Changing all character of a STRING to lowercase using built-in function

#include<stdio.h>
#include<ctype.h>
#include<string.h>
void changeCase(char s[]);

int main()
{
    char a[100];
    printf("Enter a string:");
    gets(a);
    changeCase(a);
    puts(a);


}

void changeCase(char str[])
{

    int i;
    for(i=0; str[i]!='\0'; i++)
    {
        if (isupper(str[i]))
        {
            str[i]=tolower(str[i]);
        }
        else if (islower(str[i]))
        {
            str[i]=tolower(str[i]);
        }
    }

}