Convert vector<int> to integer

Question

I was looking for pre-defined function for converting a vector of integers into a normal integer but i din't find one.

vector<int> v;
v.push_back(1);
v.push_back(2);
v.push_back(3);

Need this:

int i=123 //directly converted from vector to int

Is there a possible way to achieve this?

Answer 1

Using C++ 11:

reverse(v.begin(), v.end());
int decimal = 1;
int total = 0;
for (auto& it : v)
{
    total += it * decimal;
    decimal *= 10;
}

EDIT: Now it should be the right way.

EDIT 2: See DAle's answer for a shorter/simpler one.

For the sake of wrapping it into a function to make it re-usable. Thanks @Samer

int VectorToInt(vector<int> v)
{
    reverse(v.begin(), v.end());
    int decimal = 1;
    int total = 0;
    for (auto& it : v)
    {
        total += it * decimal;
        decimal *= 10;
    }
    return total;
}

Answer 2

If elements of vector are digits:

int result = 0;
for (auto d : v)  
{
    result = result * 10 + d;
}

If not digits:

stringstream str;
copy(v.begin(), v.end(), ostream_iterator<int>(str, ""));
int res = stoi(str.str());

Answer 3

One liner with C++11 using std::accumulate():

auto rz = std::accumulate( v.begin(), v.end(), 0, []( int l, int r ) {
    return l * 10 + r; 
} );

live example

Answer 4

In conjunction with the answer provided by deepmax in this post Converting integer into array of digits and the answers provided by multiple users in this post, here is a complete test program with a function to convert an integer to a vector and a function to convert a vector to an integer:

// VecToIntToVec.cpp

#include <iostream>
#include <vector>

// function prototypes
int vecToInt(const std::vector<int> &vec);
std::vector<int> intToVec(int num);

int main(void)
{
  std::vector<int> vec = { 3, 4, 2, 5, 8, 6 };

  int num = vecToInt(vec);

  std::cout << "num = " << num << "\n\n";

  vec = intToVec(num);

  for (auto &element : vec)
  {
    std::cout << element << ", ";
  }

  return(0);
}

int vecToInt(std::vector<int> vec)
{
  std::reverse(vec.begin(), vec.end());

  int result = 0;

  for (int i = 0; i < vec.size(); i++)
  {
    result += (pow(10, i) * vec[i]);
  }

  return(result);
}

std::vector<int> intToVec(int num)
{
  std::vector<int> vec;

  if (num <= 0) return vec;

  while (num > 0)
  {
    vec.push_back(num % 10);
    num = num / 10;
  }

  std::reverse(vec.begin(), vec.end());

  return(vec);
}

Answer 5

Working solution for negative numbers too!

#include <iostream>
#include <vector>
using namespace std;

template <typename T> int sgn(T val) {
    return (T(0) < val) - (val < T(0));
}

int vectorToInt(vector<int> v) {
  int result = 0;
  if(!v.size()) return result;
  result = result * 10 + v[0];
  for (size_t i = 1; i < v.size(); ++i) {
    result = result * 10 + (v[i] * sgn(v[0]));
  }
  return result;
}

int main(void) {
  vector<int> negative_value = {-1, 9, 9};
  cout << vectorToInt(negative_value) << endl;

  vector<int> zero = {0};
  cout << vectorToInt(zero) << endl;

  vector<int> positive_value = {1, 4, 5, 3};
  cout << vectorToInt(positive_value) << endl;
  return 0;
}

Output:

-199
0
1453

Live Demo

The other answers (as of May '19) seem to assume positive integers only (maybe 0 too). I had negative inputs, thus, I extended their code to take into account the sign of the number as well.