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Changing elements of lists

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Join Date: Jul 2009
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wiczerd wiczerd is offline Offline
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Changing elements of lists

 
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  #1
Jul 4th, 2009
I am reading lines from a CSV and don't know what size it'll be, so I place read lines into a list of a structure "obs". Then I want to iterate through this list, put it into another list and then modify contents of the original list so that I can print that later. I am, however, not sure if I'm actually modifying the original.

C++ Syntax (Toggle Plain Text) 
  1. using namespace std;
  2. struct obs{
  3. 	int date;
  4. 	int p;
  5. 	int attr_price;
  6. 	int q;
  7. 	double bcode;
  8. 	double uniq_store_id;
  9. };
  10.  
  11. int main(){
  12. 	string line;
  13. 	ifstream fdata("jumboa_corrected.csv");
  14.         obs last_obs;
  15.         getline(fdata,line);
  16. 	int scanned = sscanf(line.c_str(),"%i,%i,%i, %lf, %lf",&last_obs.q,&last_obs.date,&last_obs.p,
  17. 		&last_obs.bcode,&last_obs.uniq_store_id);
  18. 	list<obs> this_series(1,last_obs);
  19. 	obs this_obs;
  20. 	while(getline(fdata,line)){
  21. 		++gi;
  22. 		sscanf(line.c_str(),"%i, %i,%i, %lf, %lf",&this_obs.q,&this_obs.date,&this_obs.p,&this_obs.bcode,&this_obs.uniq_store_id);
  23. 		this_series.push_front(this_obs);
  24.  
  25. ... stuff that fills in a bunch of this_obs into the list
  26. 	if(condition to start unloading){
  27. 		list<obs*> qtr_ser_obs;
  28. 		for (list<obs>::reverse_iterator it=this_series.rbegin() ; it != this_series.rend(); ++it )
  29. 		{
  30. 			qtr_ser_obs.push_front(&*it);
  31.                          for (list<obs*>::reverse_iterator qtr_obs_it =qtr_ser_obs.rbegin(); qtr_obs_it !=qtr_ser_obs.rend();qtr_obs_it++){
  32. 				(*qtr_obs_it)->attr_price= computed_attr_price;
  33.                         }
  34.                   }
  35. }

In this last line, what am I modifying?
Any help would be greatly appreciated. I guess I'm really not sure what is being placed with the line " qtr_ser_obs.push_front(&*it); "
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Re: Changing elements of lists

 
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  #2
Jul 4th, 2009
Welcome wiczerd,

wiczerd said,
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I am, however, not sure if I'm actually modifying the original.
...
In this last line, what am I modifying?
Print the list after modification. However, there is no reason to add list item into another list to modify and see; qtr_ser_obs list will add more and more list items and the inner loop is started from the very first element - qtr_ser_obs.rbegin().

C++ Syntax (Toggle Plain Text) 
  1. list<obs*> qtr_ser_obs;
  2. for (list<obs>::reverse_iterator it=this_series.rbegin() ; it != 
  3.                               this_series.rend(); ++it )
  4.  {
  5.     qtr_ser_obs.push_front(&*it);
  6.     for (list<obs*>::reverse_iterator qtr_obs_it =qtr_ser_obs.rbegin
  7.                  (); qtr_obs_it !=qtr_ser_obs.rend();qtr_obs_it++)
  8.           {
  9. 	(*qtr_obs_it)->attr_price= computed_attr_price;
  10.                 }
  11.    }

I think it should be:
C++ Syntax (Toggle Plain Text) 
  1. for (list<obs>::reverse_iterator it=this_series.rbegin() ; it != 
  2.                               this_series.rend(); ++it )
  3.  {
  4.     list<obs*> qtr_ser_obs;
  5.     qtr_ser_obs.push_front(&*it);
  6.     for (list<obs*>::reverse_iterator qtr_obs_it =qtr_ser_obs.rbegin
  7.                  (); qtr_obs_it !=qtr_ser_obs.rend();qtr_obs_it++)
  8.           {
  9. 	(*qtr_obs_it)->attr_price= computed_attr_price;
  10.                 }
  11.    }
Last edited by adatapost; Jul 4th, 2009 at 4:51 am.
Failure is not fatal, but failure to change might be. - John Wooden
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Re: Changing elements of lists

 
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  #3
Jul 4th, 2009
Thanks very much. The suggestion to print my list was very helpful, and it seems that I am modifying the original objects.
My syntax got a bit screwed up because I tried cutting and pasting from a much larger program, but your catch was good.
The reason I am moving things from one list to the other is because I need to do calculations on a moving window of the data in the larger list. So I was reading everything into the larger list, and then putting snippets into the smaller one and hoping to modify the initial obs by referencing them through the subset list.
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