# How to Convert Decimal to Binary in C++

Jinku Hu Feb 02, 2024

This article will introduce several methods of how to convert decimal numbers to binary representation in C++.

## Use Custom Defined Function to Convert Decimal Number to Binary in C++

The flexible method defines a function that takes the `int` value, converts it to corresponding binary representation, and returns it as a string value. In this case, we implemented the algorithm using modulo `%` operator and `while` loop, where we decrease the value of integer by half each iteration.

``````#include <iostream>

using std::cout;
using std::endl;
using std::string;

string toBinary(int n) {
string r;
while (n != 0) {
r += (n % 2 == 0 ? "0" : "1");
n /= 2;
}
return r;
}

int main() {
int number = 15;

cout << "decimal: " << number << endl;
cout << "binary : " << toBinary(number) << endl;

return EXIT_SUCCESS;
}
``````

Output:

``````decimal: 15
binary : 1111
``````

## Use `std::bitset` Class to Convert Decimal Number to Binary in C++

Alternatively, we can directly use the `bitset` class from the STL library. `bitset` represents a fixed-size sequence of N bits, and it offers multiple built-in methods to manipulate the binary data efficiently. The following example shows the `bitset` object construction by passing the `string` value and an `int` value.

``````#include <bitset>
#include <iostream>

using std::bitset;
using std::cout;
using std::endl;
using std::string;

string toBinary(int n) {
string r;
while (n != 0) {
r += (n % 2 == 0 ? "0" : "1");
n /= 2;
}
return r;
}

int main() {
int number = 15;

bitset<32> bs1(toBinary(number));
cout << "binary:  " << bs1 << endl;
bitset<32> bs2(number);
cout << "binary:  " << bs2 << endl;

return EXIT_SUCCESS;
}
``````

Output:

``````binary:  00000000000000000000000000001111
binary:  00000000000000000000000000001111
``````

Notice that we specified 32 bits to be allocated during the `bitset` declaration in the previous code. We can specify a different number of bits to suit their needs better. Multiple scenarios are demonstrated in the following code segment:

``````#include <bitset>
#include <iostream>

using std::bitset;
using std::cout;
using std::endl;
using std::string;

int main() {
int number = 15;

bitset<32> bs1(number);
cout << "binary:  " << bs1 << endl;
bitset<16> bs2(number);
cout << "binary:  " << bs2 << endl;
bitset<8> bs3(number);
cout << "binary:  " << bs3 << endl;
bitset<5> bs4(number);
cout << "binary:  " << bs4 << endl;
cout << endl;

return EXIT_SUCCESS;
}
``````

Output:

``````binary:  00000000000000000000000000001111
binary:  0000000000001111
binary:  00001111
binary:  01111
``````

`bitset` class has several useful methods to operate on its content. These methods can be employed to invert all bits of the set (`flip` function) or to `reset`/`set` the specified bits in the sequence. The core binary operations like `AND`, `OR`, `XOR`, `NOT` and `SHIFT` are also supported. We show a couple of them in the example below, but you can see the full manual of the `bitset` class here.

``````#include <bitset>
#include <iostream>

using std::bitset;
using std::cout;
using std::endl;
using std::string;

int main() {
int number = 15;

bitset<32> bs(number);
cout << "binary : " << bs << endl;
cout << "flipped: " << bs.flip() << endl;
cout << "shift>4: " << (bs >>= 6) << endl;
cout << "shift<5: " << (bs << 2) << endl;
cout << "reset  : " << bs.reset() << endl;
cout << "set    : " << bs.set(16) << endl;

return EXIT_SUCCESS;
}
``````

Output:

``````binary : 00000000000000000000000000001111
flipped: 11111111111111111111111111110000
shift>4: 00000011111111111111111111111111
shift<5: 00001111111111111111111111111100
reset  : 00000000000000000000000000000000
set    : 00000000000000010000000000000000
``````
Author: Jinku Hu

Founder of DelftStack.com. Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.