Substring to Int C-CodePudding

I am trying to convert a string that is only 1s and 0s to a decimal value. The variable value is initialized to 0 and is never updated. I suspect the problem is that binaryString[i] is treated as a string and therefore the athematic function doesn't work. How can I fix this?

void binaryToDec(string binaryString, int value)
{
  int binaryStringLength = binaryString.length();
  for (int i = 0; i < binaryStringLength; i  )
    {
      value  = pow(2,i)  binaryString[i];
    }
}

I tried to use basic type casting like int(binaryString[i]) but that doesn't work.

CodePudding user response：

Firstly binaryString[i] is a character, not an integer. To convert a digit character to an integer you can just subtract '0'.

binaryString[i] - '0'

Secondly pow(2,i) returns a floating point number, when you want an integer. This is inefficient, and even more seriously might be subject to rounding errors. Instead you should use a shift operator, which efficiently and accurately calculates integer powers of two.

1 << i

Thirdly you have where you need *. The two terms should be multiplied not added.

Putting all that together you get

value  = (1 << i) * (binaryString[i] - '0');

But the most serious error of all is that your function does not return a value. It should look like this

int binaryToDec(string binaryString)
{
    int value = 0;
    ...
    return value;
}

Your version passes value as a parameter, that's the wrong way round, binaryString is a parameter, but value should be returned from the function. Not sure why but this is a difference a lot of newbies struggle with.

CodePudding user response：

You could construct a bitset from the string and then back to a ullong.

But, limiting the solution to a maximum binary string size.

[Demo]

#include <bitset>
#include <fmt/core.h>
#include <stdexcept>  // out_of_range

auto binaryToDec(const std::string& binary) {
    if (binary.size() > 64) {
        throw std::out_of_range{ "binary string is too big" };
    }
    return std::bitset<64>{binary}.to_ullong();
}

int main() {
    try {
        std::string binary_1{"101010"};
        fmt::print("binary: {}, decimal: {}\n", binary_1, binaryToDec(binary_1));
        std::string binary_2{
            "1111""1111""1111""1111"
            "1111""1111""1111""1111"
            "1111""1111""1111""1111"
            "1111""1111""1111""1111"
            "1"
        };
        fmt::print("{}\n", binaryToDec(binary_2));
    } catch (const std::exception& e) {
        fmt::print("Error: {}.\n", e.what());
    }
}

// Outputs:
//
//   binary: 101010, decimal: 42
//   Error: binary string is too big.