Vowel Count in a String

CODING:
#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;
}