CODEWITHSUNDEEP
c++
Ayush Bali
Ayush Bali

Reputation: 21

I wrote a program to convert a negative number to binary, is this code correct?

I have used the 2's compliment approach to convert a negative number to binary I am getting the answer right

after I converted the number to binary let's say n = -6

  1. I ignored the negative sign (by making the number positive)
  2. I took it's 2's compliment

Now if the MSB (Most Significant Bit) is 1 that means the number is negative this ans is stored in newAns

But my doubt is, for me to print the negative binary number since MSB of newAns was 1 do I have to find 2's compliment of newAns again or not?

#include<iostream>
#include <math.h>
using namespace std;

int decimalToBinary(int n){
    int ans = 0;
    int i = 0;
    while(n!=0){
        int bit = n&1; 
        ans = (bit * pow(10,i)) + ans;
        n = n >> 1;
        i++;
    }
    return ans;
}
int main(){
    int n;
    cin >> n;

    if(n<0){
        // if number is negative
        n = n*(-1);
        int ans = decimalToBinary(n);
    // Find 2's compliment of the number
    // 1's comp
    int newAns = (~ans);
    // 2's comp
    newAns = newAns+1;
    cout << newAns << endl;

    } else {
        // if number is positive 
        int ans = decimalToBinary(n);
        cout << ans << endl;
    }
}

Upvotes: 1

Views: 11540

Answers (3)

vishal singh
vishal singh

Reputation: 9

Here is the Correct Solution

#include <iostream>
using namespace std;

void print_binary(int num)
{
    // `result` stores the binary notation of `num` in decimal format
    int result = 0;

    // It ignores leading zeros and leading ones
    int place_value = 1;
    while (!(num == 0 | num == -1))
    {
        // Extracting the rightmost bit from `num`
        int bit = num & 1;

        // appending `bit` to `result`
        result += bit * place_value;

        // Shifting `num` to the right
        // so that second bit (from right) now become the rightmost bit
        num = num >> 1;
        place_value *= 10;
    }

    cout << result << endl;
}

int main()
{
    int number = -6;
    int neg_number = ~number + 1; // Took 2's compliment of number

    print_binary(number);
    print_binary(neg_number);
}

Upvotes: 0

Eric Postpischil
Eric Postpischil

Reputation: 222354

“Binary” and “decimal” are representations of numbers as sequences of digits. For example, “123” interpreted as decimal means 1 hundred, 2 tens, and 3 ones, and “101” interpreted as binary means 1 four, 0 twos, and 1 one.

Your decimalToBinary routines does not convert to binary, because it does not produce a normal sequence of binary digits. For the number five, it produces the number one-hundred-and-one. When one-hundred-and-one is converted to decimal and printed, the output is “101”. This could be called decimal-coded-binary.

Because the value returned by decimalToBinary, which is stored in ans, is not a binary representation of n, ~ans does not take its one’s complement. When ans has the value one-hundred-and-one, decimal 101, its binary representation is 000…0001100101. The ~ operator takes the one’s complement of those bits, yielding 111…1110011010.

This is not a useful way to compute the two’s complement representation of a negative number. Depending on what you are trying to learn with this assignment, two methods of computing and displaying the two’s complement representation are:

  • Once the number is read with cin >> n, it is already in binary form, as the input routines invoked by cin >> n convert it. Your C++ representation almost certainly uses two’s complement, but you can be sure by using unsigned int u = n;, as the conversion from int to unsigned int will effectively produce a two’s complement representation. Then you can simply write code to print the bits of u: For each bit in it, print “0” or “1” according to what the value of the bit is.
  • Alternatively, if you want to work with the mathematics of binary representations, write code to convert n to an array of characters, in which each character is “0” or “1”, according to the binary representation of n. If n is negative, you can first convert its negation to binary, and then you can iterate through the array of characters to change each “0” to a “1” and vice-versa, and finally you can write code to add “1” to the binary numeral in the array. That will include writing code to carry from position to position.

Upvotes: 3

tenfour
tenfour

Reputation: 36896

I think the biggest issue is a confusion about representing numbers in bases.

int values are always "binary" in memory because memory is fundamentally binary. There's no such thing as a "decimal int" vs. "binary int". Representation in a specific base is a text/string/printing concept. The int is the value itself, and a "decimal int" is how you format it for someone to read. If you want to represent that number as a "decimal" or "binary", then you should convert it to a string.

A lot of weirdness in this code. Instead of n=n*(-1);decimalToBinary(n) why not just decimalToBinary(-n)? Very confusing. But also, why are we using twos-complement at all? It's just confusing to devs (i.e. bad). Use unary operator -().

Upvotes: 3

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