C Exercises: Convert Octal number to a Decimal number

C For Loop: Exercise-51 with Solution

Write a C program to convert an octal number to a decimal without using an array.

Visual Presentation:

Convert Octal number to a Decimal number

Sample Solution:

C Code:

#include <stdio.h>
int main() {
    int n1, n5, p = 1, k, ch = 1;
    int dec = 0, i = 1, d;
    printf("\n\nConvert Octal to Decimal:\n ");
    printf("Input an octal number (using digits 0 - 7): ");
    scanf("%d", &n1);
    n5 = n1;
    // Checking if any digit in the input number is greater than or equal to 8
    for (; n1 > 0; n1 = n1 / 10) {
        k = n1 % 10;
        if (k >= 8) {
            ch = 0;
            break;  // No need to continue checking if one invalid digit is found
    // Switch-case to determine if the input is a valid octal number or not
    switch (ch) {
        case 0:
            printf("\nThe number is not an octal number.\n\n");
        case 1:
            n1 = n5;
            // Calculating the decimal equivalent of the octal number
            for (; n1 > 0; n1 = n1 / 10) {
                d = n1 % 10;
                dec += d * p;
                p *= 8;
            // Displaying the input octal number and its equivalent decimal number
            printf("\nThe Octal Number: %d\nThe equivalent Decimal Number: %d\n\n", n5, dec);
    return 0;

Sample Output:

Convert Octal to Decimal:                                                                                     
Input an octal number (using digit 0 - 7) :745                                                                
The Octal Number : 745                                                                                        
The equivalent Decimal  Number : 485  


Here's a brief explanation.

  • Header and Declaration:
    • Includes the necessary header file (stdio.h).
    • Declare variables 'n1' (input octal number), 'n5' (backup of the input), 'p' (power of 8 multiplier), 'k' (temporary variable for digit checking), 'ch' (flag for octal validity check), 'dec' (resulting decimal number), 'i' (loop variable), and 'd' (temporary variable for digit extraction).
  • User Input:
    • Asks the user to input an octal number (using digits 0 - 7).
    • Reads the input and stores it in the variable 'n1'.
  • Octal Validity Check:
    • Uses a for loop to iterate through each digit of the input octal number ('n1').
    • Checks if any digit is greater than or equal to 8, marking the number as invalid if true.
  • Switch-Case for Octal Validity:
    • Uses a switch statement based on the validity check ('ch').
    • If invalid, prints a message indicating that the input is not an octal number.
    • If valid, calculate the decimal equivalent.
  • Decimal Equivalent Calculation:
    • Reset 'n1' to the backup ('n5').
    • Use a for loop to extract each digit of the octal number, multiplying it by the corresponding power of 8 ('p') and accumulating the result in the 'dec' variable.
  • Result Display:
    • Prints the input octal number and its equivalent decimal number.
  • Return Statement:
    • Returns 0 to indicate successful execution.


Flowchart : Convert decimal number to octal without using array

C Programming Code Editor:

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