f1（）函数将如何执行？

Question

#include <stdio.h>
void f1(char* str, int index)
{
   *(str + index) &= ~32;
}
int main()
{
   char arr[] = "gatecsit";
   f1(arr, 0);
   printf("%s", arr);
   return 0;
}

How is function f1() working?
Being specific *(str + index) &= ~32; this one.... thanks

Answer 1

I think f1() capitalizes the first letter of the string by exploiting a property of ASCII that means that corresponding lower and upper-case letters differ by 32. For example the code for 'A' is 65, while that for 'a' is 97. The '&= ~32' bit of the code will turn of bit-5 of the ASCII representation of the character str[index], which should turn the 'g' into 'G'. This should be fine for strings that contain only ordinary letters, but will have strange effects on digits and punctuation characters.

Answer 2

The expression

*(str + index)

is equivalent to

str[index]

So the character at position index is changed the following way

*(str + index) &= ~32;

In the ASCII table lower case letters differ from upper case letters by having one more set bit. For example the lower case letter 'a' has the code in hex 61 while the upper case letter 'A" has the code in hex 41 . So the difference is equal to the value in hex 20 that in decimal is equal to 32 .

So the original expression resets the corresponding bit in the character to 0 converting a lower case letter to the upper case letter.

Answer 3

The code removes 1 bit from the character effectively subtracting 32 from the byte or 0x20.

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

void f1(char* str, int index)
{
 // The code removes 1 bit from the character at the position `str[index]`
 // effectively subtracting 32 from that character
 // Capital letters in ASCII are apart by 32 (0x20) from small letters 

 // Since 'a' = 0x61 and 'A' = 0x41  'a' - 32 = 'A'
 // Since 'b' = 0x62 and 'B' = 0x42  'b' - 32 = 'B'

 // `~` is a binary negation operator 0 -> 1;  1 -> 0
 // `&` is a binary AND
 // x &= y; is equivalent to x = x & y;

 // ~0x20 = 0b11011111    

   *(str + index) &= ~0x20; // 0x20 = 32;
}


int main()
{
   int i;
   char arr[] = "gatecsit";
   size_t len = strlen(arr);


  for(i = 0; i< len; i++)
      printf(" %c " , arr[i]);   

   printf("\n");

  for(i = 0; i< len; i++)
      printf(" %X" , arr[i]);

   printf("\n");

  // convert all letters:  
   for(i = 0; i< len; i++)
      f1(arr, i);

   printf("\n");

  for(i = 0; i< len; i++)
      printf(" %c " , arr[i]);  

   printf("\n");

  for(i = 0; i< len; i++)
      printf(" %X" , arr[i]);

   return 0;
 }

Output: The small and capital are letters apart by 0x20 (or 32 decimal). This can be clearly seen from this printout:

 g  a  t  e  c  s  i  t 
 67 61 74 65 63 73 69 74

 G  A  T  E  C  S  I  T 
 47 41 54 45 43 53 49 54