Vowel count

/*A program to  Count vowel*/

#include<stdio.h>

int count_vowels(char []);
int check_vowel(char);

main()
{
char array[100];
int c;

printf("Enter a string\n");
gets(array);

c = count_vowels(array);

printf("Number of vowels: %d\n", c);

return 0;
}

int count_vowels(char a[])
{
int count = 0, c = 0, flag;
char d;

do
{
d = a[c];

flag = check_vowel(d);

if ( flag == 1 )
count++;

c++;
}while( d != '\0' );

return count;
}

int check_vowel(char a)
{
if ( a >= 'A' && a <= 'Z' )
a = a + 'a' - 'A'; /* Converting to lower case */

if ( a == 'a' || a == 'e' || a == 'i' || a == 'o' || a == 'u')
return 1;

return 0;
}