need for operator[] in map

[subramanian100in]

For inserting new elements in map, we can use insert member function.

To know if an element exists or not in a map, we can use count or find
member function.

Also, we can use the iterator returned by find to modify the mapped
value of an existing key.

When we use operator[], it may add an element into the map if the key
already doesn't exist, which may not be always wanted.

Given this, I am unable to understand the reason as to why we have
operator[] in map whose functionality can be achieved by other member
functions.

Kindly clarify.

Thanks
V.Subramanian

 

[[rob desbois]]

For inserting new elements in map, we can use insert member function.

To know if an element exists or not in a map, we can use count or find
member function.

Also, we can use the iterator returned by find to modify the mapped
value of an existing key.

When we use operator[], it may add an element into the map if the key
already doesn't exist, which may not be always wanted.

Given this, I am unable to understand the reason as to why we have
operator[] in map whose functionality can be achieved by other member
functions.

Kindly clarify.

Thanks
V.Subramanian

You've already got a response on your original post - if that doesn't
answer the question, reply and give some information as to how it
didn't help.
Don't just post the same question again, especially on the same day.

Rob.

 

[subramanian100in]

For inserting new elements in map, we can use insert member function.

To know if an element exists or not in a map, we can use count or find
member function.

Also, we can use the iterator returned by find to modify the mapped
value of an existing key.

When we use operator[], it may add an element into the map if the key
already doesn't exist, which may not be always wanted.

Given this, I am unable to understand the reason as to why we have
operator[] in map whose functionality can be achieved by other member
functions.

Kindly clarify.

Thanks
V.Subramanian

You've already got a response on your original post - if that doesn't
answer the question, reply and give some information as to how it
didn't help.
Don't just post the same question again, especially on the same day.

Rob.

If you are referring to the other thread with the subject
"using operator[] or insert in map ?",
then I would like to say that I got clarified for THAT doubt.

If you are referring to some other thread, please give the subject of
the thread you are referring to.

In this particular thread ie the thread with the subject
"need for operator[] in map",
I want to know the reason for having operator[] in map.

Since the modification of mapped-value for a given key can be done
through dereferencing and modifying the iterator returned by find()
operation, I - as a beginner in STL, am unable to understand the
reason for having operator[] in map.

I still think the question I have posted here is different from the
other thread with the subject: "using operator[] or insert in map ?"

Please provide me an answer for this thread.

Thanks
V.Subramanian

 

[tragomaskhalos]

For inserting new elements in map, we can use insert member function.

To know if an element exists or not in a map, we can use count or find
member function.

Also, we can use the iterator returned by find to modify the mapped
value of an existing key.

When we use operator[], it may add an element into the map if the key
already doesn't exist, which may not be always wanted.

Given this, I am unable to understand the reason as to why we have
operator[] in map whose functionality can be achieved by other member
functions.

My guess is: syntactic elegance, convenience,
familiarity, symmetry. IOW it's nice to be
able to be able to do:

foomap["bar"] = "baz";
// then
s = foomap["bar"];

You only need to look at code using [] vs
the alternatives to see this for yourself.

 

[James Kanze]

For inserting new elements in map, we can use insert member function.

To know if an element exists or not in a map, we can use count or find
member function.

Also, we can use the iterator returned by find to modify the mapped
value of an existing key.

When we use operator[], it may add an element into the map if the key
already doesn't exist, which may not be always wanted.

Given this, I am unable to understand the reason as to why we have
operator[] in map whose functionality can be achieved by other member
functions.

Convenience. It's a bit awkward, in that the semantics which
would be most convenient vary according to what you're using the
map for, and in practice, I rarely use []. (But then, most of
my maps are const, so I can't.) The real question is, of
course, if [] doesn't find the element, what does it do?

In the most general case, because of such uncertainty surronding
the semantics, one could argue that map shouldn't support an
operator[] at all---that its a case of operator overloading
abuse. In practice, however, smaller, text based languages like
AWK and perl use it effectively, and it seems reasonable to
support it in the same way they do---if you are using std::map
like you'd use an array in AWK or perl, you use it (and not much
else of the interface of std::map); if you are using std::map in
some other context, you don't use it.

 

[Kira Yamato]

For inserting new elements in map, we can use insert member function.

To know if an element exists or not in a map, we can use count or find
member function.

Also, we can use the iterator returned by find to modify the mapped
value of an existing key.

When we use operator[], it may add an element into the map if the key
already doesn't exist, which may not be always wanted.

Given this, I am unable to understand the reason as to why we have
operator[] in map whose functionality can be achieved by other member
functions.

Kindly clarify.

Beside providing some nice syntactic alternatives, it also have some
real functional values.

For example, you can write template algorithms that assume its input to
have random access like that of an array using the operator[] with an
indexing set that is not necessarily integer-based. Then, since
std::map supports operator[], std::map can participate in such
algorithms.

An example of such algorithm may be accessing edges of a directed graph
based on their terminating vertices as the index.

 

[Kai-Uwe Bux]

For inserting new elements in map, we can use insert member function.

To know if an element exists or not in a map, we can use count or find
member function.

Also, we can use the iterator returned by find to modify the mapped
value of an existing key.

When we use operator[], it may add an element into the map if the key
already doesn't exist, which may not be always wanted.

Given this, I am unable to understand the reason as to why we have
operator[] in map whose functionality can be achieved by other member
functions.

Convenience. It's a bit awkward, in that the semantics which
would be most convenient vary according to what you're using the
map for, and in practice, I rarely use []. (But then, most of
my maps are const, so I can't.) The real question is, of
course, if [] doesn't find the element, what does it do?

Well, the language does not really give you a chance here. Suppose you
wanted to not insert a default or dummy value into the map, what would you
return? Maybe a proxy like so:


#include <map>
#include <cassert>
#include <iostream>
#include <utility>


template < typename Key, typename Mapped >
struct does_not_work
: public std::map< Key, Mapped > // hack!
{

typedef typename std::map< Key, Mapped >::iterator iterator;

class proxy {

does_not_work * map_ptr;
Key const * key_ptr;
iterator where;

public:

proxy ( Key const & the_key, does_not_work & the_map )
: map_ptr ( &the_map )
, key_ptr ( &the_key )
, where ( map_ptr->find( the_key ) )
{}


operator Mapped & ( void ) {
assert( where != map_ptr->end() );
return ( where->second );
}

operator Mapped const & ( void ) const {
assert( where != map_ptr->end() );
return ( where->second );
}

Mapped & operator= ( Mapped const & other ) {
if ( where == map_ptr->end() ) {
where = map_ptr->insert( std::make_pair( *key_ptr, other ) ).first;
} else {
where->second = other;
}
return ( where->second );
}

Mapped & operator= ( proxy const & other ) {
return ( this->operator=( static_cast< Mapped const & >( other ) ) );
}

};

proxy operator[] ( Key const & key ) {
return ( proxy( key, *this ) );
}

};


int main ( void ) {
does_not_work< int, int > my_map;
my_map[3] = 5;
std::cout << my_map[3] << '\n';
std::cout << my_map[0] << '\n'; // triggers assert, could be made throwing
}


However, that is not really satisfying when it comes to syntactic sugar:

my_map[key].some_method();

for instance cannot be done since we cannot overload the dot-operator.
Because of that, we cannot have smart-references as a complement to smart
pointers and proxy object can fall short of desires and expectations.


[snip]


Best

Kai-Uwe Bux

 

[Daniel T.]

For inserting new elements in map, we can use insert member function.

To know if an element exists or not in a map, we can use count or find
member function.

Also, we can use the iterator returned by find to modify the mapped
value of an existing key.

When we use operator[], it may add an element into the map if the key
already doesn't exist, which may not be always wanted.

Given this, I am unable to understand the reason as to why we have
operator[] in map whose functionality can be achieved by other member
functions.

Kindly clarify.

For the answer, you will have to ask the standards body, or at least
Alex Stepanov (the creator of the STL.)

For now, maybe the explanation from SGI's website will do?

Note that the definition of operator[] is extremely simple: m[k] is
equivalent to (*((m.insert(value_type(k,
data_type()))).first)).second. Strictly speaking, this member
function is unnecessary: it exists only for convenience.
(http://www.sgi.com/tech/stl/Map.html)

 

[James Kanze]

[...]

Convenience. It's a bit awkward, in that the semantics which
would be most convenient vary according to what you're using the
map for, and in practice, I rarely use []. (But then, most of
my maps are const, so I can't.) The real question is, of
course, if [] doesn't find the element, what does it do?

Well, the language does not really give you a chance here.

A chance for what? There are really three different reasonable
semantics for a "get" function on an associative container:
1. return a pointer to the value, returning a null pointer if
the value isn't found;
2. throw an exception if the value isn't present, or
3. create a new entry, with a default value (in which case, the
"get" function can't be const---in the other two cases, you
would probably overload the function on const).

The interface of std::map basically implements the first, with
the "getter" called find(), not get (or operator[]), except that
it returns an iterator, rather than a pointer (and the iterator
is to the attribute value pair, not the value). The second
option isn't directly available, and operator[] implements the
third.

I suspect that the reason behind this is that it is easier to
simulate the others when you have the first, rather than vice
versa---with the second, you need a wrapper with a try block to
simulate the others, and with the third, you need some way of
testing whether an element is present to simulate the other two.

I suspect that the operator[] was added mainly to support the
perl/AWK idiom. If you want perl/AWK like associative arrays,
you use it (and you pretty much ignore const). Otherwise, you
use find() and insert().