C++ bit string to int conversion - c++

Question

I am trying to convert a bit string (bitString) of length 'sLength' to an int.
The following code works fine for me in my computer. Is there any case where it may not work?
int toInt(string bitString, int sLength){
int tempInt;
int num=0;
for(int i=0; i<sLength; i++){
tempInt=bitString[i]-'0';
num=num+tempInt * pow(2,(sLength-1-i));
}
return num;
}
Thanks in advance

Answer 1

pow works with doubles. Result may be inaccurate. Use bit arithmetic instead
num |= (1 << (sLength-1-i)) * tempInt;
Don't also forget about cases when bitString contains symbols other than '0' and '1' or too long

Answer 2

Or, you can let the standard library do the heavy lifting:
#include <bitset>
#include <string>
#include <sstream>
#include <climits>
// note the result is always unsigned
unsigned long toInt(std::string const &s) {
static const std::size_t MaxSize = CHAR_BIT*sizeof(unsigned long);
if (s.size() > MaxSize) return 0; // handle error or just truncate?
std::bitset<MaxSize> bits;
std::istringstream is(s);
is >> bits;
return bits.to_ulong();
}

Answer 3

Why not change your for loop to the more efficient and far more simple C++11 version:
for (char c : bitString)
num = (num << 1) | // Shift the current set of bits to the left one bit
(c - '0'); // Add in the current bit via a bitwise-or
By the way, you should also check that the number of bits specified does not overrun an int and you may want to make sure that each char in the string is either a '0' or '1'.

Answer 4

Answer and notice about inaccuracy of floating-point numbers already given; here's a more readable implementation with integer arithmetic, though:
int toInt(const std::string &s)
{
int n = 0;
for (int i = 0; i < s.size(); i++) {
n <<= 1;
n |= s[i] - '0';
}
return n;
}
Notes:
You don't need an explicit length. That's why we have std::string::length().
Counting from zero results in cleaner code, because you don't have to do the subtraction every time.

Answer 5

for (std::string::reverse_iterator it = bitString.rbegin();
it != bitString.rend(); ++it) {
num *= 2;
num += *it == '1' ? 1 : 0;
}

Answer 6

I see directly three cases where it may not work :
pow Works with double, your result may be inaccurate, you can fix it with :
num |= tempInt * ( 1 << ( sLength - 1 - i ) );
If bitString[i] is not a '0' or '1',
If your number in the string in bigger than the int limit.
If you have control over the last two points, your resulting code could be :
int toInt( const string& bitString )
{
int num = 0;
for ( char c : bitString )
{
num <<= 1;
num |= ( c - '0' );
}
return num;
}
Don't forget the const reference as a parameter.