int firstClass[5] = {0};
int economy[5] = {0};
// or
int seats[10] = {0};

#include <stdio.h>

int main()
{
	char answer = 0;
	int i, start, end, seat = 0, tclass = -1, found = 0;
	int seats[10] = {0};

	for(;;)
	{
		do
		{
			printf("Please type 1 for 'first class'\n");
			printf("Please type 2 for 'economy class'\n");
			printf("Please type anything else to quit: ");
			rewind(stdin);
		}
		while(scanf("%c", &answer) != 1);

		if(answer == '1' || answer == '2')
		{
			if(answer == '1')
			{
				tclass = 0;
				start = 0;
				end = 5;
			}
			else
			{
				tclass = 1;
				start = 5;
				end = 10;
			}

			for(i = start; i < end; i++)
			{
				if(seats[i] == 0)
				{
					seat = i;
					seats[i] = 1;
					break;
				}

				/* check if seats in current class are taken */
				if(i == end - 1)
				{
					for(i = 0; i < 10; i++)
					{
						if(seats[i] == 0)
						{
							/* still a seat free */
							found = 1;
							break;
						}
					}

					if(!found)
					{
							printf("Sorry flight is full, next flight leaves in 3 hours.\n\n");
							return 0;
					}
					found = 0;

					if(tclass == 0)
					{
						printf("\nThere is no seat left in first class.\n");
						printf("Would you like to get a seat in the economy class?\n");
					}
					else
					{
						printf("\nThere is no seat left in the economy class.\n");
						printf("Would you like to get a seat in first class?\n");
					}
					printf("Please type 'y' for yes, anything else for no: ");
					rewind(stdin);
					scanf("%c", &answer);

					if(answer != 'y')
					{
						printf("Next flight leaves in 3 hours.\n\n");
						return 0;
					}

					/* switch to first class */
					if(tclass == 1)
					{
						tclass = 0;
						start = 0;
						end = 5;
						i = start - 1; /* continue doesn't change for loop */
					}

					/* switch to economy class */
					else
					{
						tclass = 1;
						start = 5;
						end = 10;
						i = start - 1; /* continue doesn't change for loop */
					}
				}
				continue;
			}
			printf("Your seat number is %d in %s class\n\n", seat+1, tclass ? "economy" : "first");
		}

		else
		{
			printf("Goodbye!\n");
		}
	}
	return 0;
}