Write a C program to input Octal number from user and convert to Decimal number system. How to convert from Octal number system to Decimal number system in C program. Logic to convert octal to decimal number system in C programming.

**Example**

**Input**

Input octal number: 172

**Output**

Decimal number: 122

## Required knowledge

Basic C programming, While loop

Must know - Program to find last digit of a number.

### Octal number system

Octal number system is a base 8 number system. Octal number system uses 8 symbols to represent all its numbers i.e. `01234567`

### Decimal number system

Decimal number system is a base 10 number system. Decimal number system uses 10 symbols to represent all its numbers i.e. `0123456789`

## Logic to convert from octal to decimal

Algorithm Conversion Octal to Binarybegin:read(octal);decimal← 0;rem← 0;place← 0;While(octal!=0)begin:rem←octal% 10;decimal←decimal+ (8^{place}*rem);octal←octal/ 10;place←place+ 1;end;write('Decimal ='decimal);end;

## Program to convert octal to decimal number system

```
/**
* C program to convert Octal number system to Decimal number system
*/
#include <stdio.h>
#include <math.h>
int main()
{
long long octal, tempOctal, decimal;
int rem, place;
/* Input octal number from user */
printf("Enter any octal number: ");
scanf("%lld", &octal);
tempOctal = octal;
decimal = 0;
place = 0;
/*
* Convert octal to decimal
*/
while(tempOctal > 0)
{
/* Extract the last digit of octal */
rem = tempOctal % 10;
/* Convert last octal digit to decimal */
decimal += pow(8, place) * rem;
/* Remove the last octal digit */
tempOctal /= 10;
place++;
}
printf("Octal number = %lld\n", octal);
printf("Decimal number = %lld", decimal);
return 0;
}
```

Output

Enter any octal number: 172 Octal number = 172 Decimal number = 122

Happy coding 😉

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