Thread: Some problems with bitwise operators and char

Hi guys this is my first post in here and I hope to be an active user

I got some problems with this code.
It is supposed to rotate of n_bit the input char and print the bit rappresentation

Now here is the not working code:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>


char rotate_c(char c, short n_bit);
void arr_bit (short arr[], char c);


int main() {
    char c;
    char c_rot;
    short n, arr[8], arr_rot[8];


   printf("Insert char\n-> ");
   c = getchar();
   printf("%c = ",c);
   arr_bit(arr,c);
   for (n = 0; n < 8; n++)
      printf("%d",arr[n]);

   printf("\n [(-7,-1)(1-7)]\n-> ");
   scanf("%d", &n);
   c_rot = rotate_c(c,n);

   printf("Your char: %c\n", c);
   printf("modified: %c\n", c_rot);
   arr_bit(arr_rot,c_rot);

   for (n = 0; n < 8; n++)
      printf("%d",arr_rot[n]);
}


char rotate_c(char ch, short n_bit) {
    char car = ch;
    char temp;


    if (n_bit < 0) {
        n_bit = abs(n_bit);
        temp = car;
        car = car<<n_bit;
        temp = temp>>(8-n_bit);
        car = car|temp;
    }


    if (n_bit > 0) {
        temp = car;
        car = car>>n_bit;
        temp = temp<<(8-n_bit);
        car = car|temp;
    }


    return car;
}


void arr_bit (short arr[], char c) {
    short i = 7;


    while (i >= 0) {
        if (c != 0) {
            arr[i--] = c&1;
            c = c>>1;
        }
        else {
            arr[i--] = 0;
        }
    }


}

In this case, the input variable is modified by the rotate_c function and the bit rappresentation of the character is 1111 1111
always.
Now if I change the variable n from short to int it is fully working!
And I just can't figure out why!!
Anyone can help me understanding why?
Sorry for the long post and see ya soon

A quick lesson on how to use a debugger.

Code:

$ gcc -g bar.c
$ gdb -q ./a.out 
Reading symbols from /home/sc/Documents/a.out...done.
(gdb) break rotate_c 
Breakpoint 1 at 0x400760: file bar.c, line 38.
(gdb) run
Starting program: /home/sc/Documents/a.out 
Insert char
-> @
@ = 01000000
 [(-7,-1)(1-7)]
-> -1

Breakpoint 1, rotate_c (ch=64 '@', n_bit=-1) at bar.c:38
38        char car = ch;
(gdb) n
42        if (n_bit < 0) {
(gdb) n
43          n_bit = abs(n_bit);
(gdb) n
44          temp = car;
(gdb) print n_bit
$1 = 1
(gdb) n
45          car = car<<n_bit;
(gdb) n
46          temp = temp>>(8-n_bit);
(gdb) print car
$2 = -128 '\200'
(gdb) print/x car
$3 = 0x80
(gdb) n
47          car = car|temp;
(gdb) print temp
$4 = 0 '\000'
(gdb) n
51        if (n_bit > 0) {
(gdb) print car
$5 = -128 '\200'
(gdb) print n_bit
$6 = 1

So, n_bit is suddenly 1 and you're about to enter the other if statement in rotate_c.

Hi and thanks for the answer!
You're right! But after the run and the call to the function rotate_c, even if I correct the if-else statement, the input character is modified using the variable n as short and I can't figure out why! If I compile and run the same code but with the variable n as int it seems to be fully working!
Thank you and sorry for the stupid questions if they are!

EDIT
Found the solution. It is in the scanf process waiting for an int ("%d"). I changed that to ("%hd") and it's working now even with n declared as a short. Thank you for the interest!