [EMAIL PROTECTED] we use the int-based template approach for a couple of years now in our AGV controller software. We actually sometimes reach the stage that it works when succesfully compiled and linked. Since our control software is physics throughout (field of robotics), the type safety is very important. However, besides the basic SI units we also have 'angle' as a dimension which allows us to distinguish 'velocity' and 'angular velocity' for example. Hence, from out 'real user' experience (engineering point of view) it would be a necessity to add 'angle' as a dimension without breaking already defined quantities. Most (all?) units libraries already define 'angle' to be dimensionless, which is true in scientifically spoken, but pragmatically (engineering ;-) less handy. There's another question. If we add angle as a dimension, then what kind of angle is it? There are several kinds of angles, such as radians (plane angle) and steradians (solid angle). If both were represented by the same angle dimension, then it probably wouldn't make much sense to add radians and steradians (and what would be the resulting quantity?), yet, the library would allow it. in our implementation (plane) angle is sufficient, so we don't bother with solid angle for the moment. However, I think it should have it's own dimension since steradian != radian^2 (although m^2/m^2 = (m/m)^2 ;-) but I think a unit library would reach a much broader audience if it allows to choose what dimensions are handled and how. For my application field it is necessary to incorporate everything related to robotics (time, length, angle, charge, mass), but temp, luminious intensity, amount of substance not really. I think it would be nice to be able to select only those you need in a unit lib and add your own dimensions if needed renej Regards, Terje ___ Unsubscribe other changes: http://lists.boost.org/mailman/listinfo.cgi/boost ___ Unsubscribe other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
[boost] Quantity library: any interest?
I wrote Quantity library some time ago and wonder whether it may get added
into Conversion library. If anyone gets interested I will Boostify and
submit it.
Quantity library is used to get safe conversions between different physical
units (like between Celsius and Farhernheit degrees). Instead of bare number
you have specific type and conversion rules are applied automatically.
Example:
typedef struct { float value; } speed_km_per_hour_t;
typedef Quantity
float, // numeric type to represent speed in meter/sec
speed_km_per_hour// alternative representation
speed_meters_per_sec_t;
// conversion rule: define them when it makes sense
void convert_quantity(const speed_meters_per_sec_t from,
speed_km_per_hour_t to)
{
to.value = from.value * 3.6;
}
int main()
{
speed_meters_per_sec_t m_p_s(10);
speed_km_per_hour_tkm_p_h;
km_p_h = m_p_s;// OK
// m_p_s = km_p_h; ERROR - this conversion is not defined
km_p_h = static_castspeed_km_per_hour_t(m_p_s); // OK
}
You can manipulate Quantity value like any other value (add, multiply,
compare), you may add e.g. 1 inch to value of 2.1 meters. This library
helps to avoid really hard to track down conversion errors.
Some conversions may not be needed: e.g. $1.5 money may be converted to
string one dollar, fifty cents but not vice versa. Unimplemented and used
conversions result in (relatively) readable linker error.
Ability e.g. to divide length quantity by time quantity and get speed
quantity may be added into the library.
The library is simple and uses Boost.Preprocessor to emulate variable number
of template parameters.
-
Shortened snippet of implementation:
templatetypename MainType, typename OtherType1
class Quantity
{
public:
Quantity() {}
Quantity(const Quantity other) : value(other.value) {}
Quantity(const MainType val) : value(val) {}
Quantity(const OtherType1 val) { convert_quantity(val, *this); }
~Quantity() {}
Quantity operator=(const Quantity other) { value = other.value; return
*this; }
Quantity operator=(const MainType other) { value = other; return
this*; }
Quantity operator=(const OtherType1 other) { convert_quantity(other,
*this); return *this; }
operator MainType() { return value; }
operator OtherType1() { OtherType1 result; convert_quantity(*this,
result); return result; }
// operators like ==, /, * etc. not shown here
MainType value;
};
-
Name and inspiration was taken from Martin Fowler's article
http://www.martinfowler.com/ap2/quantity.html.
/Pavel
___
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