C++ Vector Sort String Issue

Question

Okay I have a little assignment I need to sort a vector of strings using std::sort but it does not sort any "numbers" above two digest correctly. It is critical I use this API for the assignment.

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
int main()
{
    vector<string> Nums = { "1", "5", "34", "3", "6", "12", "21" };
    sort(Nums.begin(), Nums.end());
    for (int i = 0; i < Nums.size(); i++)
    {
        cout << Nums[i] << endl;
    }
    system("PAUSE");
}

Result:

1
12
21
3
34
5
6
Press any key to continue . . .

Want:

Do you want to sort the strings numerically, or lexicographically? i.e. should the list start("1", "3", ... ) or ("1", "12", ...)? — Tristan Brindle
– Tristan Brindle, Commented Dec 1, 2015 at 2:00
This is using string sort. If you want to sort them by numeric value, use a custom comparator in the call to sort. More info here: en.cppreference.com/w/cpp/algorithm/sort — paddy
– paddy, Commented Dec 1, 2015 at 2:01

Tristan Brindle · Accepted Answer · 2015-12-01 02:05:52Z

2

std::sort is doing exactly what you're asking it to: it's sorting a list of strings in lexicographical order. To sort numerically, you're going to need to pass a custom comparator to sort which compares the strings as if they were numbers. For example:

std::sort(std::begin(Nums), std::end(Nums)
          [] (const std::string& lhs, const std::string& rhs) {
              return std::stoi(lhs) < std::stoi(rhs);
          });

This uses the function stoi from the standard library to convert the strings to ints for comparison.

answered Dec 1, 2015 at 2:05

Tristan Brindle

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Comments

user2736738 · Accepted Answer · 2015-12-01 06:48:05Z

0

I have provided the same thing in a more simple manner. You have to use custom comparison in order to achieve this.

bool cmp(string a,string b)
{
     int p=convertToNum(a); //can use custom function or std::stoi
     int q=convertToNum(b); // converting string to integer
     return p<q;
}
sort(Nums.begin(), Nums.end(),cmp);

edited Dec 1, 2015 at 6:48

answered Dec 1, 2015 at 2:06

user2736738

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