Incorrect addition in a Function - c++

Question

I am writing a function in C++ to convert a number from some base to decimal.
It works fine when the number of digits is even, but when it is odd it gives wrong answer.
For example:
Number to convert : 100
Base to convert to: 10
Correct answer : 100
Function's output : 99
Here is the code:
unsigned long long convertToDecimal(const std::string& number, const unsigned base)
{
std::string characters = "0123456789abcdef";
unsigned long long res = 0;
for(int i = 0, len = number.size(); i<len; ++i)
{
res += characters.find(number.at(i))*std::pow(base, len-1-i);
}
return res;
}
I'm using g++ C++11.

Answer 1

I can't reproduce your particular issue, but std::pow returns a floating point number and your implementation may have introduced some sort of rounding error which leaded to a wrong result when converted to unsigned long long.
To avoid those errors, when dealing with integer numbers, you should consider to avoid std::pow at all. Your function, for example, could have been written like this:
#include <iostream>
#include <string>
#include <cmath>
unsigned long long convertToDecimal(const std::string& number, const unsigned base)
{
std::string characters = "0123456789abcdef";
unsigned long long res = 0;
unsigned long long power = 1;
for(auto i = number.crbegin(); i != number.crend(); ++i)
{
// As in your code, I'm not checking for erroneous input
res += characters.find(*i) * power;
power *= base;
}
return res;
}
int main ()
{
std::cout << convertToDecimal("100", 2) << '\n'; // --> 4
std::cout << convertToDecimal("1234", 8) << '\n'; // --> 668
std::cout << convertToDecimal("99999", 10) << '\n'; // --> 99999
std::cout << convertToDecimal("fedcba", 16) << '\n'; // --> 16702650
}