Thread: Averaging numbers using a sentinal to stop the loop

Hello, I did do a search and found two posts on the same program, but they didn't quite get me working like I'd like.

The assignment: The user will enter either an integer to be averaged or a sentinel of 999 indicating that no more numbers are to be entered. When the sentinel is entered, the program will exit the repetition loop. It will then compute and display the average of numbers entered.

My program works well ( I think) so far, but I would like for the program to loop back and allow the user to input another integer should they decide to type an invalid character rather than an integer. Currently the program stops if scanf !=1 and averages the numbers correctly.

There are no comments yet, and the prompts are not finished as this is how I test my programs currently.

Code:

 #include <stdio.h>
 
 int main()
 {
     
  int uInput;
  int count = 0;
  int sum = 0;
  int sentinel = 999;
  float average;
     
  printf("purpose, %d \n\n", sentinel);
  printf("enter you first number\n");
     
  while (scanf("%d", &uInput)==1)
   {
       if (uInput == sentinel)
       break;
       printf("enter another integer or %d to average the values\n", sentinel);
       sum += uInput;
       count ++;
       printf("the sum is now %d\n", sum);
         
     }
     
     if ( count != 0)
     {
     average= sum/count;
     printf("%d numbers input, %d sum, %.3f average", count, sum, average);
  }
  else 
  printf ("No valid integers were entered, please restart the program and try again");
 
     
         

     
  //The section below is to stop the program until a key is pressed.
  fflush(stdin); //clears the input area so you can pause
  printf("\n\n\nPress any key to exit the program."); //prints the message
  getchar(); //force the computer to pause until you press a key
  return 0; //returns a value of 0 if the program works correct    
 }

I also have been told that last section is garbage, but our instructor said we have to include those lines.

I can create external functions if this would be easier, but have only written one simple program using pointers and addressing. Any help is greatly appreciated! Thanks in advance!

If the user enters invalid characters, scanf() will stop there. It will return a value (probably zero) but leave the invalid input waiting to be read. So it will loop forever in your code.

There are various ways to address that, such as using fgets() to read a line of input, and something like sscanf() [note additional s in the name] to interpret that line of input.

That is an all-or-nothing proposition. If you use fgets() to read from a stream, don't use scanf() or getchar(). The three styles of input (line-oriented, formatted, character-oriented) don't mix well.

Also, don't use fflush(stdin). It has undefined results.

Originally Posted by jesterisdead

Grumpy,
Again, thee fflush(stdin) is required by our instructor, and the 2 lines after it. I'm a student and only have learned what I've read in the book as it is an online course. I have been told by laserlight that these few lines are crap and could give errors, but he wants them.

One of my lecturers insists on using fflush(stdin) and after reading numerous articles on why it shouldn't be used i showed him the c standard.After reading it to himself a couple of times and then out loud a few time's.He muttered some gibberish and left.After a few days he came back to me saying the compiler we use "accepts" this way of flushing the input buffer. I went on to ask him about portability. He hasn't gotten back to me yet, its been over a month so i feel its safe to say he has been educated. I advice you to follow suit although you might want to present yourself a little better than i did(do not ask in presence of your peers....with a cocky smile on your face).

Originally Posted by jesterisdead

Anyone? should I just stick with what I have since it's technically not within the scope of the problem? I just hate not having error checking built in.

If you read the second line of my previous post, there is an approach to deal with that. The third line of my previous post describes some caveats (things you need to be aware of when addressing problems like you describe).

No, I'm not going to supply code to do that. I've given you enough pointers so, with a little effort, you would be able to write your own code to address the problem.

Our FAQ tells you how to clean up input without any messy reliance on stupid tutors or "works for me" compiler behaviour.
FAQ > Flush the input buffer - Cprogramming.com

Originally Posted by jesterisdead

Grumpy, if you read my post after that, I can't use strings yet as we have not studied them. I have to use the scanf() function only to get user input.

Yeh okay. Technically, it is possible to fix the problem by checking the return value of scanf(), and taking different actions for different return values.

If your code detects the presence of an unwanted character (say, your scanf() call returns zero) scanf("%c", &some_character) can be used to read that unwanted character, after which you can go back to trying to read an int again. Bear in mind the set of possible return values from scanf() are the same (an error can occur on reading that single character too).


Practically, unless it is a specific requirement of your homework exercise, I wouldn't do it. It is unnecessarily complicated and error prone - and only being allowed to use scanf() is a very artificial constraint. The requirement is to use the simplest technique can read the intended input, and recover sensibly if unintended characters are input.


The problem with such a combined approach is that, if it gets in trouble, there is virtually no way your program can recover.