# C Program to convert a decimal number to an octal number

Last updated on July 27, 2020

The following is a C program to convert a decimal number to an octal number:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 | ```
/*******************************************************
Program to convert a decimal number to an octal number
******************************************************/
#include<stdio.h> // include stdio.h library
#include<math.h> // include math.h library
int main(void)
{
long long num, oct = 0;
int i = 0, rem;
printf("Enter a decimal number: ");
scanf("%lld", &num);
while(num != 0)
{
rem = num % 8; // get the last digit
oct = rem * (long long)pow(10, i++) + oct;
num /= 8; // get the quotient
}
printf("0o");
printf("%lld", oct);
return 0; // return 0 to operating system
}
``` |

**Expected Output:** 1st run:

1 2 | ```
Enter a decimal number: 74
0o112
``` |

2nd run:

1 2 | ```
Enter a decimal number: 2545
0o4761
``` |

## How it works #

To convert a decimal number to an octal number, we follow these steps:

Step 1: Divide the decimal number continuously by 8 and write the remainder on the right-hand side of the dividend. We repeat this process until we get the quotient 0.

Step 2: Write the remainders from bottom to top.

Let's take some examples:

**Example 1**: Convert decimal `125`

to an octal number:

Step 1:

Quotient | Remainder | |
---|---|---|

`125/8` |
`15` |
`5` |

`15/8` |
`1` |
`7` |

`1/2` |
`0` |
`1` |

Step 2:

\(\mathtt{125_{10} = 175_{8}}\)

**Example 2**: Convert decimal `500`

to an octal number:

Step 1:

Quotient | Remainder | |
---|---|---|

`500/8` |
`62` |
`4` |

`62/8` |
`7` |
`6` |

`1/2` |
`0` |
`7` |

Step 2:

\(\mathtt{500_{10} = 764_{8}}\)

The following table shows what happens at each iteration of the loop (assuming `num = 74`

):

Iteration | rem | bin | num | i |
---|---|---|---|---|

After 1st iteration | `rem=74%8=2` |
`oct=2*(10^0)+0=2` |
`num=74/8=9` |
`i=2` |

After 2nd iteration | `rem=9%8=1` |
`oct=1*(10^1)+2=12` |
`num=9/8=1` |
`i=3` |

After 3rd iteration | `rem=1%8=1` |
`oct=1*(10^2)+12=112` |
`num=1/8=0` |
`i=4` |

**Recommended Reading**

- C Program to convert a decimal number to a hexadecimal number
- C Program to convert a decimal number to a binary number
- C Program to Convert a Binary Number to a Decimal Number
- C Program to multiply two numbers using Russian peasant method
- C Program to check whether the number is a Palindrome

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