Using Ascii characters to count characters-CodePudding

Why does the code I commented out does not work to count the digits 0-9? for example when the input is happy 34567 fans#. The number counter should be 5 and letter counter should be 9.

#include <stdio.h>
int main() 
{
    /*edit*/
   /* Write your code here */
   char ch;
   int letter_counter=0, num_counter=0;
   printf("Enter your characters (# to end):\n");
   while((ch = getchar()) != '#'){
       //if( ('A'<=ch<='Z') || ('a'<= ch <= 'z')){
       if ((ch >= 'A' && ch <= 'Z')  ||  (ch >= 'a' && ch <= 'z')){
           letter_counter  ;
       }
       //else if('0' <=ch <= '9'){
       else if(ch >= '0' && ch <= '9') {
           num_counter  ;
       }
   }
   printf("The number of digits: %d\n", num_counter);
   printf("The number of letters: %d", letter_counter);


    /*end_edit*/
   return 0;
}

CodePudding user response：

The expressions

'0' <=ch <= '9'

and

'A' <= ch <= 'Z'

and

'a' <= ch <= 'z'

will all evaluate to true, irrespective of the value of ch.

According to the rules of operator precedence, the expression

'A' <= ch <= 'Z'

is equivalent to:

('A'<=ch) <= 'Z'

The sub-expression 'A'<=ch will evaluate to either true (1) or false (0), so the entire expression will be equivalent to either

0 <= 'Z'

or:

1 <= 'Z'

Assuming that you are using an ASCII-compatible character set, 'Z' will have the character code 90, so the entire expression will be equivalent to either

0 <= 90

or:

1 <= 90

In both cases, the expression will evaluate to true.

That is why you must write

ch >= 'A' && ch <= 'Z'

'A' <= ch && ch <= 'Z'

instead of:

'A' <= ch <= 'Z'

Also, it is worth noting that you can simplify your code a bit, by using the functions isalpha and isdigit. Both of these functions require you to #include <ctype.h>.

#include <stdio.h>
#include <ctype.h>

int main() 
{
   int ch;
   int letter_counter=0, digit_counter=0;

   printf( "Enter your characters (# to end):\n" );
   while ( (ch=getchar()) != '#') {
       
       if ( isalpha( ch ) ) {
           letter_counter  ;
       }
       else if ( isdigit( ch ) ) {
           digit_counter  ;
       }
   }

   printf( "The number of digits: %d\n", digit_counter);
   printf( "The number of letters: %d\n", letter_counter);

   return 0;
}

CodePudding user response：

you could do it like that :

#include <stdio.h>
int main() 
{
   char ch;
   int letter_counter=0, num_counter=0;

   printf("Enter your characters (# to end):\n");
   while((ch = getchar()) != '#'){
       if (ch >= 'a' && ch <= 'z')
           letter_counter  = 1;
       if (ch >= 'A' && ch <= 'Z')
           letter_counter  = 1;
       if (ch >= '0' && ch <= '9')
           num_counter  = 1;
   }
   printf("The number of digits: %d\n", num_counter);
   printf("The number of letters: %d", letter_counter);
   return 0;
}