Conversion of Binary to Decimals - c++

Question

I am writing a c++ program where the program takes the t test cases as input and asks the binary input t times, and after that displays the output.
I tried to code but for 1st test case it displays right output in decimal format but for further test cases its not displaying correct output result.
#include<iostream>
#include<cmath>
using namespace std;
int main() {
long int num;
int r=0,i=0,t,dec=0;
cin>>t;
while(t--) {
cin>>num;
while (num!=0) {
r = num% 10;
num /= 10;
dec += r * pow(2, i);
++i;
}
cout<<dec<<"\n";
}
return 0;
}
This code if run for lets say 2 test cases , lets say first one is 101 , it displays 5 which is fine but on the second input , lets say 111 , it displays output 61.
I am not sure where I am getting it wrong.

Answer 1

You initialize i, dec outside the outer while loop. Initialize them to 0 inside the outer while loop and it works:
#include <iostream>
#include <cmath>
using namespace std;
int main()
{
int t;
cin >> t;
while(t--)
{
int num, r, dec = 0, i = 0;
cin >> num;
while (num != 0) {
r = num % 10;
num /= 10;
dec += r * pow(2, i);
//cout << r << " " << num << " " << i << " " << dec << endl;
++i;
}
cout << dec << "\n";
}
return 0;
}

Answer 2

There is C function strtoull, which is doing the job
Example:
cat bin2dec.cpp
#include <iostream>
#include <string>
#include <cstdlib>
using namespace std;
int main() {
string num;
int t;
cin>>t;
while(t--) {
cin>>num;
auto dec = strtoull(num.c_str(), NULL, 2);
cout << dec << endl;
}
return 0;
}
g++ -O2 -o bin2dec bin2dec.cpp
2
111
7
100000000000
2048
g++ --version | head -1
g++ (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0

Answer 3

While you can use the division and remainder method, C++ provides std::bitset and the string conversion std::stoul, std::stoull that can automate the conversion for you.
The following example loops prompting the user for input of a binary value and converts the value to unsigned long so long as the binary digits entered are 64 or less. Simply press Enter without entering a value to end the program:
#include <iostream>
#include <string>
#include <bitset>
int main() {
for (;;) { /* loop continually */
std::string input{}; /* binary input from user */
unsigned long value = 0; /* value to hold conversion to ulong */
/* prompt for input, read/validate input */
std::cout << "\nenter a binary value: ";
if (!getline(std::cin, input) || input.size() == 0) {
break;
}
if (input.size() > 64) { /* validate 64 binary digits or less */
std::cerr << "error: input exceeds 64-bit conversion limit.\n";
continue;
}
value = std::bitset<64>(input).to_ulong(); /* convert to ulong */
std::cout << input << ": " << value << '\n'; /* output result */
}
}
Note: you would also want to validate the user has entered only '0's and '1's which you can do simply with std::basic_string::find_first_not_of, e.g.
if (input.find_first_not_of("01") != std::string::npos) {
std::cerr << "error: invalid input not '0' and '1'\n";
continue;
}
(simply add the if statement before the conversion and assignment to value)
Example Use/Output
./bin/bitset_to_ulong
enter a binary value: 1010
1010: 10
enter a binary value: 10000000
10000000: 128
enter a binary value: 1111
1111: 15
enter a binary value: 1234
error: invalid input not '0' and '1'
enter a binary value:
Just an alternative way of doing the same that C++ has provided a convenient way to do in C++11 and forward.