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]

Jellicle wrote:

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];
You mean

double minvalue = s1[0].keyField;

??
for (int i=0; i< 100; i++ ) // for (int i=0; i< vecS1.size(); i++)
You mean

for (int i=1; 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?

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]

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:

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!

[Jellicle]

Victor Bazarov wrote:

Jellicle wrote:

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

double minvalue = s1[0];

You mean

double minvalue = s1[0].keyField;

??

for (int i=0; i< 100; i++ ) // for (int i=0; i< vecS1.size(); i++)

You mean

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

??

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]

Shimin wrote:

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

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]

Shimin wrote:

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

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]

Shimin 写�：

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

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]

Jellicle wrote:

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".

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]

Jellicle wrote:

Shimin 写�：

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

Thank you!

Would you explain it in detail?

To repeat from my post above, this trick should be avoided. See my post
and Markus Schoder's for two better options.
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.

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

Cheers! --M

[Jellicle]

Markus Schoder wrote:

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).

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!