How to pass the field name of a struct as a parameter?

[Jellicle]

There are some sturct arrays (or struct vectors), such as:

struct S1_t
{
double keyField ;

..// other fields
}

S1_t s1[100];
std::vector<S1_ t> vecS1;



I need to find the index of the item in array "s1" or vector "vecS1"
which
has the minimal value in field "keyField".

Some codes could be:
//----------------------------------------------

double minvalue = s1[0];
for (int i=0; i< 100; i++ ) // for (int i=0; i< vecS1.size(); i++)
....
{
if (s1[i].keyField < minvalue )
{
minvalue =s1[i].keyField;
index = i;
}
}
//----------------------------------------------

However, since there are many such arrays(vectors) with different
struct type
(so, with different "field" name)

How can I define a function (or MACRO?) to get the index
of the minumal item (by comparing the the value of a key field)?

such as MIN(struct_arra y_variable, field_name)

e.g.

index = MIN(s1, keyField);

to get the index of array "s1" with a minimal value of "keyField".

And ,

index = MIN(s2,f2) get that of "s2" (defined as follows:)

struct s2_t
{
int f2;

// other fields

}

s2_t s2[100]




A similar function is FIND(struct_arr ay, field, value)
to find the index from array "struct_arr ay", where

struct_array[index].field == value;



I think that the sticking point is that how to pass the field name
as a parameter? isn't it?

Could anyone help me?

Thank you very much!

[Victor Bazarov]

Re: How to pass the field name of a struct as a parameter?

Jellicle wrote:[color=blue]
> There are some sturct arrays (or struct vectors), such as:
>
> struct S1_t
> {
> double keyField ;
>
> ..// other fields
> }
>
> S1_t s1[100];
> std::vector<S1_ t> vecS1;
>
>
>
> I need to find the index of the item in array "s1" or vector "vecS1"
> which
> has the minimal value in field "keyField".
>
> Some codes could be:
> //----------------------------------------------
>
> double minvalue = s1[0];[/color]

You mean

double minvalue = s1[0].keyField;

??
[color=blue]
> for (int i=0; i< 100; i++ ) // for (int i=0; i< vecS1.size(); i++)[/color]

You mean

for (int i=1; i< ...

??
[color=blue]
> ...
> {
> if (s1[i].keyField < minvalue )
> {
> minvalue =s1[i].keyField;
> index = i;
> }
> }
> //----------------------------------------------
>
> However, since there are many such arrays(vectors) with different
> struct type
> (so, with different "field" name)
>
> How can I define a function (or MACRO?) to get the index
> of the minumal item (by comparing the the value of a key field)?
>
> such as MIN(struct_arra y_variable, field_name)
>
> e.g.
>
> index = MIN(s1, keyField);
>
> to get the index of array "s1" with a minimal value of "keyField".
>
> And ,
>
> index = MIN(s2,f2) get that of "s2" (defined as follows:)
>
> struct s2_t
> {
> int f2;
>
> // other fields
>
> }
>
> s2_t s2[100]
>
>
>
>
> A similar function is FIND(struct_arr ay, field, value)
> to find the index from array "struct_arr ay", where
>
> struct_array[index].field == value;
>
>
>
> I think that the sticking point is that how to pass the field name
> as a parameter? isn't it?[/color]

No, it isn't. The sticking point is to learn about pointers to
members and how they are used.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Shimin]

Re: How to pass the field name of a struct as a parameter?

you can get the offset of a field f in a struct s using the following
well-known trick:

#define OFFSET(s,f) &((struct s *) 0)->f

Once you have the offset, it becomes straightforward to solve your problem.

-Shimin

Jellicle wrote:[color=blue]
> There are some sturct arrays (or struct vectors), such as:
>
> struct S1_t
> {
> double keyField ;
>
> ..// other fields
> }
>
> S1_t s1[100];
> std::vector<S1_ t> vecS1;
>
>
>
> I need to find the index of the item in array "s1" or vector "vecS1"
> which
> has the minimal value in field "keyField".
>
> Some codes could be:
> //----------------------------------------------
>
> double minvalue = s1[0];
> for (int i=0; i< 100; i++ ) // for (int i=0; i< vecS1.size(); i++)
> ...
> {
> if (s1[i].keyField < minvalue )
> {
> minvalue =s1[i].keyField;
> index = i;
> }
> }
> //----------------------------------------------
>
> However, since there are many such arrays(vectors) with different
> struct type
> (so, with different "field" name)
>
> How can I define a function (or MACRO?) to get the index
> of the minumal item (by comparing the the value of a key field)?
>
> such as MIN(struct_arra y_variable, field_name)
>
> e.g.
>
> index = MIN(s1, keyField);
>
> to get the index of array "s1" with a minimal value of "keyField".
>
> And ,
>
> index = MIN(s2,f2) get that of "s2" (defined as follows:)
>
> struct s2_t
> {
> int f2;
>
> // other fields
>
> }
>
> s2_t s2[100]
>
>
>
>
> A similar function is FIND(struct_arr ay, field, value)
> to find the index from array "struct_arr ay", where
>
> struct_array[index].field == value;
>
>
>
> I think that the sticking point is that how to pass the field name
> as a parameter? isn't it?
>
> Could anyone help me?
>
> Thank you very much!
>[/color]

[Jellicle]

Re: How to pass the field name of a struct as a parameter?

Victor Bazarov wrote:
[color=blue]
> Jellicle wrote:[/color]
[color=blue][color=green]
> > Some codes could be:
> > //----------------------------------------------
> >
> > double minvalue = s1[0];[/color]
>
> You mean
>
> double minvalue = s1[0].keyField;
>
> ??
>[color=green]
> > for (int i=0; i< 100; i++ ) // for (int i=0; i< vecS1.size(); i++)[/color]
>
> You mean
>
> for (int i=1; i< ...
>
> ??
>[/color]

You are right exactly :-).
(and I should check if vecS1[0] exists in a vector :-) )

But what I puzzled is that how to handle the various types of
structs and fields in different arrays or vectors.

[Default User]

Re: How to pass the field name of a struct as a parameter?

Shimin wrote:
[color=blue]
> you can get the offset of a field f in a struct s using the following
> well-known trick:[/color]


Please don't top-post. See item four in the FAQ below:

<http://www.parashift.c om/c++-faq-lite/how-to-post.html#faq-5.4>




Brian

[mlimber]

Re: How to pass the field name of a struct as a parameter?

Shimin wrote:[color=blue]
> you can get the offset of a field f in a struct s using the following
> well-known trick:
>
> #define OFFSET(s,f) &((struct s *) 0)->f
>
> Once you have the offset, it becomes straightforward to solve your problem.
>
> -Shimin[/color]

First, don't top-post. It is considered rude. Second, don't use this
trick unless you must. (Generally, if you must, it means your design is
flawed.)

A better solution would use a function to extract the proper key.
Something like:

struct S1 { int key1; /*...*/ };
struct S2 { int key2; /*...*/ };

int GetKey( const S1& s ) { return s.key1; }
int GetKey( const S2& s ) { return s.key2; }

template<class T>
int MinKey( const vector<T>& v )
{
if( v.size() == 0 ) return -1;
int min = GetKey( v[0] );
for( vector<T>::cons t_iterator i = v.begin()+1; i != v.end(); ++i )
{
if( GetKey( *i ) < min ) min = GetKey( *i );
}
return min;
}

If the keys are also different types, you could still do it, but it
would have to be a little fancier.

Cheers! --M

[Jellicle]

Re: How to pass the field name of a struct as a parameter?


Shimin 写道：
[color=blue]
> you can get the offset of a field f in a struct s using the following
> well-known trick:
>
> #define OFFSET(s,f) &((struct s *) 0)->f
>
> Once you have the offset, it becomes straightforward to solve your problem.
>
> -Shimin[/color]


Thank you!

Would you explain it in detail?
1)how to implement something like :
if (s1[i].keyField < minvalue )

2) Is it possible to return a value, so
I can write a compact and beautiful statement in my codes:

index = MIN(s1, field);

But I am afrad that a macro can not return a value.

[Markus Schoder]

Re: How to pass the field name of a struct as a parameter?

Jellicle wrote:[color=blue]
> There are some sturct arrays (or struct vectors), such as:
>
> struct S1_t
> {
> double keyField ;
>
> ..// other fields
> }
>
> S1_t s1[100];
> std::vector<S1_ t> vecS1;
>
>
>
> I need to find the index of the item in array "s1" or vector "vecS1"
> which
> has the minimal value in field "keyField".
>
> Some codes could be:
> //----------------------------------------------
>
> double minvalue = s1[0];
> for (int i=0; i< 100; i++ ) // for (int i=0; i< vecS1.size(); i++)
> ...
> {
> if (s1[i].keyField < minvalue )
> {
> minvalue =s1[i].keyField;
> index = i;
> }
> }
> //----------------------------------------------
>
> However, since there are many such arrays(vectors) with different
> struct type
> (so, with different "field" name)
>
> How can I define a function (or MACRO?) to get the index
> of the minumal item (by comparing the the value of a key field)?
>
> such as MIN(struct_arra y_variable, field_name)
>
> e.g.
>
> index = MIN(s1, keyField);
>
> to get the index of array "s1" with a minimal value of "keyField".[/color]

A STL style solution using iterators would be:

#include <iterator>

template<class Iterator, typename T> Iterator minfield(Iterat or start,
const Iterator &end, T std::iterator_t raits<Iterator> ::value_type::* p)
{
T acc = (*start).*p;
Iterator min_i(start);
++start;
while(start != end)
{
const T &r = (*start).*p;
if(r < acc)
{
acc = r;
min_i = start;
}
++start;
}
return min_i;
}

Usage would be:

index = minfield(s1, s1 + 100, &S1_t::keyfield ) - s1;

or

index = minfield(vecS1. begin(), vecS1.end(), &S1_t::keyfield ) -
vecS1.begin();

Note that the function assumes that there is at least one value in the range
[start, end).

[mlimber]

Re: How to pass the field name of a struct as a parameter?

Jellicle wrote:[color=blue]
> Shimin 写道：
>[color=green]
> > you can get the offset of a field f in a struct s using the following
> > well-known trick:
> >
> > #define OFFSET(s,f) &((struct s *) 0)->f
> >
> > Once you have the offset, it becomes straightforward to solve your problem.
> >
> > -Shimin[/color]
>
>
> Thank you!
>
> Would you explain it in detail?[/color]

To repeat from my post above, this trick should be avoided. See my post
and Markus Schoder's for two better options.
[color=blue]
> 1)how to implement something like :
> if (s1[i].keyField < minvalue )
>
> 2) Is it possible to return a value, so
> I can write a compact and beautiful statement in my codes:
>
> index = MIN(s1, field);
>
> But I am afrad that a macro can not return a value.[/color]

Technically, a macro doesn't return anything. Macros are not functions
(despite the similar looking syntax); they are text substitutions. If
the text you substitute evaluates to a value, then it can be assigned
to some variable like you want to here. If it does not, then it cannot
be. For instance:

#define A 42
#define B { cout << "Hello"; }

int main()
{
int val1 = A; // Ok: becomes "int val1 = 42;"
int val2 = B; // Error: becomes "int val2 = { cout << "Hello"; };"
}

However, macros should be avoided when possible. See this FAQ and its
references for reasons why:

http://www.parashift.com/c++-faq-lite/inline-functions.html#faq-9.5

Cheers! --M

[Jellicle]

Re: How to pass the field name of a struct as a parameter?


Markus Schoder wrote:

[color=blue]
>
> A STL style solution using iterators would be:
>
> #include <iterator>
>
> template<class Iterator, typename T> Iterator minfield(Iterat or start,
> const Iterator &end, T std::iterator_t raits<Iterator> ::value_type::* p)
> {
> T acc = (*start).*p;
> Iterator min_i(start);
> ++start;
> while(start != end)
> {
> const T &r = (*start).*p;
> if(r < acc)
> {
> acc = r;
> min_i = start;
> }
> ++start;
> }
> return min_i;
> }
>
> Usage would be:
>
> index = minfield(s1, s1 + 100, &S1_t::keyfield ) - s1;
>
> or
>
> index = minfield(vecS1. begin(), vecS1.end(), &S1_t::keyfield ) -
> vecS1.begin();
>
> Note that the function assumes that there is at least one value in the range
> [start, end).[/color]


Thank you and everyone!

But I cannot compile the codes successfully in Visual C++ 6.0.
It said that
"error C2653: 'value_type' : is not a class or namespace name"
on this line:
const Iterator &end, T
std::iterator_t raits<Iterator> ::value_type::* p)

if I remove "::" between "value_type " and "*p", it says:
"error C2143: syntax error : missing ',' before '*' "

Since I don't understand this code line exactly, I don't know how to
correct it.

Would you help me?

Thank you!

[Markus Schoder]

Re: How to pass the field name of a struct as a parameter?

Jellicle wrote:[color=blue]
> But I cannot compile the codes successfully in Visual C++ 6.0.[/color]

You probably need a newer compiler.

[mlimber]

Re: How to pass the field name of a struct as a parameter?

Markus Schoder wrote:[color=blue]
> Jellicle wrote:[color=green]
> > But I cannot compile the codes successfully in Visual C++ 6.0.[/color]
>
> You probably need a newer compiler.[/color]

Try this (I didn't test it, but it compiles :-) ). I think it should
work with VC++ 6, and it works with keys of different types and
different names:

#include <vector>
#include <cassert>

using namespace std;

template<class Iterator, typename key_type, typename T>
Iterator minfield(
Iterator start,
const Iterator &end,
key_type T::*p)
{
key_type acc = (*start).*p;
Iterator min_i(start);
++start;
while(start != end)
{
const key_type &r = (*start).*p;
if(r < acc)
{
acc = r;
min_i = start;
}
++start;
}
return min_i;
}

struct S1
{
typedef int key_type;
key_type key1;
S1( key_type key ) : key1(key) {}
};

struct S2
{
typedef double key_type;
key_type key2;
S2( key_type key ) : key2(key) {}
};

int main()
{
std::vector<S1> v1;
v1.push_back( S1(1) );
v1.push_back( S1(5) );
v1.push_back( S1(-10) );
v1.push_back( S1(3) );

vector<S1>::con st_iterator m1 = minfield(
v1.begin(), v1.end(), &S1::key1 );
assert( m1->key1 == -10 );

std::vector<S2> v2;
v2.push_back( S2(1.1) );
v2.push_back( S2(5.1) );
v2.push_back( S2(-1.1) );
v2.push_back( S2(3.1) );

vector<S2>::con st_iterator m2 = minfield(
v2.begin(), v2.end(), &S2::key2 );
assert( m2->key2 == -1.1 );

return 0;
}

Cheers! --M

[Jellicle]

Re: How to pass the field name of a struct as a parameter?


mlimber 写道：
[color=blue]
> Jellicle wrote:[color=green]
> > Shimin 写道：
> >[color=darkred]
> > > you can get the offset of a field f in a struct s using the following
> > > well-known trick:
> > >
> > > #define OFFSET(s,f) &((struct s *) 0)->f
> > >
> > > Once you have the offset, it becomes straightforward to solve your problem.
> > >
> > > -Shimin[/color]
> >
> >
> > Thank you!
> >
> > Would you explain it in detail?[/color]
>
> To repeat from my post above, this trick should be avoided. See my post
> and Markus Schoder's for two better options.
>[color=green]
> > 1)how to implement something like :
> > if (s1[i].keyField < minvalue )
> >
> > 2) Is it possible to return a value, so
> > I can write a compact and beautiful statement in my codes:
> >
> > index = MIN(s1, field);
> >
> > But I am afrad that a macro can not return a value.[/color]
>
> Technically, a macro doesn't return anything. Macros are not functions
> (despite the similar looking syntax); they are text substitutions. If
> the text you substitute evaluates to a value, then it can be assigned
> to some variable like you want to here. If it does not, then it cannot
> be. For instance:
>
> #define A 42
> #define B { cout << "Hello"; }
>
> int main()
> {
> int val1 = A; // Ok: becomes "int val1 = 42;"
> int val2 = B; // Error: becomes "int val2 = { cout << "Hello"; };"
> }
>
> However, macros should be avoided when possible. See this FAQ and its
> references for reasons why:
>
> http://www.parashift.com/c++-faq-lit...s.html#faq-9.5
>[/color]

Thank you!
And I found the url link you gave was quite useful for me:)

I think that the (only?) good feature of a macro is that it works on
"compiling time".
so I can replace the codes ( not the data). It maybe save some similar
codes.

for example,

#define FIELDNAME( field) field

mystruct.FIELDN AME(myfield)
==> mystruct.myfiel d




[color=blue]
> Cheers! --M[/color]

[Jellicle]

Re: How to pass the field name of a struct as a parameter?


mlimber 写道：
[color=blue]
> Markus Schoder wrote:[color=green]
> > Jellicle wrote:[color=darkred]
> > > But I cannot compile the codes successfully in Visual C++ 6.0.[/color]
> >
> > You probably need a newer compiler.[/color]
>
> Try this (I didn't test it, but it compiles :-) ). I think it should
> work with VC++ 6, and it works with keys of different types and
> different names:
>
> #include <vector>
> #include <cassert>
>
> using namespace std;
>
> template<class Iterator, typename key_type, typename T>
> Iterator minfield(
> Iterator start,
> const Iterator &end,
> key_type T::*p)
> {
> key_type acc = (*start).*p;
> Iterator min_i(start);
> ++start;
> while(start != end)
> {
> const key_type &r = (*start).*p;
> if(r < acc)
> {
> acc = r;
> min_i = start;
> }
> ++start;
> }
> return min_i;
> }
>
> struct S1
> {
> typedef int key_type;
> key_type key1;
> S1( key_type key ) : key1(key) {}
> };
>
> struct S2
> {
> typedef double key_type;
> key_type key2;
> S2( key_type key ) : key2(key) {}
> };
>
> int main()
> {
> std::vector<S1> v1;
> v1.push_back( S1(1) );
> v1.push_back( S1(5) );
> v1.push_back( S1(-10) );
> v1.push_back( S1(3) );
>
> vector<S1>::con st_iterator m1 = minfield(
> v1.begin(), v1.end(), &S1::key1 );
> assert( m1->key1 == -10 );
>
> std::vector<S2> v2;
> v2.push_back( S2(1.1) );
> v2.push_back( S2(5.1) );
> v2.push_back( S2(-1.1) );
> v2.push_back( S2(3.1) );
>
> vector<S2>::con st_iterator m2 = minfield(
> v2.begin(), v2.end(), &S2::key2 );
> assert( m2->key2 == -1.1 );
>
> return 0;
> }
>
> Cheers! --M[/color]


Thank you, mlimber. I am really grateful for you kind help!

Yes, it works well in VC6 :)

However, my question is that the structs were defined by other people,
and were used
somewhere else. So I cannot redefine them.

[mlimber]

Re: How to pass the field name of a struct as a parameter?


Jellicle wrote:[color=blue]
> mlimber 写道：
>[color=green]
> > Markus Schoder wrote:[color=darkred]
> > > Jellicle wrote:
> > > > But I cannot compile the codes successfully in Visual C++ 6.0.
> > >
> > > You probably need a newer compiler.[/color]
> >
> > Try this (I didn't test it, but it compiles :-) ). I think it should
> > work with VC++ 6, and it works with keys of different types and
> > different names:
> >
> > #include <vector>
> > #include <cassert>
> >
> > using namespace std;
> >
> > template<class Iterator, typename key_type, typename T>
> > Iterator minfield(
> > Iterator start,
> > const Iterator &end,
> > key_type T::*p)
> > {
> > key_type acc = (*start).*p;
> > Iterator min_i(start);
> > ++start;
> > while(start != end)
> > {
> > const key_type &r = (*start).*p;
> > if(r < acc)
> > {
> > acc = r;
> > min_i = start;
> > }
> > ++start;
> > }
> > return min_i;
> > }
> >
> > struct S1
> > {
> > typedef int key_type;
> > key_type key1;
> > S1( key_type key ) : key1(key) {}
> > };
> >
> > struct S2
> > {
> > typedef double key_type;
> > key_type key2;
> > S2( key_type key ) : key2(key) {}
> > };
> >
> > int main()
> > {
> > std::vector<S1> v1;
> > v1.push_back( S1(1) );
> > v1.push_back( S1(5) );
> > v1.push_back( S1(-10) );
> > v1.push_back( S1(3) );
> >
> > vector<S1>::con st_iterator m1 = minfield(
> > v1.begin(), v1.end(), &S1::key1 );
> > assert( m1->key1 == -10 );
> >
> > std::vector<S2> v2;
> > v2.push_back( S2(1.1) );
> > v2.push_back( S2(5.1) );
> > v2.push_back( S2(-1.1) );
> > v2.push_back( S2(3.1) );
> >
> > vector<S2>::con st_iterator m2 = minfield(
> > v2.begin(), v2.end(), &S2::key2 );
> > assert( m2->key2 == -1.1 );
> >
> > return 0;
> > }
> >
> > Cheers! --M[/color]
>
>
> Thank you, mlimber. I am really grateful for you kind help!
>
> Yes, it works well in VC6 :)
>
> However, my question is that the structs were defined by other people,
> and were used
> somewhere else. So I cannot redefine them.[/color]

There should be no need to alter the structs (I had initially added the
key_type to them but realized I didn't need to). If the structs looked
like this, it would work the same:

struct S1
{
int key1;
// ... whatever else
};

struct S2
{
double key2;
// ... whatever else
};

Cheers! --M