How do I reset variables in loops and prevent incorrect-input-caused infinite loops?

[Daria Shmaria]

Hi! I'm very new to C++ and have two questions whose answers are probably comically obvious. Any suggestions you have will be accepted gratefully.

I'm writing a program that calculates the average of integers (grades) input by the user. Once the average is computed, the program should ask if the user wants to find the average of another set (start over again).

1. How do I use cin.clear() to prevent incorrect input types from causing infinite loops? I know it has to do with cin.clear() and cin.ignore(), but I'm not sure where to put them.

2. How do I reset the sum variable to 0 if the user wants to input another set of integers to be averaged? As it is now, when the user wants to input another set of numbers, the program is automatically including additional, unwanted integers in the set of numbers to be averaged.

#include <iostream> // Allows the user to input data and view output 

using namespace std; // Allows us to use the standard C++ library 

int main() {
    int stillentering = 1; // Whether or not the user is still entering grades to be averaged
    int grade; // Grade input by user
    int sum = 0; // Summation of grades
    int gradecount = 0; // Number of grades
    char moregrades; // Whether or not the user is still entering grades to be averaged
    float average;
    int playagain; // Whether or not the user wants to compute another set of averages
    
    do
    {
    do 
    {
       do
       {
       cout << "Please enter a number from 0 to 100." << endl; // Prompts the user to enter a grade
       cin >> grade; // Stores the input as variable "grade"
       cin.clear();
       cin.ignore(1000,'\n');
       } while ( !( grade <= 100 ) || !( grade >= 0 ) );
       if ( grade <= 100 && grade >= 0 )
       {
          sum = sum + grade;
          gradecount++; 
          cout << "Would you like to input another grade? (y/n)" << endl;
          do
          {
             cout << "Please type y if you would like to input another grade,\nor type n if you are done entering grades." << endl;
             cin >> moregrades;
          } while ( ( moregrades != 'y' ) && ( moregrades != 'n' ) && ( moregrades != 'Y' ) && ( moregrades != 'N' ) );
          if ( ( moregrades == 'y' ) || ( moregrades == 'Y' ) )
             {
             stillentering = 1;
             cout << "You are still entering grades." << endl;
             }
          if ( ( moregrades == 'n' ) || ( moregrades == 'N' ) )
             {
             stillentering = 0;
             cout << "You are done entering grades." << endl;
             }
       }
    } while ( stillentering == 1 );
    
    average = (float)sum / gradecount; // calculate the average
    cout << "The average of these numbers is " << average << "." << endl;;// output the average
    cout << "Would you like to compute the average for another set of numbers? Type 1 for yes, 0 for no." << endl;
    cin >> playagain;
    cin.clear();
    sum = 0;
    cout << "sum should be 0" << endl;
    } while ( playagain == 1);
    
    return 0;}

[Narue]

>I'm very new to C++ and have two questions whose answers are probably comically obvious.
I/O in C++ is rarely obvious.

1) When an input request fails, cin is put into an error state, which basically means that any further input requests will also fail. This includes a request to ignore characters in the stream. So you use cin.clear() after testing cin for failure but before calling cin.ignore(). The following is the skeleton for what you should be doing in your input loop:

while ( !( cin>> variable ) ) { // Test for failure
  cin.clear(); // Clear the error state
  cin.ignore ( 1024, '\n' ); // Discard all characters in the stream
}

2) A good rule of thumb is to reset your variables to a pristine state either at the beginning of each complete operation, or immediately after the user signals that they want to do something again.

[Daria Shmaria]

Thank you so much! I'm still a little confused about one part, though. Even though I reset sum=0 after the user indicates that they want to compute another set of scores, it doesn't seem to have worked. Should I have put sum=0 somewhere else?

#include <iostream> // Allows the user to input data and view output 

using namespace std; // Allows us to use the standard C++ library 

int main() {
    int stillentering = 1; // Whether or not the user is still entering grades to be averaged
    int grade; // Grade input by user
    int sum = 0; // Summation of grades
    int finalsum = 0;
    int gradecount = 0; // Number of grades
    char moregrades; // Whether or not the user is still entering grades to be averaged
    float average;
    int playagain; // Whether or not the user wants to compute another set of averages
    
    do
    {
    do 
    {
/*    cout << "Please enter a number from 0 to 100." << endl; // Prompts the user to enter a grade
    cout << "sum is " << sum << endl;
    while ( !(cin >> grade) ) 
          {
          cin.clear();
          cin.ignore (1024, '\n');
          }       
    cin >> grade;
*/       // nothing is output after "enter a num..."

       do {
          cout << "Please enter a number from 0 to 100." << endl; // Prompts the user to enter a grade
          cout << "sum is " << sum << endl;
            while ( !(cin >> grade) )
            {
                  cin.clear();
                  cin.ignore (1024, '\n');
            }
          cin >> grade; // Stores the input as variable "grade"
       } while ( !( grade <= 100 ) || !( grade >= 0 ) );
       
       if ( grade <= 100 && grade >= 0 )
       {
          sum = sum + grade;
          gradecount++; 
          cout << "Would you like to input another grade? (y/n)" << endl;
          do
          {
             cout << "Please type y if you would like to input another grade,\nor type n if you are done entering grades." << endl;
             cin >> moregrades;
          } while ( ( moregrades != 'y' ) && ( moregrades != 'n' ) && ( moregrades != 'Y' ) && ( moregrades != 'N' ) );
          if ( ( moregrades == 'y' ) || ( moregrades == 'Y' ) )
             {
             stillentering = 1;
             cout << "You are still entering grades." << endl;
             }
          if ( ( moregrades == 'n' ) || ( moregrades == 'N' ) )
             {
             stillentering = 0;
             cout << "sum is " << sum << endl;
             finalsum = sum;
             sum = 0;
             cout << "You are done entering grades." << endl;
             }
       }
    } while ( stillentering == 1 );
    
    average = (float)finalsum / gradecount; // calculate the average
    cout << "The average of these numbers is " << average << "." << endl;;// output the average
    cout << "Would you like to compute the average for another set of numbers? Type 1 for yes, 0 for no." << endl;
    while ( !(cin >> playagain) ) 
          {
          cin.clear();
          cin.ignore (1024, '\n');
          }
    sum = 0;
    cout << sum << endl;
    } while ( playagain == 1);
    
    return 0;}

[Lerner]

You use two counters in the course of the program. Both are set to zero before the first run of scores, but only one is reset to zero if the user wants to do another set of scores. Fix that and see if your problem goes away.

[Daria Shmaria]

Thank you very much!