Segmentation fault while declaring vector string array C++ - c++

Question

#include<iostream>
#include<string>
#include<stdio.h>
#include<stdlib.h>
//#include<set>
#include<list>
#include<cmath>
//#include<algorithm>
#include<fstream>
#include<vector>
#define max 10000.0
using namespace std;
int main()
{
int n;//number of rooms
cin>>n;
vector<string> room_name(n,"");
for(int i=0;i<n;i++)
{
string tmp;
cin >> tmp;
room_name.push_back(tmp);
}
}
The error Segmentation fault: 11 still occurs.
Unable to find the point of error. Any idea why this might be happening?

Answer 1

It is not enough to declare a vector: unlike an array, it could grow, so by default it starts with a fixed size of zero.
If you know the size of the vector upfront, you can request a vector of a specific size (you can grow it after that, too)
vector<string> room_name(n, ""); // You can supply a different default, or no default here
This makes a vector of 100 empty strings, letting you access elements 0..n-1 in the subsequent loop.
Alternatively, you could make your vector grow one element at a time by using push_back, like this:
for(int i=0;i<n;i++)
{
string tmp;
cin >> tmp;
room_name.push_back(tmp);
}

Answer 2

Unless the vector is already sized, you need to use push_back to append to the end of the vector.
std::vector<int> foo;
for (int i = 0; i < 10; ++i)
{
foo.push_back(i);
}
Otherwise you need to ensure that the vector is already sized, either
std::vector<int> foo(10);
Or
std::vector<int> foo;
foo.resize(10);
Then you could use indexing as you did
for (int i = 0; i < 10; ++i)
{
foo[i] = i;
}
If you try to write to each index before you sized your array, you are writing to memory out of bounds.

Answer 3

Are you going indeed to enter 1000 strings manually in the vector?:)
Also it is a good idea to define canstant names using letters of upper case. So instead of
#define max 10000.0
it would be better to write
#define MAX 10000.0
And it is not clear why max is defined as a float number instead of an integer.
Take into account that variable n was not declared in your program. So the program does not compile. Maybe you mean max instead of n.
As the vector is defined as an empty vector
vector<string> room_name;
that is it does not have elements then you may not use the subscript operator to enter values in the vector.
You could use method push_back instead the following way
vector<string> room_name;
room_name.reserve( n );
for ( int i = 0; i < n; i++ )
{
string s;
cin >> s;
room_name.push_back( s );
}
And instead of the loop you could use standard algorithm std::copy_n declared in header <algorithm>
#include <iostream>
#include <string>
#include <iterator>
#include <algorithm>
#define N 2
int main()
{
std::vector<std::string>room_name;
room_name.reserve( N );
std::copy_n( std::istream_iterator<std::string>( std::cin ), N,
std::back_inserter( room_name ) );
for ( const std::string &s : room_name ) std::cout << s << std::endl;
return 0;
}
If to enter strings
my_room
your_room
then the program will output them from the vector
my_room
your_room