Program to calculate 2’s complement of a binary number.

2’s complement of a number is obtained by scanning it from right to left and complementing all   the bits after the first appearance of a 1. Thus 2’s complement of 11100 is 00100.

#include <stdio.h>
#include<conio.h>

void complement (char *a);
void main()
{
char a[16];
int i;
clrscr();
printf("Enter the binary number");
gets(a);
for(i=0;a[i]!=''; i++)
{
if (a[i]!='0' && a[i]!='1')
{
printf("The number entered is not a binary number. Enter the correct number");
exit(0);
}
}
complement(a);
getch();
}
void complement (char *a)
{
int l, i, c=0;
char b[16];
l=strlen(a);
for (i=l-1; i>=0; i--)
{
if (a[i]=='0')
b[i]='1';
else
b[i]='0';
}
for(i=l-1; i>=0; i--)
{
if(i==l-1)
{
if (b[i]=='0')
b[i]='1';
else
{
b[i]='0';
c=1;
}
}
else
{
if(c==1 && b[i]=='0')
{
b[i]='1';
c=0;
}
else if (c==1 && b[i]=='1')
{
b[i]='0';
c=1;
}
}
}
b[l]='';
printf("The 2's complement is %s", b);
}
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