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if vector is empty

Posted on 2003-02-23
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Last Modified: 2010-04-17
i'm using the std vector.h
i want to know how to prove if vector is empty.
because at certain stage i erase elements. i want to exit if the vector is emtpy.
sort of like if (vector == NULL)
that doesn't work.
thanks
also why can't i do this? and what's the alternative
for( i=0; ........)
if(points0[i] == points1[j])
{
points0.erase(i);
}
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Question by:kencj
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5 Comments
 

Accepted Solution

by:
colmstea earned 80 total points
ID: 8005973
There is a function called empty() that returns a boolean indicating whether the vector is empty..
ie. if (vector.empty()) return -1;

You cant call vector.erase(x) on the index because vectors store their values as pointers.. You need to pass the actual pointer value of the stored element to the erase function.
ie:
for (i=0; ....)
if (points0[i]==points1[j])
{
   points0.erase(&points0[i]);
}

hope that answers your question!

cliff
0
 

Author Comment

by:kencj
ID: 8025629
that helped. but i have this access violation. something to do with that.
template<class _II, class _OI> inline
     _OI copy(_II _F, _II _L, _OI _X)
     {for (; _F != _L; ++_X, ++_F)
          *_X = *_F;
     return (_X); }
          // TEMPLATE FUNCTION copy_backward

and also that empty function,
     if (points0.empty())
     {
          fprintf(p_points, "%3c\n", "end");
     }
this should work right? thanks. still learning so abit slow
0
 

Expert Comment

by:colmstea
ID: 8028879
That copy function assumes that the output vector already has enough elements pushed onto it to hold the elements being copied...  
So you should either have a FOR loop that pushes enough elements on right before calling the function OR rewrite the copy function to use vectors instead of iterators and just push_back each element onto vector X from vector _F.

Let me know if you need me to elaborate or provide code.

Also:
That last part with the empty() will not crash assuming p_points is an open file.  It will not write "end" to the file either though! (if thats your intention, you need to use fprintf(p_points, "%s\n", "end")  ) . let me know

cliff

0
 

Expert Comment

by:colmstea
ID: 8030646
That copy function assumes that the output vector already has enough elements pushed onto it to hold the elements being copied...  
So you should either have a FOR loop that pushes enough elements on right before calling the function OR rewrite the copy function to use vectors instead of iterators and just push_back each element onto vector X from vector _F.

Let me know if you need me to elaborate or provide code.

Also:
That last part with the empty() will not crash assuming p_points is an open file.  It will not write "end" to the file either though! (if thats your intention, you need to use fprintf(p_points, "%s\n", "end")  ) . let me know

cliff

0
 

Author Comment

by:kencj
ID: 8031815
great help!
0

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