need for operator[] in map

[subramanian100in@yahoo.com, India]

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]]

Re: need for operator[] in map

On Feb 7, 2:56 pm, "subramanian10. ..@yahoo.com, India"
<subramanian10. ..@yahoo.comwro te:

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@yahoo.com, India]

Re: need for operator[] in map

On Feb 7, 10:00 am, "[rob desbois]" <rob.desb...@gm ail.comwrote:

On Feb 7, 2:56 pm, "subramanian10. ..@yahoo.com, India"
>
>
>
<subramanian10. ..@yahoo.comwro te:

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]

Re: need for operator[] in map

On Feb 7, 2:56 pm, "subramanian10. ..@yahoo.com, India"
<subramanian10. ..@yahoo.comwro te:

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]

Re: need for operator[] in map

On Feb 7, 3:56 pm, "subramanian10. ..@yahoo.com, India"
<subramanian10. ..@yahoo.comwro te:

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.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[Daniel T.]

Re: need for operator[] in map

<subramanian100 in@yahoo.com wrote:

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(va lue_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)