replacing structs in a vector

[Joe Van Dyk]

I have a vector of structs. Say I get a new struct that's supposed to
replace one of the structs in the vector. I believe I could use
replace_if() to do this, but I'm not sure what the syntax would be.
Something to do with bind2nd or something, I bet.

If there's another struct in the vector who's "id" data member is the
same as the new struct, that's the one that should be replaced.
Otherwise, I just want to add the struct to the end of the vector.

(not all these structs have a valid 'id' field at all times, otherwise
I'd use a map)

Thanks,
Joe

[Rolf Magnus]

Joe Van Dyk wrote:

I have a vector of structs. Say I get a new struct that's supposed to
replace one of the structs in the vector. I believe I could use
replace_if() to do this, but I'm not sure what the syntax would be.
Something to do with bind2nd or something, I bet.
That depends on how you want to find the element to be replaced.
If there's another struct in the vector who's "id" data member is the
same as the new struct, that's the one that should be replaced.
Ah, ok. Then replace_if might work. I think you would need to write your own
predicate to compare the id fields of two objects.

Something like:
(untested)

struct id_cmp
{
bool operator()(const my_struct& a, const my_struct& b) const
{
return a.id == b.id;
}
};

my_struct new_obj;
replace_if(my_vec.begin(), my_vec.end(), bind2nd(id_cmp, new_obj), new_obj);
Otherwise, I just want to add the struct to the end of the vector.

(not all these structs have a valid 'id' field at all times, otherwise
I'd use a map)

I just wanted to suggest that ;-)

[Joe Van Dyk]

Rolf Magnus wrote:

Joe Van Dyk wrote:

I have a vector of structs. Say I get a new struct that's supposed to
replace one of the structs in the vector. I believe I could use
replace_if() to do this, but I'm not sure what the syntax would be.
Something to do with bind2nd or something, I bet.

That depends on how you want to find the element to be replaced.

If there's another struct in the vector who's "id" data member is the
same as the new struct, that's the one that should be replaced.

Ah, ok. Then replace_if might work. I think you would need to write your own
predicate to compare the id fields of two objects.

Something like:
(untested)

struct id_cmp
{
bool operator()(const my_struct& a, const my_struct& b) const
{
return a.id == b.id;
}
};

my_struct new_obj;
replace_if(my_vec.begin(), my_vec.end(), bind2nd(id_cmp, new_obj), new_obj);

I'm slighly confused, probably mostly because I don't understand how
bind2nd works. You're creating a new struct that contains a bool
operator that takes two my_structs and returns their equality status.

Why is the id_cmp struct needed? Can't you just have a function called
compare_my_structs or something and use that function in replace_if?

Joe

Otherwise, I just want to add the struct to the end of the vector.

(not all these structs have a valid 'id' field at all times, otherwise
I'd use a map)

I just wanted to suggest that ;-)

[Daniel T.]

In article <Iy********@news.boeing.com>,
Joe Van Dyk <jo********@boeing.com> wrote:

I have a vector of structs. Say I get a new struct that's supposed to
replace one of the structs in the vector. I believe I could use
replace_if() to do this, but I'm not sure what the syntax would be.
Something to do with bind2nd or something, I bet.

If there's another struct in the vector who's "id" data member is the
same as the new struct, that's the one that should be replaced.
Otherwise, I just want to add the struct to the end of the vector.

How many objects are to get replaced? If only one, I would use find_if
to find it and then replace what is found rather than replace_if (which
goes through the entire container even if it replaces the first element.)

struct pred : unary_function< my_struct, bool >
{
int id;
pred( int id ): id( id ) { }
bool operator()( my_struct st ) {
return st.id == id;
}
};
Then in the code where you have:

my_struct st;
vector< my_struct > vec;
int id;

Do this:

vector< my_struct >::iterator it =
find_if( vec.begin(), vec.end(), pred( id ) );
if ( it != vec.end() )
*it = st;

[Daniel T.]

In article <Iy********@news.boeing.com>,
Joe Van Dyk <jo********@boeing.com> wrote:

Rolf Magnus wrote:

Joe Van Dyk wrote:

I have a vector of structs. Say I get a new struct that's supposed to
replace one of the structs in the vector. I believe I could use
replace_if() to do this, but I'm not sure what the syntax would be.
Something to do with bind2nd or something, I bet.

That depends on how you want to find the element to be replaced.

If there's another struct in the vector who's "id" data member is the
same as the new struct, that's the one that should be replaced.

Ah, ok. Then replace_if might work. I think you would need to write your own
predicate to compare the id fields of two objects.

Something like:
(untested)

struct id_cmp
{
bool operator()(const my_struct& a, const my_struct& b) const
{
return a.id == b.id;
}
};

my_struct new_obj;
replace_if(my_vec.begin(), my_vec.end(), bind2nd(id_cmp, new_obj), new_obj);

I'm slighly confused, probably mostly because I don't understand how
bind2nd works. You're creating a new struct that contains a bool
operator that takes two my_structs and returns their equality status.

Why is the id_cmp struct needed? Can't you just have a function called
compare_my_structs or something and use that function in replace_if?

You could, but then the two my_struct objects would have to be exactly
identical... Which makes one wonder why it's being replaced. :-)

[Noah Roberts]

Daniel T. wrote:

In article <Iy********@news.boeing.com>,
Joe Van Dyk <jo********@boeing.com> wrote:

I have a vector of structs. Say I get a new struct that's supposed to
replace one of the structs in the vector. I believe I could use
replace_if() to do this, but I'm not sure what the syntax would be.
Something to do with bind2nd or something, I bet.

If there's another struct in the vector who's "id" data member is the
same as the new struct, that's the one that should be replaced.
Otherwise, I just want to add the struct to the end of the vector.
How many objects are to get replaced? If only one, I would use find_if
to find it and then replace what is found rather than replace_if (which
goes through the entire container even if it replaces the first element.)

struct pred : unary_function< my_struct, bool >
{
int id;
pred( int id ): id( id ) { }
bool operator()( my_struct st ) {

This could be improved by accepting "const my_struct&" instead of
passing by value.
return st.id == id;
}
};

The op might also consider if map<> would serve them better.

[Joe Van Dyk]

Daniel T. wrote:

In article <Iy********@news.boeing.com>,
Joe Van Dyk <jo********@boeing.com> wrote:

I have a vector of structs. Say I get a new struct that's supposed to
replace one of the structs in the vector. I believe I could use
replace_if() to do this, but I'm not sure what the syntax would be.
Something to do with bind2nd or something, I bet.

If there's another struct in the vector who's "id" data member is the
same as the new struct, that's the one that should be replaced.
Otherwise, I just want to add the struct to the end of the vector.

How many objects are to get replaced? If only one, I would use find_if
to find it and then replace what is found rather than replace_if (which
goes through the entire container even if it replaces the first element.)

Just one of them.

struct pred : unary_function< my_struct, bool >
{
int id;
pred( int id ): id( id ) { }
bool operator()( my_struct st ) {
return st.id == id;
}
};
I'm not familar with unary_function. I'll go look it up.
Then in the code where you have:

my_struct st;
vector< my_struct > vec;
int id;

Do this:

vector< my_struct >::iterator it =
find_if( vec.begin(), vec.end(), pred( id ) );
if ( it != vec.end() )
*it = st;

Hm.

Here's the code I currently have:
std::vector<my_struct>::iterator i;
for (i = vec.begin(); i != vec.end(); ++i)
{
if (i->id == st.id)
{
*i = st;
break;
}
}
if (i == vec.end())
vec.push_back(st);

Would people rather see that, or something like what Daniel wrote?

Perhaps I need to redo my design, and use a map for when my structs have
a valid id, and another container for the structs that are waiting for
their id.

[Daniel T.]

In article <Iy********@news.boeing.com>,
Joe Van Dyk <jo********@boeing.com> wrote:

Hm.

Here's the code I currently have:
std::vector<my_struct>::iterator i;
for (i = vec.begin(); i != vec.end(); ++i)
{
if (i->id == st.id)
{
*i = st;
break;
}
}
if (i == vec.end())
vec.push_back(st);

Would people rather see that, or something like what Daniel wrote?
Now that I know what's supposed to happen if none of the objects have
that id, let me update:

struct has_id : unary_function< my_struct, bool >
{
int id;
has_id( int id_ ): id( id_ ) { }
bool operator()( my_struct st ) const {
return st.id == id;
}
};

The above is just boilerplate code... Now let's look at the actual
location where the job is done...

vector<my_struct>::iterator i =
find_if( vec.begin(), vec.end(), has_id( st.id ) );
if ( i == vec.end() )
vec.push_back( st )
else
*i = st;
Let's compare the two examples. My call explicitly says: "find_if...
has_id" so it's easy to tell what the block of code is supposed to do.
Now I admit that you have to get used to interpreting "i == vec.end()"
as "not found" but otherwise, what I wrote is rather straight forward
and requires little further interpretation from the reader.

Now notice that no where in your loop does the subject of finding
anything ever come up. In fact, we have to read two blocks in before we
even learn that the loop will terminate before reaching the end of the
container!

Also, my example has a Cyclomatic Complexity of 2 (i.e. there are only
two paths of execution.) Yours has a complexity of 4 and 'i' must be
compared to vec.end() at least twice.

Perhaps I need to redo my design, and use a map for when my structs have
a valid id, and another container for the structs that are waiting for
their id.

That would probably be a better choice, and remove the 'id' from the
struct. Instead use it as a key to the map.

[Daniel T.]

In article <11**********************@e56g2000cwe.googlegroups .com>,
"Noah Roberts" <ro**********@gmail.com> wrote:

Daniel T. wrote:

In article <Iy********@news.boeing.com>,
Joe Van Dyk <jo********@boeing.com> wrote:

I have a vector of structs. Say I get a new struct that's supposed to
replace one of the structs in the vector. I believe I could use
replace_if() to do this, but I'm not sure what the syntax would be.
Something to do with bind2nd or something, I bet.

If there's another struct in the vector who's "id" data member is the
same as the new struct, that's the one that should be replaced.
Otherwise, I just want to add the struct to the end of the vector.

How many objects are to get replaced? If only one, I would use find_if
to find it and then replace what is found rather than replace_if (which
goes through the entire container even if it replaces the first element.)

struct pred : unary_function< my_struct, bool >
{
int id;
pred( int id ): id( id ) { }
bool operator()( my_struct st ) {

This could be improved by accepting "const my_struct&" instead of
passing by value.

Maybe, it all depends on the size of the struct. It's a little more
flexible with pass by value because one doesn't need to worry about the
reference to reference problem if the pred is wrapped in another functor
like "not1".

return st.id == id;
}
};