Decimal integer to binary in C

Question

Please give me some feedback on how to make my code better or more efficient. It should convert a decimal integer to binary.

#include <stdio.h>

binarydigits(int div, int dis)
{
    int numit;
    numit=0;

    do
    {
        ++numit;
        div /= dis;
    }
    while (div!=1);
    ++numit;
    return numit;
}

main()
{
    int x, nb, i;

    printf("\n Input an decimal integer number to be converted: ");
    scanf("%d", &x);
    fflush(stdin);

    if (x==0 || x==1)
    {
        printf("\n\n %d in binary : %d", x, x);
    }

    else
    {
        printf("\n\n %d in binary : ", x);
        nb = binarydigits(x, 2);
        // the function 'binarydigits' returns how many binary digits are needed to represent 'x'
        int remind[nb];
        // an array of 'nb' elements is declared. Each element of this array will hold a binary digit
        for(i=(nb-1) ; i>=0 ; --i, x/=2)
        {
            remind[i] = x%2;
        }
        //this 'for' structure saves the remainder of 'x/2' (x%2) in an element of the 'remind[nb]' array

        for (i=nb ; i>0 ; --i)
        {
            printf("%d", remind[nb-i]);
        }
        //this 'for' structure prints the elements of the 'remind[nb]' array in increasing order

    }

    getch();
    return 0;


}

Any tips on how to make this better would be nice.

Answer 1

Firstly, binarydigits should have a return type int. This is because you return an integer variable numit at the end of this function. Change your function header to:

int binarydigits(int div, int dis)

Secondly, the main() function needs to have a return type int by definition in C, and C++ for that matter. Without it, your compiler will produce a warning, something similar to:

main.c:18:1: warning: return type defaults to 'int' [-Wimplicit-int]
 main()
 ^~~~

Here is a snippet from the the C11 standard (ISO/IEC 9899:2011) on the definition of the main() function:

The function called at program startup is named main. The implementation declares no prototype for this function. It shall be defined with a return type of int and with no parameters: - Return Type of main()

int main(void) { /* ... */ }

Thirdly, you should remove fflush(stdin) because using the fflush() for stdint is undefined behavior as it is not a part of standard C. From C11 7.21.5.2, fflush works only with output/update stream, not input stream:

If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined. - fflush(stdin)

Answer 2

How to make my code better or more efficient?

My advice to you is to stop trying to learn C by trial-and-error method. You should obtain a good book and study it first. It is impossible to create a fast and efficient C program without mastering pointers, bitwise operators and memory manipulations.

Simply, to make your code fast, you should completelly delete your code (I am not going to list all of your bad-practice things here) and start understanding my example:

int main(void){
  char *s = (char*)malloc(33);
  char *t = s;
  int a;

  s += 33;
  *s = 0;

  printf("Enter a number: ");
  scanf("%d", &a);
  printf("That number in binary: ");
  while(a){
    *(--s) = a & 1 ? '1' : '0';
    a >>= 1;
  }
  printf("%s\n", s);

  free(t);
  return 0;
}

Explanation: we have pointer (if you don't know pointers, well you should probably first learn them) s which points to the end of a string. While number from input (number a) is nonzero, we put its last binary digit in the string, decrease pointer and divide a integrally by 2 (this is a >>= 1 instruction). When a is 0, just print the string.