Convert vector<int> to integer - c++

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

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 2

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 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.