Read a string from user using gets(). Now travel the whole string and check it is an alphabet or Not. If it is an alphabet then check is it a upper case character [ASCII between 65 - 90], if it is a upper case character then add 32 with it and convert into lower case character. Now print the string using puts().
int main()
{
char str[30],i;
//READ A STRING
printf("Enter A String: ");
gets(str);
for(i=0;str[i]!='\0';i++)
{
if((str[i]>=65 && str[i]<=90) ||(str[i]>=97 && str[i]<=122))
{
if(str[i]>=65 && str[i]<=90)
{
str[i]=str[i]+32;
}
}
}
//PRINT THE OUTPUT
printf("Result : ");
puts(str);
return 0;
}