The five letters A, E, I, O and U are called vowels. All other alphabets except these 5 vowels are called consonants.

This program assumes that the user will always enter an alphabet character.

Program to Check Vowel or consonant

#include <stdio.h>
int main(){
    char c;
    int lowercase_vowel, uppercase_vowel;
    printf("Enter an alphabet: ");
    scanf("%c", &c);

    // evaluates to 1 if variable c is a lowercase vowel
    lowercase_vowel = (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u');

    // evaluates to 1 if variable c is a uppercase vowel
    uppercase_vowel = (c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U');

    // evaluates to 1 (true) if c is a vowel
    if (lowercase_vowel || uppercase_vowel)
        printf("%c is a vowel.", c);
    else
        printf("%c is a consonant.", c);
    return 0;
}

Output

Enter an alphabet: G
G is a consonant.

The character entered by the user is stored in variable c.

The lowercase_vowel variable evaluates to 1 (true) if c is a lowercase vowel and 0 (false) for any other characters.

Similarly, the uppercase_vowel variable evaluates to 1 (true) if c is an uppercase vowel and 0 (false) for any other character.

If either lowercase_vowel or uppercase_vowel variable is 1 (true), the entered character is a vowel. However, if both lowercase_vowel and uppercase_vowel variables are 0, the entered character is a consonant.

Note: This program assumes that the user will enter an alphabet. If the user enters a non-alphabetic character, it displays the character is a constant.

To fix this, we can use the isalpha() function. The islapha() function checks whether a character is an alphabet or not.

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

int main(){
   char c;
   int lowercase_vowel, uppercase_vowel;
   printf("Enter an alphabet: ");
   scanf("%c", &c);

   // evaluates to 1 if variable c is a lowercase vowel
   lowercase_vowel = (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u');

   // evaluates to 1 if variable c is a uppercase vowel
   uppercase_vowel = (c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U');

   // Show error message if c is not an alphabet
   if (!isalpha(c))
      printf("Error! Non-alphabetic character.");
   else if (lowercase_vowel || uppercase_vowel)
      printf("%c is a vowel.", c);
   else
      printf("%c is a consonant.", c);

   return 0;
}

Now, if the user enters a non-alphabetic character, you will see:

Enter an alphabet: 3
Error! Non-alphabetic character.