How to check if a string is a letter(a-z or A-Z) in c

Question

I am getting user input, and I want to determine if the user has entered a letter , an integer, or an operator. I can successfully determine if it is an integer using sscanf, but I am stumped on how to determine if it is a letter.

By letter, I mean: A-Z, a-z.

int main(){
    char buffer[20];
    int integer; 
    printf("Enter expression: ");
    while (fgets(buffer, sizeof(buffer), stdin) != NULL){
        char *p = strchr(buffer, '\n'); //take care of the new line from fgets
        if (p) *p = 0;

        //Buffer will either be a integer, an operator, or a variable (letter).
        //I would like a way to check if it is a letter
        //I am aware of isalpha() but that requires a char and buffer is a string

         //Here is how I am checking if it is an integer
         if (sscanf(buffer, "%d", &integer) != 0){
             printf("Got an integer\n");
         }
         else if (check if letter)
             // need help figuring this out
         } 
         else{
             // must be an operator
         }
    }
}

Solution 1

You can use the isalpha() and isdigit() standard functions. Just include <ctype.h>.

     if (isdigit(integer)) != 0){
         printf("Got an integer\n");
     }
     else if (isalpha(integer))
         printf"Got a char\n"); 
     } 
     else{
         // must be an operator 
     }

Solution 2

To find out if the input is a letter or a digit:

• int isalpha ( int c ); function to verify whether c is an alphabetic letter.
• int isalnum ( int c ); function to verify whether c is either a decimal digit or an uppercase or lowercase letter.
• int isdigit ( int c ); function to verify whether c is a decimal digit character.

To find out if the letter is uppercase or lowercase:

• int islower ( int c ); to checks whether c is a lowercase letter: a-z
• int isupper ( int c ); to checks whether c is a uppercase letter: A-Z

Put them into if statements which do something (true or false), depending on the result.

PS You can find out more about standard library here: Character handling functions: ctype.h

Comments

[chux - Reinstate Monica]

BTW: If you entered only spaces, (sscanf(" ", "%d", &integer) would yield -1 and pass your "Got an integer\n" test. Recommend in the future use affirmative sscanf() comparison tests such as sscanf(buffer, "%d", &integer) == 1 for sscanf(..., "%d",...) may be expected to return 0, 1 or EOF.

[Talen Kylon]

Thanks for the suggestion, but how do change buffer to an integer?

[Giuseppe Pes]

just use the first character of the buffer and cast it to int.