Thread: loop

Is it possible to create a do-while loop to restrict input to numbers only and not letters?

I looked into this and understand that it is the getchar command. But I have tried several ways of doing it but cannot seem to get it to work right. I have a program that is supposed to accept numbers until a negative number is inputed and then it will ask to average the numbers, or exit. I put a letter in and it crashes. Cannot figure out how to change my "scanf" command to the "getchar" command to make it take only numbers. Let me know if the code needs to be posted or if it is more proper here to email it.

This program is for a class so I do not want the exact answer. I would greatly appreciate a nudge in the right direction though. I have tried to put the "getchar" command in but had no success. this is just a snippet of the original code with the "scanf" command.

Code:

float n, count = 0, average = 0, sum = 0;
	int op;
	printf("This program asks the user to input a series of test scores\n");
		printf("and tells the user whether the grades are PASSING or FAILING.\n");
		printf("Next, the user can select from other menu options that will\n");
		printf("allow additional grade queries or allow the user to exit from\n");
		printf("the program.\n\n");
	printf("Please enter test scores.  After each score, press the return key.\n");
		printf("When you are finished entering scores please enter a negative number.\n\n");
	scanf("%f", &n);
	while (n >= 0)
	{
		++count;
	sum += n;
	scanf("%f", &n);
	}
	average = sum / count;
	printf("\n");
	if (average >= 60.0)
		printf("****PASSING****\n\n\n");
	else
		printf("****FAILING****\n\n\n");

It takes a lot of code to make scanf() bomb-proof against erroneous user input. As you've probably seen, entering characters to that loop causes it to loop indefinitely.

This approach is generally a lot simpler to get right, because it separates input from conversion.

Code:

char buff[100];
while ( fgets( buff, sizeof buff, stdin ) != NULL ) {
  if ( sscanf( buff, "%f", &n ) == 1 ) {
    if ( n >= 0 ) {
      sum += n;
      count++;
    } else {
      // end of input signalled
      break;
    }
  } else {
    // not a float, complain
  }
}

That is quite advanced to what I should be using right now. As far as I was told by one instructer, I should be using the 'getchar' command. Isn't there are more simple way of doing this?

that code only displays the number you type in. if you type in a letter, it will only show 0.00 and end the program. If you look at the original code I posted, you will see that it is an infinite loop if a letter is typed in. I am trying to use the 'getchar' command to prevent this from happening. So I am pretty much looking at an additional loop inside a loop.

You said you didn't want the exact answer, so I didn't write your program for you.

You can simply check the input inside the getchar() loop with isdigit() or whatever you like.

FINALLY figured it out. but it sorta messed up some other function within the program. I was able to put things into a do-while loop with 'getchar'. still working on it. Didn't have to use 'isdigit()'. I appreciate everyones help. Thank you and should have it done soon. Will post it after tomorrow since it is due tomorrow. I just need to figure out the new bugs I created. It is supposed to average and count the number of grades entered but after solving the 'getchar' issue, it won't count or average right anymore.

Is there something I am missing? I can't seem to find the issue.

Code:

float count, num, sum, average;
	int n, op;
	printf("please enter your numbers: \n");
	
	do {
		
		
		//********************************
		
		
		while (scanf("%f", &num) != 1)
		{
			while (getchar() != '\n');
			printf ("Try again: \n");
		}
		
		//********************************

		++count;
		sum += num;
		//scanf("%f", &a);
	}
	
	while (num >= 0);

	
	average = sum / count;
	printf("average:  %.2f\n", average);
	printf("sum:  %.2f\n", sum);
	printf("count:  %.2f\n", count);

Thank you for everyones help. I finally figured out all the bugs and got it working just right. And just in time.