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; }