Why wont isalpha() work in my program?

[337higgin]

I wrote this program for my class that will display how many toilets are in a community and how much water is saved by switching to a low-flush toilet, but when i check to make sure the user doesn't enter an alphabet character for the population it isn't working.

If you could tell me how i am using this function incorrectly it would be greatly appreciated.

#include <stdio.h>
#include <ctype.h>

void main(void)
{

	int low_flush = 2; //liters per flush
	int old_flush = 15; //liters per flush
	int flush_dif = 0;
	int total_people = 0;
	float toilets = 0;
	int toilet_flush = 14; //times a day
	int cost_to_install = 150; //dollars
	int cost_of_toilets = 0;
	int water_saved = 0; //Amount of water saved

	printf("\nThis program will tell you how much your community will");
	printf("\nsave by switching to the low-flush model toilets.");

	printf("\n\nHow many people are in your community?: ");
	scanf("%d", &total_people);

	if(isalpha(total_people))
	{
		printf("\nYou entered a non-numeric value. Please try again\n");
	}
	else
	{
		if(total_people > 0)
		{
			toilets = total_people / 3;

			if((total_people % 3) != 0)
				toilets += 1;

			cost_of_toilets =  cost_to_install * toilets;
			flush_dif = old_flush - low_flush;
			water_saved = flush_dif * toilets;

			if(total_people <=3)
			{
				printf("\nThere is %.0f toilet in your community.", toilets);
			}
			else
				printf("\nThere are %.0f toilets in your community.", toilets);
				printf("\nWith the low flush models your community will save %d liters", water_saved);
				printf("\nof water per flush.\n\n");
		}
		else
		{
			printf("\nIt is impossible to have this amount of people in a community.");
			printf("\nPlease try again.");
			printf("\n\n");
		}
	}

	return;

}

Thanks

[cas]

When you do scanf("%d", &total_people), you're explicitly asking for a number. If the user enters something that isn't a number (and an integral one at that), then total_people will not be touched by scanf(). What that means is that there is absolutely no use for isalpha() here. In fact, isalpha() should only be used when you have a character that you need to check (such as returned by getc() or through scanf("%c")). It's not a general purpose "did this input contain alphabetic characters?"; rather, it's more like, "is this particular character an alphabetic character?"

If scanf() fails to read a number (e.g. you typed in a letter), it will return 0, which in this case means it performed no conversions, i.e. it didn't process anything. You're in a bad spot in this case, because the next time you try to read, that letter will still be there. If you do scanf("%d") again, it will fail, and leave the letter there for the next input function. And so on. I suggest reading about scanf()—especially its return value—in your documentation.

Reading text input in C is tricky business. Generally speaking the best combination of robustness and ease-of-use is to read an entire line with fgets() and then pick it apart (with sscanf(), strtol(), etc). This isn't perfect, but it's a decent way to start.

[AndrewHunter]

A quick and dirty example with scanf() and checking input:

#include <stdio.h>

int main(void){

	int number;

	printf("Enter a number: ");
	
	while(scanf("%d", &number) != 1){ //scanf returns number of successful inputs
		printf("Incorrect input. Enter a number: ");
		getchar(); //get rid of trailing newline
	}

	printf("You entered %d", number);

	return(0);
}