Binary addition in c with character arrays

Question

I am trying to add two binary numbers in a c function, and I can't get the result I'm looking for.

#include <string.h>
#include <stdio.h>

char * sadd(char bn1[], char bn2[], char s[]){ //Function to add the binary numbers

        int i;
        int co = 0;
        int num = 0;

        for(i = 15; i>0; i++){
                num = (int)bn1[i] + (int)bn2[i] + co;
                co = 0;
                if(num == 2){
                        co = 1;
                        s[i] = '0';
                }
                else{
                        s[i] = (char)num;
                }
        }
        return s;
}

The base specifies how we *write* a number, not usually the operations that can be performed on that number. — Neil, Mar 13 '22 at 19:04

Vlad from Moscow · Accepted Answer · 2022-03-13T18:11:11.007

2

This for loop

for(i = 15; i>0; i++){

is incorrect. Instead of decreasing the variable i it is increased. It seems you mean

for(i = 15; i>=0; i--){

This statement

num = (int)bn1[i] + (int)bn2[i] + co;

should be rewritten like

num = bn1[i] - '0' + bn2[i] - '0' + co;

And this statement

s[i] = (char)num;

should be rewritten like

s[i] = num + '0';

And you are not processing the case when num is equal tp 3 when co is equal to 1 and bn[i] and bn2[i] are both equal to '1'.

Instead of this if statement

co = 0;
if(num == 2){
        co = 1;
        s[i] = '0';
}
else{
        s[i] = (char)num;
}

you could just write

s[i] = num % 2 + '0';
cp = num / 2;

edited Mar 13 '22 at 18:11

answered Mar 13 '22 at 18:06

Vlad from Moscow

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also OP doesnt deal with the case of '1' + '1' + (carry = '1') – pm100 Mar 13 '22 at 18:08
So, I think your answer should be the accepted one, sorry for stealing your thunder here. I +1'ed you though. – einpoklum Mar 13 '22 at 18:29

score 0 · Answer 2 · answered Mar 13 '22 at 18:23

@VladFromMoscow has provided a patient and valid answer. But - you could have figured this out on your own, if you were to step through a run of your program, using a debugger.

Here are two Stackoverflow questions about debugging on Linux and on Windows:

If you were to debug the program, stepping through individual commands, you would notice that:

The loop runs for more than 15 iterations, with j increasing.
The value of num is rarely between 0 and 3.

and then you could have thought about why that happens, and reached the same conclusion as in Vlad's answer.

Also, and for future reference - most StackOverflow users expect question askers to perform their "due diligence", making reasonable efforts to figure out their problems before asking us for a solution. Now that you know about debuggers - please use one before asking "Why does my program not do what I expected".

Binary addition in c with character arrays

2 Answers2