使用Vector按降序对子字符串进行排序，但是出现分段错误

Question

I want to sort the string from N to M where N indicates the starting index and M indicates the ending index.

However, my code is getting failed with segmentation fault. Here is my code:

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

using namespace std;

int main()
{   
    string s;
    int M=0,N=0;

    cout<<"Enter String"<<endl;
    getline(cin,s);

    vector<char> data(s.begin(), s.end());

    cout<<"Enter start_index and end_index for sorting";
    cin>>N>>M;    //Passed externally as N=start_index, M=end_index

    std::sort(data.begin()+N, data.begin()+M, std::greater<char>());

    for (std::vector<char>::const_iterator i = data.begin(); i != data.end(); ++i)
        std::cout << *i << ' ';

    return 0;
}

Answer 1

This example does work fine for me:

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

using namespace std;

int main()
{   
    string s = "ABCDEFG";
    int N = 1;
    int M = 5;

    vector<char> data(s.begin(), s.end());

    std::sort(data.begin() + N, data.begin() + M, std::greater<char>());

    for (auto& character : data)
        std::cout << character << ' ';
    return 0;
}

http://coliru.stacked-crooked.com/a/ee7c5f05afe85115

I suspect you get an empty string with cin, and therefore your data.begin() is invalid. Be cautious with user entered data. Always do proper checking for input that may break your code.

Additonally your templated greater is the comparing function for the wrong type.

Answer 2

The answer to the above question is from the guidance of Trevir. In order to avoid the segmentation fault, check the size of the input string and then apply operations on it.

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

using namespace std;

int main()
{   
    string s;
    int M,N;

    cout<<"Enter String"<<endl;
    getline(cin,s);

    if ( s.size() == 0 )
    {
       std::cout << "Empty Input" << std::endl;   
       return 0;
    }

    vector<char> data(s.begin(), s.end());

    cout<<"Enter start_index and end_index for sorting";
    cin>>N>>M;    //Passed externally as N=start_index, M=end_index

    std::sort(data.begin()+N, data.begin()+M, std::greater<char>());

    for (std::vector<char>::const_iterator i = data.begin(); i != data.end(); ++i)
        std::cout << *i;

    return 0;
}