In this example, you will learn to convert binary number to octal, and octal number to binary manually by creating a user-defined function.

To understand this example, you should have the knowledge of following C++ programming topics:

In this program, we will first convert the binary number to decimal. Then, the decimal number is converted to octal.

```
#include <iostream>
#include <cmath>
using namespace std;
int convertBinarytoOctal(long long);
int main()
{
long long binaryNumber;
cout << "Enter a binary number: ";
cin >> binaryNumber;
cout << binaryNumber << " in binary = " << convertBinarytoOctal(binaryNumber) << " in octal ";
return 0;
}
int convertBinarytoOctal(long long binaryNumber)
{
int octalNumber = 0, decimalNumber = 0, i = 0;
while(binaryNumber != 0)
{
decimalNumber += (binaryNumber%10) * pow(2,i);
++i;
binaryNumber/=10;
}
i = 1;
while (decimalNumber != 0)
{
octalNumber += (decimalNumber % 8) * i;
decimalNumber /= 8;
i *= 10;
}
return octalNumber;
}
```

**Output**

Enter a binary number: 10001 10001 in binary = 21 in octal

The binary number entered by the user is passed to `convertBinaryToOctal()`

function. And, this function converts the number to octal and returns to the `main()`

function

In this program, the octal number is converted to decimal at first. Then, the decimal number is converted to binary number.

```
#include <iostream>
#include <cmath>
using namespace std;
long long convertOctalToBinary(int);
int main()
{
int octalNumber;
cout << "Enter an octal number: ";
cin >> octalNumber;
cout << octalNumber << " in octal = " << convertOctalToBinary(octalNumber) << "in binary";
return 0;
}
long long convertOctalToBinary(int octalNumber)
{
int decimalNumber = 0, i = 0;
long long binaryNumber = 0;
while(octalNumber != 0)
{
decimalNumber += (octalNumber%10) * pow(8,i);
++i;
octalNumber/=10;
}
i = 1;
while (decimalNumber != 0)
{
binaryNumber += (decimalNumber % 2) * i;
decimalNumber /= 2;
i *= 10;
}
return binaryNumber;
}
```

**Output**

Enter an octal number: 54 54 in octal = 101100

The octal number entered by the user is passed to `convertOctalToBinary()`

function. And, this function converts the number to binary and returns the `main()`

function

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