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C++ loops: how to get program to repeat "error" message more than once anytime user enters invalid input
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Hello all,
I'm ALMOST done with this assignment, but still having trouble getting the code to display the error messages continuously until the correct input is entered.

For example, if the user enters "4" for operation (which should be between 1-3), it correctly displays: "Your operation choice isn't valid! Please try again, using 1, 2, or 3." However, if the user enters another invalid number for the operation (such as 5), it doesn't repeat the error message, but just continues forward.

Anyone able to help me figure out how to get each error message to repeat until valid numbers or characters are entered for each prompt?

Note: I am very new to coding, so please use simple explanations or examples in code, if possible. THANK YOU!!

What I have tried:

#include <iostream>
#include <cstdio>
#include <time.h>
#include <stdlib.h>
using namespace std;

int main()
{
    int operation, num3, guess, num1, num2, temp;
    char play;
    srand(time(0));

    do
    {
      num1 = rand() % 10;
      num2 = rand() % 10;

      if (num1 < num2)
      {
          temp = num1;
          num1 = num2;
          num2 = temp;
      }
        do
        {
            cout << "Choose an operation." << endl;
            cout << "Enter 1 to add, 2 to subtract, or 3 to multiply: " <<
endl;
            cout << "" << endl;
            cin >> operation;
            cout << "" << endl;

            if (operation > 3 || operation < 1)
            {
                cout << "Your operation choice isn't valid!  Please try
again, using 1, 2, or 3." << endl;
                cout << "" << endl;
                cout << "Choose an operation." << endl;
                cout << "Enter 1 to add, 2 to subtract, or 3 to multiply: "
<< endl;
                cout << "" << endl;
                cin >> operation;
                cout << "" << endl;
            }
        }
        while (operation > 3 || operation < 1);

        switch(operation)
        {
            case 1:
            cout << "You chose addition." << endl;
            num3 = num1 + num2;
            cout << "" << endl;
            cout << "What is " <<  num1 << " + " << num2 << " ?: " << endl;
            cout << "" << endl;
            cin >> guess;
            cout << "" << endl;

                if (guess != num3)
                    {
                    cout << "That is incorrect. Please try again." << endl;
                    cout << "" << endl;
                    cout << "What is " <<  num1 << " + " << num2 << " ?: "
<< endl;
                    cout << "" << endl;
                    cin >> guess;
                    cout << "" << endl;
                    }

                else if (guess == num3)
                    {
                    cout << "That is correct!" << endl;
                    cout << "" << endl;
                    }
            break;

            case 2:
                cout << "You chose subtraction." << endl;
                num3 = num1 - num2;
                cout << "What is " <<  num1 << " - " << num2 << " ?: " <<
endl;
                cout << "" << endl;
                cin >> guess;
                cout << "" << endl;

                    if (guess != num3)
                        {
                        cout << "That is incorrect. Please try again." <<
endl;
                        cout << "" << endl;
                        cout << "What is " <<  num1 << " - " << num2 << " ?:
" << endl;
                        cout << "" << endl;
                        cin >> guess;
                        }

                    else if (guess == num3)
                        {
                        cout << "That is correct!" << endl;
                        cout << "" << endl;
                        }
                break;

            case 3:
                cout << "You chose multiplication." << endl;
                num3 = num1 * num2;
                cout << "What is " <<  num1 << " * " << num2 << " ?: " <<
endl;
                cout << "" << endl;
                cin >> guess;
                cout << "" << endl;

                    if (guess != num3)
                        {
                        cout << "That is incorrect. Please try again." <<
endl;
                        cout << "" << endl;
                        cout << "What is " <<  num1 << " * " << num2 << " ?:
" << endl;
                        cout << "" << endl;
                        cin >> guess;
                        }

                    else if (guess == num3)
                        {
                        cout << "That is correct!" << endl;
                        cout << "" << endl;
                        }
            break;
        }

        do
        {
             cout << "Would you like to play again? Press Y for yes or Q for
quit" << endl;
             cout << "" << endl;
             cin >> play;

           if (play != 'Y' && play != 'Q')

            {
                cout << "That is not a valid choice. Please choose Y for yes
or Q to quit. " << endl;
                cout << "" << endl;
            }

        }

        while(play !='Y' && play !='Q');

        if (play == 'Y')
        {
        cout << "Thank you for playing! Let's play again!" << endl;
        cout << "" << endl;
        }

        else
        {
        cout << "Thank you for playing! See you next time!" << endl;
        cout << "" << endl;
        }

     }
     while(play=='Y');
return 0;
}
/*Sample Run:
Choose an operation.
Enter 1 to add, 2 to subtract, or 3 to multiply:

3

You chose multiplication.
What is 4 * 1 ?:

4

That is correct!

Would you like to play again? Press Y for yes or Q for quit

Y
Thank you for playing! Let's play again!

Choose an operation.
Enter 1 to add, 2 to subtract, or 3 to multiply:

1

You chose addition.

What is 6 + 1 ?:

7

That is correct!

Would you like to play again? Press Y for yes or Q for quit

Y
Thank you for playing! Let's play again!

Choose an operation.
Enter 1 to add, 2 to subtract, or 3 to multiply:

2

You chose subtraction.
What is 5 - 0 ?:

5

That is correct!

Would you like to play again? Press Y for yes or Q for quit

Y
Thank you for playing! Let's play again!

Choose an operation.
Enter 1 to add, 2 to subtract, or 3 to multiply:

1

You chose addition.

What is 7 + 1 ?:

9

That is incorrect. Please try again.

What is 7 + 1 ?:

10

Would you like to play again? Press Y for yes or Q for quit

Y
Thank you for playing! Let's play again!

Choose an operation.
Enter 1 to add, 2 to subtract, or 3 to multiply:

2

You chose subtraction.
What is 7 - 3 ?:

5

That is incorrect. Please try again.

What is 7 - 3 ?:

6
Would you like to play again? Press Y for yes or Q for quit

Q
Thank you for playing! See you next time!


Process returned 0 (0x0)   execution time : 43.057 s
Press any key to continue.
*/
Posted
Updated 25-May-18 0:36am
Add a Solution

2 solutions

hi,

the problem is the do- while loop

 do
        {
            cout << "Choose an operation." << endl;
            cout << "Enter 1 to add, 2 to subtract, or 3 to multiply: " <<
endl;
            cout << "" << endl;
            cin >> operation;
            cout << "" << endl;

            if (operation > 3 || operation < 1)
            {
                cout << "Your operation choice isn't valid!  Please try
again, using 1, 2, or 3." << endl;
                cout << "" << endl;
                cout << "Choose an operation." << endl;
                cout << "Enter 1 to add, 2 to subtract, or 3 to multiply: "
<< endl;
                cout << "" << endl;
                cin >> operation;
                cout << "" << endl;
            }
        }
        while (operation > 3 || operation < 1);



you can try changing the position of do just before the if condition as 


       cout << "Choose an operation." << endl;
            cout << "Enter 1 to add, 2 to subtract, or 3 to multiply: " <<
endl;
            cout << "" << endl;
            cin >> operation;
            cout << "" << endl;

do
        {
      
            if (operation > 3 || operation < 1)
            {
                cout << "Your operation choice isn't valid!  Please try
again, using 1, 2, or 3." << endl;
                cout << "" << endl;
                cout << "Choose an operation." << endl;
                cout << "Enter 1 to add, 2 to subtract, or 3 to multiply: "
<< endl;
                cout << "" << endl;
                cin >> operation;
                cout << "" << endl;
            }
        }
        while (operation > 3 || operation < 1);


in your code the program is waiting for the input 2 times in the do - while (one before if condition and one inside if condition.)
 
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Posted
Comments
Rick York 25-May-18 0:56am    
The prompting of the user should be in a function I think. Something like this:
int GetOperation()
{
    cout << "Choose an operation." << endl;
    cout << "Enter 1 to add, 2 to subtract, or 3 to multiply: ";
    cin >> operation;
    cout << "" << endl;
}

// then the previous loop can be written as :
   int operation = GetOperation();
   while (operation > 3 || operation < 1);
   {
       cout << "Your choice isn't valid!  Please try again." << endl;
       cout << "" << endl;
       operation = GetOperation();
   }
Rick York 25-May-18 10:40am    
Similarly, prompting the user for the answer should be a function which takes the actual answer as an argument.
As suggested, use functions to factor out duplicate code and keep your main function neat. This way the program flow is easily settled, e.g.

C++
#include <iostream>
#include <cstdlib>
#include <string>
#include <cassert>
using namespace std;

struct Operation
{
  string name;
  string symbol;
};

enum
{
  eAdd = 0,
  eSub,
  eMul,
  eOperations
};

int computeResult(size_t choice, int x, int y);
size_t askOperation(const Operation op_array[], size_t size);
int askGuess(const Operation & op, int x, int y);

int main()
{
  srand(time(0));

  Operation op_array[eOperations] = { { "addition", "+" }, { "subtraction", "-"}, {"multiplication", "*"}};

  while (true)
  {
    size_t choice = askOperation( op_array, eOperations );
    if ( choice >= eOperations) break;
    int a = rand() % 10;
    int b = rand() % 10;
    if ( a < b ) swap(a,b);
    int result = computeResult(choice, a, b);
    int guess = askGuess( op_array[choice], a, b);
    cout << "your answer is " << ( guess == result ? "correct" : "incorrect") << endl << endl;

  }
  cout << "see you next time" << endl;
} // <- main

int computeResult(size_t choice, int x, int y)
{
  int result = -1;
  switch( choice)
  {
  case eAdd:
    result = x + y;
    break;
  case eSub:
    result = x - y;
    break;
  case eMul:
    result = x * y;
    break;
  default:
    assert(false);
  }
  return result;
}

size_t askOperation(const Operation op_array[], size_t size)
{
  cout << "choose an operation. " << endl << "enter";
  for (size_t n=0; n<size; ++n)
    cout << ", " << (n+1) << " for " << op_array[n].name;
  cout << " (any other value to exit): " << endl;
  int choice;
  cin >> choice;
  return (choice-1);
}

int askGuess(const Operation & op, int x, int y)
{
  cout << "you choose " << op.name << endl;
  cout << "what is " << x << " " << op.symbol <<  " " << y << " ?" << endl;
  int result;
  cin >> result;
  return result;
}


Modern C++ allowes more concision:
#include <iostream>
#include <utility>
#include <cstdlib>
#include <string>
#include <array>

using namespace std;

using Function = int (int, int);

struct Operation
{
  string symbol;
  string name;
  Function * fun;
};

size_t askOperation(const array<Operation, 3> & oper_array);
int askGuess(const Operation & oper, int x, int y);

int main()
{
  srand(time(0));

  array<Operation, 3> oper_array =
  {{
    { "+", "addition", [](int x, int y) { return x+y;} },
    { "-", "subtraction", [](int x, int y) { return x-y;} },
    { "*", "multiplication", [](int x, int y) { return x*y;}}
  }};

  while (true)
  {
    auto choice = askOperation( oper_array );
    if ( choice >= oper_array.size()) break;
    int a = rand() % 10;
    int b = rand() % 10;
    if ( a < b ) swap(a,b);
    Operation & oper = oper_array[choice];
    int result = oper.fun( a, b );
    int guess = askGuess( oper, a, b);
    cout << "your answer is " << ( guess == result ? "correct" : "incorrect") << endl;

  }
  cout << "see you next time" << endl;
}// <- main

size_t askOperation(const array<Operation, 3> & oper_array)
{
  cout << "choose an operation. " << endl << "enter";
  for (size_t n=0; n<oper_array.size(); ++n)
    cout << ", " << (n+1) << " for " << oper_array[n].name;
  cout << " (any other value to exit): " << endl;
  int choice;
  cin >> choice;
  return (choice-1);
}
int askGuess(const Operation & oper, int x, int y)
{
  cout << "you choose " << oper.name << endl;
  cout << "what is " << x << " " << oper.symbol <<  " " << y << " ?" << endl;
  int result;
  cin >> result;
  return result;
}


[update]
Or
#include <iostream>
#include <functional>
#include <map>
#include <cstdlib>
using namespace std;

int main()
{
  srand(time(0));

  using Operation = tuple< string, char , function < int (int, int) > >;

  map < int, Operation > m = {
    {1, {"addition", '+', plus<int>() }},
    {2, {"subtraction", '-', minus<int>() }},
    {3, {"multiplication", '*', multiplies<int>() }},
    };

  while (true)
  {
    cout << "\nenter choice:\n";
    for (const auto & [i, oper] : m)
      cout << '\t' << i << " " << get<0>(oper) << '\n';
    cout << "any other value to exit\n";
    int choice;
    cin >> choice;
    auto it = m.find(choice);
    if ( it == m.end()) break;

    int a = rand()%10;
    int b = rand()%10;
    if ( a < b ) swap(a,b);

    const auto & [name, symbol, fun] = it->second;

    cout << "you choose " << name << "\n";
    cout << "what is " << a <<  " " << symbol << " " << b << " ?\n";
    int guess;
    cin >> guess;
    cout << "your guess is " << (fun(a,b) == guess ? "correct\n" :  "incorrect\n");
  }
  cout << "see you next time\n";
}

[/update]
 
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Posted
Updated 25-May-18 3:36am
v2
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