C Program to Convert Decimal to Octal

Converting a decimal number to its octal counterpart is a common task in programming when dealing with different number systems. So, let’s say you want to convert the decimal number 156 to octal. This program shows you how to do that using simple iteration and base conversion.

Prerequisites for the Decimal to Octal Conversion Program in C

Before starting with the code, make sure you meet the following requirements:

• Knowledge of while loops for control flow
• Understanding of the % and / operators for arithmetic expressions
• Include the <stdio.h> library
• Any C compiler such as GCC 9 or newer — or just run it in the EduSeekho online compiler

Source Code of C Program to Convert Decimal to Octal

Here's the complete C code to convert a decimal number to octal. It uses a while loop to repeatedly divide the number by 8 and collect the remainders, which form the octal number.

// Program to convert decimal to octal
#include <stdio.h>

int main() {
    int decimal, quotient, octal_number = 0, position = 1;

    // Input from user
    // Prompt the user to enter a decimal number
    printf("Enter a decimal number: ");
    scanf("%d", &decimal);

    // Copy the input number to quotient
    quotient = decimal;

    // Convert decimal to octal
    while (quotient != 0) {
        int remainder = quotient % 8;
        octal_number += remainder * position;
        quotient /= 8;
        position *= 10;
    }

    // Output the octal number
    printf("Octal number: %d\n", octal_number);

    return 0;
}

Output of the Decimal to Octal Conversion Program

When you run the program with the input '156', here's what you get:

Enter a decimal number: 156
Octal number: 234

Algorithm to Convert Decimal to Octal in C

Converting a decimal number to octal involves dividing the number by 8 and storing the remainders. The remainders are collected in reverse order to form the octal number. Here is the exact logic this program uses, step by step.

1. Read the decimal number from the user.
2. Initialize octal_number to 0 and position to 1.
3. Repeat while the quotient is not 0:
• Find remainder by quotient % 8.
• Update octal_number by adding remainder * position.
• Divide quotient by 8.
• Multiply position by 10.
4. Print the octal_number.

flowchart TD
  A([Start]) --> B[/Enter decimal number/]
  B --> C[quotient = decimal]
  C --> D[octal_number = 0]
  D --> E[position = 1]
  E --> F{"quotient != 0?"}
  F -->|Yes| G["remainder = quotient %% 8"]
  G --> H["octal_number = octal_number + remainder * position"]
  H --> I["quotient = quotient / 8"]
  I --> J["position = position * 10"]
  J --> F
  F -->|No| K[/Print octal_number/]
  K --> L([End])

How the C Program to Convert Decimal to Octal Works

Let's break down the code to see how each part contributes to the conversion from decimal to octal.

• #include <stdio.h>: Includes the standard input-output library for using printf and scanf.
• int main(): The main function where the program execution begins.
• int decimal, quotient, octal_number = 0, position = 1;: Declares and initializes the needed variables. Here, octal_number will store the result, and position helps place each digit correctly.
• scanf: Reads the decimal number from the user.
• quotient = decimal;: Copies the user input to quotient for processing.
• The while loop: Continues as long as there's a quotient to process.
• remainder = quotient % 8;: Finds the remainder when quotient is divided by 8.
• octal_number += remainder * position;: Adds the new digit to the growing octal number.
• quotient /= 8;: Prepares for the next iteration by reducing the quotient.
• position *= 10;: Moves the position left by one digit place.
• printf: Outputs the final octal number.
• return 0;: Ends the program.

Time and Space Complexity of the Decimal to Octal Conversion Program

Analyzing how efficient our program is can help us understand its behavior with larger inputs.

Time Complexity: O(log n). Here, n is the decimal number being converted. This is because the program repeatedly divides quotient by 8, halving the number of digits to process each time. For example, converting 156 to octal takes only a few steps.

Space Complexity: O(1). The program uses a fixed number of variables: decimal, quotient, octal_number, and position. These are four integers regardless of the size of the input number.

Key takeaway:Multiplying by position makes sure each digit in the octal number is correctly placed by its value.

FAQs About C Program to Convert Decimal to Octal

Here are some common questions students have about converting decimal to octal in C.

Related Programs

If you found this program helpful, check out these related programs:

• C Program to Convert Binary to Decimal with Code and Output
• C Program to Convert Hexadecimal to Decimal
• C Program to Convert Octal to Decimal: Full Code & Output
• C Program to Convert Binary to Octal with Sample Output
• C Program to Convert Binary to Decimal and Vice-Versa

Conclusion

Converting decimal to octal in C is easy once you get how remainders and positions work. Explore octal stuff more: try converting octal back to decimal. Practice more to really nail these concepts.

To boost your understanding, take our C language quiz or try this code in the online compiler.