how to keep insert values from a istringstream to a vector?

Question

I want to take input from the user in a string and insert those values into a vector using stringstream
my code looks like this :

#include <iostream>
#include <string>
#include <sstream>
#include <vector>
using namespace std;
int main (){
    string s;
    getline(cin ,s);
    vector<int>vec;
    istringstream iss{s};
   while(iss){
        int temp;
        iss>>temp;
        vec.push_back(temp);
    }
    for (auto val:vec){
        cout << val<<" ";
    }
    return 0;
}

I am inserting 1 2 3 in the string but the output is 1 2 3 3 . this method is inserting the last
element 2 times.
but the following is working fine :

#include <iostream>
#include <string>
#include <sstream>
#include <vector>
using namespace std;
int main (){
    string s;
    getline(cin ,s);
    vector<int>vec;
    istringstream iss{s};
    do{
        int temp;
        iss>>temp;
        vec.push_back(temp);
    }while(!iss.eof());
    
    for (auto val:vec){
        cout << val<<" ";
    }
    return 0;
}

can anyone tell why the first method is taking the last input twice?

On a side-note, if you learn about [`std::istream_iterator`](https://en.cppreference.com/w/cpp/iterator/istream_iterator) you can simplify your code quite considerably: `std::vector vec(std::istream_iterator(iss), std::istream_iterator());` The first loop isn't needed. There's a corresponding [`std::ostream_iterator`](https://en.cppreference.com/w/cpp/iterator/ostream_iterator) which can be used together with [`std::copy`](https://en.cppreference.com/w/cpp/algorithm/copy) to omit the second loop also, but your explicit loop is more clear and understandable so keep it. — Some programmer dude, Aug 27 '21 at 05:37

Ted Lyngmo · Accepted Answer · 2021-08-27T06:16:47.327

It's inserting twice because you don't check if extraction from the stream works. Do this instead:

int temp;
while(iss >> temp) {
    vec.push_back(temp);
}

You could use a for loop if you want to keep the scope of temp to a minimum:

for(int temp; iss >> temp;) {
    vec.push_back(temp);
}

Lets pretend that there is only one int in iss and walk through what happens in your loop:

while(iss) {              // 1.   4.   7.
    int temp;
    iss>>temp;            // 2.   5.
    vec.push_back(temp);  // 3.   6.
}

iss is in a good state, the condition is true.
Extracting an int into temp works.
The newly extracted int is stored.
iss is still in a good state, the condition is true.
Extracting an int into temp fails and sets iss in a failed state and leaves temp uninitialized
The previously extracted int is stored¹
The condition is now false.

¹Reading uninitialized values makes your program have undefined behavior. In this case it happens to give you the value stored in the temp from the previous loop - presumably because each temp created in the loop shares the same memory address.

how to keep insert values from a istringstream to a vector?

1 Answers1