Loop Exercises
Write a C program to input binary number from user and find twos complement of the binary number. How to find 2s complement of a binary number in C. Logic to find twos complement of a binary number in C programming.
Required knowledge
Basic C programming, If else, For loop, String
What is twos complement?
Twos complement of an N-bit number is defined as the complement with respect to 2N. It is the result of subtracting the number from 2N, which in binary is one followed by N zeroes.
In simple words twos complement is defined as sum of ones complement of a binary number and 1.
Logic to find twos complement
Step by step descriptive logic to find twos complement of a binary string.
- Input a binary string from user. Store it in a variable say binary.
- Find ones complement of the binary string. Store the result in some variable say onesComp.
- Declare and initialize another binary string variable to store twos complement, say twosComp = "".
- Initialize a variable to store carry bit during addition, say carry = 1.
You may think now, why initialize carry to 1, why not 0? Since, we need to add only 1 to the binary string hence, I have initially assumed carry bit as the bit to be added. - Run a loop from length of binary string to 1, decrement 1 in each iteration. The loop structure should look like for(i=SIZE-1; i>=1; i--) (where SIZE is length of the binary string).
You may think, why iterating in reverse order? Because addition is carried out in right to left order. - Inside the loop there can be three cases.
- If both current binary bit and carry bit are 1 then, put 0 to twos complement. Which is if(onesComp[i]=='1' && carry==1) then, twosComp[i] = '0', carry is still preserved to 1.
- If current binary bit is 0 and carry bit is 1 then, put 1 to twos complement and set carry bit to 0. Which is if(onesComp[i]=='0' && carry==1) then, twosComp[i] = '1' and carry = 0.
- If carry bit is 0, then assign the value of onesComp to twosComp.
Program to find twos complement of a number
/** * C program to find twos complement of a binary number */ #include <stdio.h> #define SIZE 8 int main() { char binary[SIZE + 1], onesComp[SIZE + 1], twosComp[SIZE + 1]; int i, carry=1; printf("Enter %d bit binary value: ", SIZE); /* Input 8-bit binary string */ gets(binary); /* Find ones complement of the binary number */ for(i=0; i<SIZE; i++) { if(binary[i] == '1') { onesComp[i] = '0'; } else if(binary[i] == '0') { onesComp[i] = '1'; } } onesComp[SIZE] = '\0'; /* * Add 1 to the ones complement */ for(i=SIZE-1; i>=0; i--) { if(onesComp[i] == '1' && carry == 1) { twosComp[i] = '0'; } else if(onesComp[i] == '0' && carry == 1) { twosComp[i] = '1'; carry = 0; } else { twosComp[i] = onesComp[i]; } } twosComp[SIZE] = '\0'; printf("Original binary = %s\n", binary); printf("Ones complement = %s\n", onesComp); printf("Twos complement = %s\n", twosComp); return 0; }
Output
Enter 8 bit binary value: 01101100
Original binary = 01101100
Ones complement = 10010011
Twos complement = 10010100