Deleting an element from Vector in C++

Question

I have a "map" that stores "int" as key and "Student" class object as value. After storing data into the map (group of maps : each map contain one key and one student class object as value) I need to store those group of maps into a vector. I have also done them. But when I try to delete a map from a particular position from the vector only the last/end map in the vector is getting deleted. I want to delete the map based on index.

My Program:

#include <map> #include <iostream> #include <string> #include <vector> #include <algorithm> using namespace std; class Student { public: string name; int id; void setName(string nam) { name = nam; } void setId(int i) { id = i; } string getName() { return name; } int getId() { return id; } }; vector<map<int,Student>> coll; typedef std::map<int,Student> Mymap; Mymap c1; map<int, Student> mine(int key,Student value) { c1.insert(Mymap::value_type( key,value)); return c1; } void print(Mymap a) { coll.push_back(a); } int main() { typedef map<int, Student> map1; Student s[5]; Student s1; map1 m1; int i=0; s[0].setName("SAI"); s[1].setName("RAVI"); s[2].setName("RAJU"); s[3].setName("HemaChandra"); s[4].setName("Tushar"); s[0].setId(10); s[1].setId(20); s[2].setId(30); s[3].setId(40); s[4].setId(50); for(int j=0;j<5;j++) { m1 = mine(j,s[j]); print(m1); } cout<<endl<<endl; cout<<"Before Deleting size = "<<coll.size()<<endl; map1 m2; Student st ; std::vector<map<int,Student>>::const_iterator k; for(k=coll.begin(); k!=coll.end(); ++k) { m2 = coll[i]; st = m2.at(i); std::cout <<" "<< st.getName()<<std::endl; i++; } coll.erase(coll.begin() + 2); //Should delete 3 element but not deleting cout<<"\nAfter Deleting :\n"<<endl; i=0; for(k=coll.begin(); k!=coll.end(); ++k) { m2 = coll[i]; st = m2.at(i); std::cout <<" "<< st.getName()<<std::endl; i++; } /* coll.erase(std::remove(coll.begin(), coll.end(), m1), coll.end()); This one also not working */ return 0; }

When I try to display the elements I can display the first 4 elements but not the last element.Actually the program should remove the 3 element from the vector(That is 3rd map) and display all other maps: 1,2,4,5 data. Please anyone help me in deleting the map from a particular position from the map.

why don't you use the value of k anywhere?

Caleth
– Caleth

2017-03-21 08:19:56 +00:00
Commented Mar 21, 2017 at 8:19 — Caleth
– Caleth, Commented Mar 21, 2017 at 8:19

Caleth · Accepted Answer · 2017-03-21 10:21:01Z

The code is working fine, not sure what you would expect but this code

for (int j = 0; j<5; j++) { m1 = mine(j, s[j]); print(m1); }

can be simplified as:

Student s[5]; s[0].setName("SAI"); s[1].setName("RAVI"); s[2].setName("RAJU"); s[3].setName("HemaChandra"); s[4].setName("Tushar"); vector<map<int, Student>> coll; map<int, Student> m1; for(int i = 0; i < 5; ++i) { m1.emplace(i, s[i]); coll.push_back(m1); // You're push_back'ing a copy of the map each time, // and each time it will have a new element in it }

so your line

coll.erase(coll.begin() + 2);

actually erases just the third map from the vector:

vector { 0: map { SAI }; 1: map { SAI, RAVI }; 2: map { SAI, RAVI, RAJU }; <- this is deleted 3: map { SAI, RAVI, RAJU, HemaChandra }; 4: map { SAI, RAVI, RAJU, HemaChandra, Tushar }; }

Printing the elements with indices 0, 1, 2, 3 will yield SAI, RAVI, RAJU, HemaCHandra. If you instead print the last ones as

 cout << "\nAfter Deleting :\n" << endl; for (k = coll.begin(); k != coll.end(); ++k) { st = k->rbegin()->second; // Get the last one in each map std::cout << " " << st.getName() << std::endl; }

you will get SAI, RAVI, HemaChandra, Tushar.

Thanks mate,it solved my problem. Could you please explain what st=m2.rbegin()->second does there. I am really a bit confused.
@HemaChandra it just takes the last element of every map. I recommend you to set up a debugger and see for yourself what are the contents of the various maps at that point in the program (set a breakpoint there).

alexeykuzmin0 · Accepted Answer · 2017-03-21 07:55:26Z

Everything is deleted just fine, you can check it in the debugger.

The problem is that in the line

st = m2.at(i);

in the second printing loop you access the elements 1, 2, 3, 4 of the maps you're storing, not the elements 1, 2, 4, 5 as you expect. This code can be fixed by changing this line to

st = m2.rbegin()->second;

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