void update_word( int id, vector<CustomStruct> &v)
{
//create iterator to loop through the vector
vector< CustomStruct >::iterator i;
//check every element in the vector starting at the beginning
for (i = v.begin(); i != v.end(); i++)
{
//get the WordStat struct
CustomStruct cs= *i;
//check to see if it's the same id
if (cs.id == id)
{
cs.count++;
}
}
}
C++ and Vector/Iterator Question
Author
Hey, sorry for the really noobish question, but I have an vector of a custom struct, and i'm trying to iterate through it, and change elements, is this possible?
That's kinda what i'm trying to do..but obviously the problem is ..
cs.count++
doesn't do anything, because i'm modifying a copy, and not the real thing.
What do I do to modify the contents of the real thing?
(I realize other containers would be better suited for this, but I want to do it with a vector for other reasons)
Thanks,
ArchG
my two pennies:
void update_word( int id, vector<CustomStruct> &v){ //check every element in the vector starting at the beginning for (int i = 0; i < v.size(); i++) { //check to see if it's the same id if (v.id == id) { v.count++; } }}
the "i" retruns a renfernce to the object in the vector.
When you used "CustomStruct cs= *i;" you created a copy in "cs"
When using "cs.count++;" you were updating the copy and not the orginal
When you used "CustomStruct cs= *i;" you created a copy in "cs"
When using "cs.count++;" you were updating the copy and not the orginal
Quote:Original post by SillyCow
the "i" retruns a renfernce to the object in the vector.
When you used "CustomStruct cs= *i;" you created a copy in "cs"
When using "cs.count++;" you were updating the copy and not the orginal
OP understood that and is looking for a work around.
Quote:Original post by rlyeh_
Ugh, no. That's taking a step backwards.
Quote:Original post by swecoder
try change to
if ((*i).id == id)
{
(*i).count++;
}
That would work, but is missing the point. Also, (*x).y, in generally, is more idiomatically written as x->y.
Quote:Original post by Sc4Freak
Use a reference.
CustomStruct& cs = *i;
We have a winner. This is a useful technique in general (although if you're using it to wrap up a chain of member access, it's a sign that the object is missing some member functionality).
But here's something else to consider:
#include <algorithm> // etc.void update_word(int id, vector<CustomStruct>& v) { // The anonymous structure lets us define a kind of function which // can have one of its arguments (the id) "bound": struct { int id; void operator()(CustomStruct& cs) { if (cs.id == id) { cs.count++; } } } updater = { id }; // And then we use a standard library algorithm to apply the function // to every element of the container: for_each(v.begin(), v.end(), updater);}
Unfortunately, I believe Zahlman's code has a problem. Template parameters have to have external linkage (Which sucks, but then so does a lot of C++) - so that isn't standard code.
For example, Microsoft's C++ compiler produces this error on that snippet (if compiled with language extensions disabled):
Moving the struct out of the function isn't considerably more work:
For example, Microsoft's C++ compiler produces this error on that snippet (if compiled with language extensions disabled):
Quote:
error C2918: 'update_word::<unnamed-type-updater>' : illegal use of local type in template instantiation
Moving the struct out of the function isn't considerably more work:
struct Updater{ int id; void operator()(CustomStruct& cs) const { if (cs.id == id) { cs.count++; } }};void update_word(int id, std::vector<CustomStruct>& v){ Updater updater = { id }; std::for_each(v.begin(), v.end(), updater);}Quote:Original post by rip-off
Unfortunately, I believe Zahlman's code has a problem. Template parameters have to have external linkage (Which sucks, but then so does a lot of C++) - so that isn't standard code.
For example, Microsoft's C++ compiler produces this error on that snippet (if compiled with language extensions disabled):Quote:
error C2918: 'update_word::<unnamed-type-updater>' : illegal use of local type in template instantiation
I don't normally write it anonymously. I thought I saw ToohrVyk do something like that before :\ But that must have been in an example without a template?
It's seriously a problem because the vector is a template instantation? Wow, that really does suck.
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