Need help making my code efficient - c

Question

So I just started learning about structures and my recent question asked me to make a check for the screen time that user enters and the only valid ranges are 2pm, 5pm 8pm and 11pm. I made a long ass code for it and works as intended but I don't think it is efficient? I tried making an array for the valid screen time and entered all 4 values in that array of size 4 and made a loop to check for each value inside the array when user enters their choice but when user enters wrong choice, it displays 4 error messages since it checks all 4 values inside the array so it was wrong. Anyways here is my code:
void main(){
typedef struct{
char movieName[20];
int screenTime;
float price;
int seatNum;
} MovieTicket;
MovieTicket ticket;
//int validTime[4] = {2, 5, 8, 11};
do{
printf("Enter your screen time: ");
scanf("%d", &ticket.screenTime);
if(ticket.screenTime != 2 && ticket.screenTime != 5 && ticket.screenTime != 8 && ticket.screenTime != 11){
printf("Invalid Screen Time! Enter again please!\n");
}
} while(ticket.screenTime != 2 && ticket.screenTime != 5 && ticket.screenTime != 8 && ticket.screenTime != 11);
system("pause");
}
Thank you for your time, I am just a beginner so bear with me :(

Answer 1

Using magic numbers like this is fairly unmaintainable (adding new allowed times isn't clean, you have to edit conditions and such), your version with the array was better. The rest of this answer focuses on getting that solution to work how you wanted it to:
It's perfectly efficient (could make it more efficient by using a std::set or a std::unordered_set, but for such a small number of allowed values it's negligible), that's not an issue. Your current version is just as fast as the version with an array, as you still compare the same amount of numbers.
If your issue is that it prints the error 4 times, then keep a boolean flag to indicate the user's entered a valid value, and check it after the loop:
const unsigned int VALID_TIMES_SIZE = 4;
bool validTime = false;
for (unsigned int i = 0; i < VALID_TIMES_SIZE; ++i)
{
if (ticket.screenTime == validTimes[i])
{
validTime = true;
break;
}
}
if (!validTime)
{
printf("Invalid Screen Time! Enter again please!\n");
// Redo the loop using continue, for example
}

Answer 2

Check the return of scanf to make sure it was successful. If not, the input stream will need to be cleaned of pending characters.
#include <stdio.h>
typedef struct{
char movieName[20];
int screenTime;
float price;
int seatNum;
} MovieTicket;
int main( void) {
MovieTicket ticket;
int result = 0;
int validTime[] = {2, 5, 8, 11};
int limit = sizeof ( validTime) / sizeof ( validTime[0]);
int match = 0;
do{
match = 0;
printf("Enter your screen time: ");
if ( 1 == ( result = scanf("%d", &ticket.screenTime))) {
//check for matching validTime
for ( int each = 0; each < limit; each++) {
if ( validTime[each] == ticket.screenTime) {
match = 1;
break;
}
}
if ( !match) {
printf("Invalid Screen Time! Enter again please!\n");
result = 0;
}
}
else {
//scanf could not parse an int
printf("Enter Screen Time! Enter again please!\n");
while ( '\n' != getchar ( )) { }//clear input stream
}
} while ( result != 1);
system("pause");
}

Answer 3

To make something easy to understand you can avoid using those magic numbers or wrap them in a meaningful structure like checkTime and then use it for this kind of check. That way suppose next time you are asked to build a check time with some different times you won't have to look for hardcoded times and replace with it new things.
For example you can do something like this ,
struct checkTime{
int *checkedTimes;
int length;
int (*checkIfTimeExists)(const struct checkTime* ch, int tm);
};
int checkTimerExistAux(const struct checkTime* ch,int tm)
{
for(int i=0;i<ch->length;i++)
if(ch->checktedTimes[i]==tm) return 1;
return 0;
}
Then you create an `initilaize` function which will allocate memory and do this hardcoded thing. (like putting times and all that)[2,5,8,11] in your case.
initialize(); // here you created the structure `checkTimeInst`
// struct checkTime* checkTimeInstPtr = malloc(sizeof(struct checkTime));
// checkTimeInstPtr->checkedTimes = malloc(sizeof(int)*N);
// checkTimeInstPtr->length = 0;
// checkTimeInstPtr->checkIfTimeExists = checkTimerExistAux;
do{
printf("Enter your screen time: ");
scanf("%d", &ticket.screenTime);
if(checkTimeInstPtr->checkIfTimeExists(checkTimeInstPtr,ticket.screenTime)==0){
printf("Invalid Screen Time! Enter again please!\n");
}
} while(checkTimeInstPtr->checkIfTimeExists(checkTimeInstPtr,ticket.screenTime)==0);
This is not about speed up but it is about maintainability.
Here there is nothing that you can do about speed up. But yes if you are trying to squeeze time even more try to use the reader or scanner yourself using getchar() or similar.
Initially the code posted assumed C++ where as OP asked for C language specific answer or modification. I have modified it to follow that criteria after Paul Griffiths comment.