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Int to Binary Conversion

cfans asked
Medium Priority
Last Modified: 2013-11-15
I'm looking for a good int to binary conversion using C.
I've searched on here for some of the examples, however didn't find any I really liked..

I'd like to convert an int into a char *.
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Is there also a way to just use printf to display an int as a binary number?
Top Expert 2006

This is a very common homework question. May we know why you need this routine?

Also ...
>I've searched on here for some of the examples, however didn't find any I really liked..
What is that you are exactly looking for? Why was the existing code not good enough?



Well actually it isn't that I didn't like it.. I tried using some of the examples.. and they just gave me seg faults.. can't really figure out why either.

An int actually is a binary number already.

You can print it in hexadecimal with:
printf( "%x", intval );
or formatted in C-style and with all 4 bytes present:
printf( "0x%.8x", intval );

to print it to a string use:
char str[ 20 ];
sprintf( str, "0x%.8x", intval );

to get a char* of the 4 bytes of an int you can simply cast the pointer to it
char * barr =  (char *) &intval;
and access the bytes this way:
char firstByte = barr[ 0 ];
but the ordering of the bytes is processor dependant if you do it this way (little/big endian)

to do this processor independant you can use:
char barr[ 4 ];
barr[ 0 ] = (char) ( intval >> 24 );
barr[ 1 ] = (char) ( intval >> 16 );
barr[ 2 ] = (char) ( intval >>  8 );
barr[ 3 ] = (char) intval;

I'm not sure if this is what you meant.
char *convert(int a)
     enum { NUM_CHARS = 33 };       /* for 32 bit ints */
     char bitarray[NUM_CHARS];
     char *p;
     unsigned value;
     value = (unsigned) a;

     p = &bitarray[NUM_CHARS - 1];
    *p ='\0';
    while(value) {
         *p = (value & 1) ? '1' : '0';
         value >>= 1;
   return strdup(p);

What algorithms did you not like?

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Top Expert 2006
General function to convert  would look like this

a = number
b = base (in this case, 2)

1. Start with an empty string.
2. If a is negative, add '-' to the string and negate a.
3. Add the digit (a modulus b) to the string.
4. a = a / b.
5. If a > 0 then goto 3.
6. Reverse the string.

Specifically for binary, you can simplify it significantly ...

for (all bits - note that this will be sizeof int * bits per byte)
       AND the value with 0x01 and store 1 in string if result was true else false
       right shift value by 1
reverse the string and return

To avoid reversing you can start generating the string from the other end ... AND with 1<<number of bits-1 and left shift by 1 instead of right shift.

A very smaller recursive version

void binary(int number) {
   int remainder;
   if(number <= 1) {
      printf ("%d", number);
   remainder = number%2;
   binary(number >> 1);    
   printf ("%d", remainder);
I apologize.
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