vector problem

[stefaan]

hi all,

i have a bit of a problem with pointers to vector. I have a map

map<int , vector<int> * > children;

now I can access all the vectors trough commands like children[i]->begin().

the question is how do I gain random acces to an element of a vector to edit the value?

I have tried *(children[i])[j] = 5; but this gives me a error C2100: illegal indirection

I have tried other options but these are just wrong. I know I can evade the problem by using iterators, but I use rather large vectors so this is not optimal.

cheers,
stefaan

[JosAH]

hi all,

i have a bit of a problem with pointers to vector. I have a map

map<int , vector<int> * > children;

now I can access all the vectors trough commands like children[i]->begin().

the question is how do I gain random acces to an element of a vector to edit the value?

I have tried *(children[i])[j] = 5; but this gives me a error C2100: illegal indirection

I have tried other options but these are just wrong. I know I can evade the problem by using iterators, but I use rather large vectors so this is not optimal.

cheers,
stefaan

Go slowly here: children[i] is a pointer to vector, so *(children[i]) is the vector
itself and (*(children[i]))[5] is the fifth child. When in doubt parenthesize.

kind regards,

Jos

[weaknessforcats]

Actually, you would code:
[code=cpp]
int main()
{
map<int , vector<int> * > children;
vector<int>* v = children[5];
int answer = (*v)[3];
}
[/code]

The key 5 will return a value (pair::second), which is a pointer to a vector<int>. So, *v is the vector, so (*v)[3] is the 4th int in that vector.

It's not going to work like a 2D array but you can bury this logic in a function that is used throughout your code.

Be careful using pointers in STL conatiners. You should use a handle instead.

[phiefer3]

Just a question, but isn't it usually better to use -> instead of (* ) when you have a pointer to an object? Such as:

children[i]->[j] = 5;

Or won't that work with []? I haven't tried it.

[Darryl]

Just a question, but isn't it usually better to use -> instead of (* ) when you have a pointer to an object? Such as:

children[i]->[j] = 5;

Or won't that work with []? I haven't tried it.

Won't work, at best you'd have to

children[i]->operator[](j) = 5; //quite ugly

but better, you can use at()

children[i]->at(j) = 5;

[stefaan]

thanks all,

(*(children[i]))[5] = 0 does the trick perfectly. Apparently the brackets do matter :)

I don't know handles for pointers, will look in to it later.

I know -> is the general way to go with pointers but ->[5] doesn't work.

the reason at is no good is becaus it just returns the value at say 5. I wanted to edit the value at 5. sorry if this wasn't clear.

anyway thanks all,

stefaan

[Darryl]

thanks all,



the reason at is no good is becaus it just returns the value at say 5. I wanted to edit the value at 5. sorry if this wasn't clear.


stefaan

Actually it returns a reference so children[i]->at(5) = 0 would edit the value at 5.

additionally at() reads cleaner than the pointer dereference and at() has bounds checking which may be desirable if speed is not a concern