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C Program to Convert a Binary Number to a Decimal Number

Last updated on July 27, 2020


The following is a C program to convert a binary number to a decimal number.

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/*******************************************************
 * Program to convert a binary number to decimal number
********************************************************/

#include<stdio.h> // include stdio.h
#include<math.h> // include stdio.h

int main()
{
    long int bin, remainder, decimal = 0, i = 0;       

    printf("Enter a binary number: ");
    scanf("%d", &bin);

    while(bin != 0)
    {
        remainder = bin % 10;
        decimal += remainder * (int)pow(2, i++);
        bin = bin / 10;        
    }

    printf("Decimal: %d", decimal);    

    return 0;
}

Expected Output: 1st run:

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Enter a binary number: 100
Decimal: 4

2nd run:

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Enter a binary number: 1000011
Decimal: 67

How it works #

Here are the steps to convert a binary number to a decimal number:

Example 1: Convert binary number 100 to its decimal equivalent.

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=> ( 1 * 2^2 ) + ( 0 * 2^1 ) + ( 0 * 2^0 )
=> 4 + 0 + 0
=> 4

Example 2: Convert binary number 1001 to its decimal equivalent.

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=> ( 1 * 2^3 ) + ( 0 * 2^2 ) + ( 0 * 2^1 ) + ( 1 * 2^0 )
=> 8 + 0 + 0 + 1
=> 9

The following table shows what happens at each iteration of the loop (assuming bin = 10101):

Iteration remainder decimal bin
After 1st iteration remainder = 10101 % 10 = 1 decimal = 0 + 1 * (2^0) = 1 bin = 10101 / 10 = 1010
After 2nd iteration remainder = 1010 % 10 = 0 decimal = 1 + 0 * (2^1) = 1 bin = 1010 / 10 = 101
After 3rd iteration remainder = 101 % 10 = 1 decimal = 1 + 1 * (2^2) = 5 bin = 101 / 10 = 10
After 4th iteration remainder = 10 % 10 = 0 decimal = 5 + 0 * 2^3 = 5 bin = 10 / 10 = 1
After 5th iteration remainder = 1 % 10 = 1 decimal = 5 + 1 * (2^4) = 21 bin = 1 / 10 = 0