facing an issue with the sort function in cpp

Question

trying to sort a dynamic array sum_array but the function is not working.also my loop used to self terminate if i dont put the size of array as 1 could there be something with my declaration

**#include<bits/stdc++.h>
#include<algorithm>
#include<vector>

using namespace std;

int main(){

  vector <int> sum_array (1);
  int n;
  cin>>n;
  int sum,marks;
  for (int i = 0; i < 5; i++) {
    cout<<"turn now is ="<<i<<endl;
    sum=0;
    for (int k = 0; k < (n-1); k++) {
    cin>>marks;
    sum=sum+marks;
    cout<<"sum="<<sum<<endl;
    }
  sum_array[i]=sum;
  }
  for (int i = 0; i < 5; i++) {
    cout<<sum_array[i]<<endl;
  }
  sort(sum_array.begin(),sum_array.end());
  cout<<"after"<<endl;
  for (int i = 0; i < 5; i++) {
    cout<<sum_array[i]<<endl;
  }
    return 0;
}**

You create sum_array with one element but then set sum_array[i] with i in the range [0, 5) without ever resizing. That's undefined behaviour. It also means sort will only consider that one item. — G.M.
– G.M., Commented Apr 20, 2019 at 12:50
sum_array is given a size of one element, but elements 0 to 4 are assigned. Assigning elements like that does not implicitly resize. Instead, your code has undefined behaviour. — Peter
– Peter, Commented Apr 20, 2019 at 12:51
i thought vector was a dynamic array that doubles up on its own.if its showing me elements from 0 to 4 that means they were fitted by resizing the array then why is it undefined when i try to sort them. how do i fix it? — ash1102
– ash1102, Commented Apr 20, 2019 at 12:58

Samleo · Accepted Answer · 2019-04-21 04:28:41Z

2

You're creating a vector of one integer:

std::vector<int> sum_array(1);

Then you access elements out of bounds of the vector (which is undefined behaviour).

for (int i = 0; i < 5; i++) {
  // ...
  sum_array[i] = sum;
}

std::sort is only sorting a vector of one element so it isn't doing anything.

Since you do not know the size of the vector, you should initialize it as an empty vector:

std::vector<int> sum_array;

and push the elements into the sum_array:

sum_array.push_back(sum);

edited Apr 21, 2019 at 4:28

Samleo

6055 silver badges17 bronze badges

answered Apr 20, 2019 at 12:59

Indiana Kernick

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