Different results on different compilers c (vector copying)-CodePudding

I need to implement a simple version of schedule for monthly tasks. For example payment of electricity bills, subscription fees for communications, etc. I want to implement a set of the following operations:

ADD(i,s) - assign a case with name s to day i of the current month.

DUMP(i) - Display all tasks scheduled for day i of the current month.

NEXT - Go to the to-do list for the new month. When this command is executed, instead of the current (old) to-do list for the current month, a (new) to-do list for the next month is created and becomes active: all tasks from the old to-do list are copied to the new list. After executing this command, the new to-do list and the next month become the current month, and work with the old to-do list is stopped. When moving to a new month, you need to pay attention to the different number of days in months:

if the next month has more days than the current one, the "additional" days must be left empty (not containing cases);

if the next month has fewer days than the current one, cases from all "extra" days must be moved to the last day of the next month.

Here is my code:

#include <iostream>
#include <string>
#include <algorithm>
#include <vector>

using namespace std;


int main(){
    //q - number of operations.
    //day - on which day to plan operation.
    //to_do - what exactly to do on some day.
    //operation - which kind of operation to perform.

    const vector<int>day_mon = {31,28,31,30,31,30,31,31,30,31,30,31};
    //m_ind - month index 
    int c_ind = 0;
    int n_ind = 0;
    int q,day;
    int days_diff;
    string operation;
    string to_do; 
    
  


    //vector of the current month and next month
    vector<vector<string> > current_month(31);
    vector<vector<string> > next_month;
    
    cin >> q;
    
    //for q operations:
    for(int i = 0;i< q;i  ){
        cin >> operation;
        if(operation == "NEXT"){
            
            //change next_month index
            if(c_ind == 11){
                n_ind = 0;
            } else{
                n_ind  =1;
            }
            next_month.resize(day_mon[n_ind]);
            copy(current_month.begin(),current_month.begin()   day_mon[n_ind],next_month.begin());

            days_diff = day_mon[c_ind] - day_mon[n_ind];

            //if next month has less days as current month, write days into the last day 
            //of the next months
            if(days_diff > 0){
                for(int i = 0; i < days_diff;i  ){
                    next_month[day_mon[n_ind]-1].insert(end(next_month[day_mon[n_ind]-1]), begin(current_month[day_mon[n_ind] i]), end(current_month[day_mon[n_ind] i]));
                }
            }

            current_month.clear();
            c_ind  =1;
            current_month.resize(next_month.size());
            current_month = next_month;   
            next_month.clear();

            
          
        } else if(operation == "ADD"){
            cin >> day >> to_do;
            current_month[day-1].push_back(to_do);
        } else if(operation == "DUMP"){
            cin >> day;
            cout << current_month[day-1].size() << ' ';
            for(int i = 0; i < current_month[day-1].size(); i  ){
                cout << current_month[day-1][i] << ' ';

            }
            cout << endl;
        }

    }

    return 0;
    
    
}

I have tested program by following inputs:

12
ADD 5 Salary
ADD 31 Walk
ADD 30 WalkPreparations
NEXT
DUMP 5
DUMP 28
NEXT
DUMP 31
DUMP 30
DUMP 28
ADD 28 Payment
DUMP 28

and on my compiler I got correct result:

1 Salary
2 WalkPreparations Walk
0
0
2 WalkPreparations Walk
3 WalkPreparations Walk Payment

but on other compiler I hot incorrect results:

1 Salary 
2 WalkPreparations Walk 
1 Salary 
1  
2 WalkPreparations Walk 
3 WalkPreparations Walk Payment

Can you help me to find my mistake?

CodePudding user response：

Look here

       copy(current_month.begin(), current_month.begin()   day_mon[n_ind], next_month.begin());

What happens when current_month has 28 days and the next month has 31 days - ie day_mon[n_ind] = 31

CodePudding user response：

As mentioned, this line:

copy(current_month.begin(), current_month.begin()   day_mon[n_ind], next_month.begin());

assumes that the current_month has at least day_mon[n_ind] days, when that is not the case when, for example, current_month has 28 days (February), but the next month (March) has 31 days (the day_mon[n_ind] will be 31).

One way to fix this is to limit the copy to the minimum of the days in the month of the current month and next month:

size_t numElementsToCopy = std::min(current_month.size(), static_cast<size_t>(day_mon[n_ind]));
std::copy_n(current_month.begin(), numElementsToCopy, next_month.begin());

Note: Usage of std::copy_n is a lot less wordy than using std::copy in this instance.