how to reach an arbitrary index in a std::list

[kkirtac]

Hello, i want to reach a specified index in a list, as we can achieve
this by the "[]" operator in a std::vector. I couldnt find a way to do
it without iterators, stepping one by one..i need to point to a
specified index and retrieve the value and then remove the value of
the entry...

Regards

 

[Victor Bazarov]

Hello, i want to reach a specified index in a list, as we can achieve
this by the "[]" operator in a std::vector. I couldnt find a way to do
it without iterators, stepping one by one..i need to point to a
specified index and retrieve the value and then remove the value of
the entry...

template<class C>
typename C::value_type& getAtIndex(C &L, size_t i)
{
if (i >= L.size())
throw "Out of bounds";
typename C::iterator it = L.begin();
std::advance(it, i);
return *it;
}

...
list<int> blah;
...
int& ref = getAtIndex(blah, 42);

V

 

[Michael DOUBEZ]

kkirtac a écrit :

Hello, i want to reach a specified index in a list, as we can achieve
this by the "[]" operator in a std::vector. I couldnt find a way to do
it without iterators, stepping one by one..i need to point to a
specified index and retrieve the value and then remove the value of
the entry...

Use the std::advance algorithm:

list<T>::iterator it=list.begin();
int n=42;

std::advance(it,n);

//it is 42th element or list.end()

Michael

 

[Michael DOUBEZ]

Michael DOUBEZ a écrit :

kkirtac a écrit :

Hello, i want to reach a specified index in a list, as we can achieve
this by the "[]" operator in a std::vector. I couldnt find a way to do
it without iterators, stepping one by one..i need to point to a
specified index and retrieve the value and then remove the value of
the entry...

Use the std::advance algorithm:

list<T>::iterator it=list.begin();
int n=42;

std::advance(it,n);

//it is 42th element or list.end()

oups it is the 43th

 

[Abdo Haji-Ali]

Hello, i want to reach a specified index in a list, as we can achieve
this by the "[]" operator in a std::vector. I couldnt find a way to do
it without iterators, stepping one by one..i need to point to a
specified index and retrieve the value and then remove the value of
the entry...

Then why don't you use a vector? Generally if you want to randomly access
elements in a collection (by index) rather than sequentially (from begin()
to end()) you'll need a vector. Or you'll risk the added overhead of
traversing the whole list to get to the specific index you want.

-
Abdo Haji-Ali
Programmer
In|Framez

 

[greg]

template<class C>
typename C::value_type& getAtIndex(C &L, size_t i)
{
if (i >= L.size())
throw "Out of bounds";
typename C::iterator it = L.begin();
std::advance(it, i);
return *it;
}

...
list<int> blah;
...
int& ref = getAtIndex(blah, 42);

I was wondering: why do you need `typename C::' instead of `C::' in
both of the cases? Also how does the compiler know what value_type is?

thanks,
Greg

 

[Victor Bazarov]

I was wondering: why do you need `typename C::' instead of `C::' in
both of the cases? Also how does the compiler know what value_type is?

FAQ, section 35. And by the time this function is instantiated, the
'C::value_type' is known because 'C' is something concrete.

V

 

[greg]

FAQ, section 35.

Doesn't say anything abut this issue?

 

[Victor Bazarov]

Doesn't say anything abut this issue?

Huh? 35.18 does. It explains why 'typename' is needed, as I see it.

V

 

[greg]

Huh? 35.18 does. It explains why 'typename' is needed, as I see it.

V

My bad.

 

[Old Wolf]

I was wondering: why do you need `typename C::' instead of `C::' in
both of the cases? Also how does the compiler know what value_type is?

The 'typename' keyword tells the compiler that C::value_type
is the name of a type (as opposed to the name of a variable),
so it can parse the source file correctly.

In this particular code it's not ambiguous, but in other
code it is; does:
C::foo & x;

declare a reference, or perform a bitwise AND ?

 

[Juha Nieminen]

Hello, i want to reach a specified index in a list, as we can achieve
this by the "[]" operator in a std::vector. I couldnt find a way to do
it without iterators, stepping one by one..

There isn't. advance() has been suggested, but it just does exactly
that.