Advice on basic number validation

This is a discussion on Advice on basic number validation within the C++ Programming forums, part of the General Programming Boards category; I have been experimenting with basic number validation and i need a little advice... Code: #include <iostream> using namespace std; ...

  1. #1
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    Advice on basic number validation

    I have been experimenting with basic number validation and i need a little advice...

    Code:
    #include <iostream>
    
    using namespace std;
    
    int main() {
     int num;
    
     start:
    
     cout << "Please enter a Number between 1 and 10... ";
     cin >> num;
     if (num < 1 || num > 10) {
      cout << "Invalid Number" << endl;
      goto start;
     }
     else {
      cout << "Valid Number";
     }
    
     cin.ignore();
     cin.get();
    }
    Is this the right approach for validating a number? How would i validate that it's a number and not any other character? I noticed that if i enter a letter instead at run time the console closes presumably crashing the program.

    And finally, have i looped correctly? If an invalid number is entered it needs to ask for a number to be inputted again, I seem to recall reading that it's not advisable to use goto anymore, but I'm not sure how else to do it.

    I would appreciate any advice you can give me

  2. #2
    Registered User Micko's Avatar
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    Quote Originally Posted by Diablo84
    I have been experimenting with basic number validation and i need a little advice...

    Code:
    #include <iostream>
    
    using namespace std;
    
    int main() {
     int num;
    
     start:
    
     cout << "Please enter a Number between 1 and 10... ";
     cin >> num;
     if (num < 1 || num > 10) {
      cout << "Invalid Number" << endl;
      goto start;
     }
     else {
      cout << "Valid Number";
     }
    
     cin.ignore();
     cin.get();
    }
    Is this the right approach for validating a number? How would i validate that it's a number and not any other character? I noticed that if i enter a letter instead at run time the console closes presumably crashing the program.

    And finally, have i looped correctly? If an invalid number is entered it needs to ask for a number to be inputted again, I seem to recall reading that it's not advisable to use goto anymore, but I'm not sure how else to do it.

    I would appreciate any advice you can give me
    It's a bad way + using goto even worse...
    Check the FAQ on this
    Last edited by Micko; 04-02-2005 at 08:10 AM.
    Gotta love the "please fix this for me, but I'm not going to tell you which functions we're allowed to use" posts.
    It's like teaching people to walk by first breaking their legs - muppet teachers! - Salem

  3. #3
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    I like to read numbers in as strings, validate and then convert them to whatever I need.

  4. #4
    Registered User Joelito's Avatar
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    @Diablo: wow! I though gotos were dead :S
    anyway:
    Code:
    #include <iostream>
    
    int main() 
    {
        using namespace std;
        
        int num;
        
        for(;;)
        {
               cout << "Please enter a Number between 1 and 10... ";
               cin >> num;
               if ( (num >= 1) && (num <= 10) ) break;
               cout << "Invalid Number" << endl;
               continue;
        }
        cout << "Valid Number\n";
        system("pause"); // not portable
        return 0; 
    }
    I try your code, but let's say I "miss with it"
    Try to use your favorite loop, and use wisely break and continue

    @caduardo21: Use some of the old CRT functions: atoi or atol.
    Last edited by Joelito; 04-02-2005 at 08:37 AM.

  5. #5
    Registered User Micko's Avatar
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    And what if user enter a letter instead of number?
    Gotta love the "please fix this for me, but I'm not going to tell you which functions we're allowed to use" posts.
    It's like teaching people to walk by first breaking their legs - muppet teachers! - Salem

  6. #6
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    OP: You approach isn't going to get you where you need to go. Take a look at this and see if it helps.



    This will work if the user enters a char and not a number, as well as if the user enters a number.

    Code:
    #include <iostream>
    
    int main()
    {
    	bool valid = 0;
    	
    	do
    	{
    		char *ans = new char;
    		int num = -1;
    		
    		std::cout << "Input a number (0-9): ";
    		std::cin >> ans;
    		
    		if (isdigit(*ans))
    		{
    			num = atoi(ans);
    		}
    		
    		if ((num < 0) || (num > 9))
    		{
    			std::cout << "\nInvalid number.\n\n";
    		}
    		
    		else
    		{
    			std::cout << "\nYour number was: " << num << ".";
    			valid = 1;
    		}
    	}
    	while (valid == 0);
    	
    	std::cin.ignore(80, '\n');
    	std::cin.get();
    	return(0);
    }

  7. #7
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    Thank you.

    Could you confirm i have understood this line correctly from the FAQ:

    Code:
     if ( scanf ( "%d", &num ) != 1 )
    The condition is that scanf did not successfully read one item and
    scanf itself attempts to read inputted data and format is as an integer?

    I don't fully understand the need for the address operator, i presume it's purpose is to specify the memory location to write the value to but can it not simply assign the value to the variable?


    EDIT: Thank you Lithorien, i will have a look through your example now

  8. #8
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    Quote Originally Posted by Diablo84
    Thank you.

    Could you confirm i have understood this line correctly from the FAQ:

    Code:
     if ( scanf ( "%d", &num ) != 1 )
    The condition is that scanf did not successfully read one item and
    scanf itself attempts to read inputted data and format is as an integer?

    I don't fully understand the need for the address operator, i presume it's purpose is to specify the memory location to write the value to but can it not simply assign the value to the variable?


    EDIT: Thank you Lithorien, i will have a look through your example now
    scanf() is not a C++ function, you're much better served to use cin(). However:

    What you are saying in that statement is "If scanf(interger, store_in_num) != One_Int_Read), if I remember my C correctly. To put it in more human terms, you are checking to see if scanf() (which reads an interger into num) only got 1 int from the user.

    So yes, you were right about the condition being that scanf() didn't read just one item.

    scanf() also stores the input into addresses - that's why you have to give it &num and not just num.

  9. #9
    Registered User Joelito's Avatar
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    @Micko: Maybe:
    Code:
    #include <iostream>
    
    int main() 
    {
        using namespace std;
        
        int num;
        
        for(;;)
        {
               cout << "Please enter a Number between 1 and 10... ";
               cin >> num;
    		   if (cin.fail())
    		   {
    			   cout << "Must be a number!!!\n" << endl;
    			   break;
    		   }
    		   else
    		   {
    				if ( (num >= 1) && (num <= 10) ) 
    				{
    					cout << "Valid Number\n";
    					break;
    				}
    				cout << "Invalid Number" << endl;
    				continue;
    		   }
        }
        system("pause"); // not portable
        return 0; 
    }

  10. #10
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    The *Goto statement* is taboo for most programmers.

    Wouldn't use it- it's bad. Use a while loop with a condition


    Code:
       //The start of your program
    
        int loop=0;
        while (loop==0)
        {
            cout<<"Enter:"<<endl;
            cin>>Enter;
    
              if (Enter==valid)
              {
               do stuff;
               condition==1;  //Thus breaking the while condition
               }
         
               if (Enter==invalid)
              {
              then condition is still 0 so will loop to beginning
               }
       
        }
    Perhaps?

  11. #11
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    I have just been doing some reading on why it's advisable to avoid goto. Initially i got the impression that it was just taboo but for no specific reason. After digging a little deeper i found it's association with spaghetti code in a poorly written program etc. It's also clear now that in the vast majority of cases there is always a better alternative.

    The only query left in my mind now is, what would happen in a situation where there was a potential good use for goto. ie. it would be a more simple, efficient and effective approach to any alternative. Would it be acceptable to use a goto statement then or is it a big no no which should always be avoided?

  12. #12
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    Quote Originally Posted by Diablo84
    The only query left in my mind now is, what would happen in a situation where there was a potential good use for goto. ie. it would be a more simple, efficient and effective approach to any alternative. Would it be acceptable to use a goto statement then or is it a big no no which should always be avoided?
    There's one case that I can think of where you would use goto.

    Code:
    while (some_condition)
    {
        while (some_other_condition)
        {
            // Code that makes the program need to break out of all loops..
            goto endloop;
        }
    }
    
    endloop:
    Basically, it's to break out of nested loops when break; wouldn't do it for you.

  13. #13
    Registered User Micko's Avatar
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    Quote Originally Posted by Joelito
    @Micko: Maybe:
    Code:
    #include <iostream>
    
    int main() 
    {
        using namespace std;
        
        int num;
        
        for(;;)
        {
               cout << "Please enter a Number between 1 and 10... ";
               cin >> num;
    		   if (cin.fail())
    		   {
    			   cout << "Must be a number!!!\n" << endl;
    			   break;
    		   }
    		   else
    		   {
    				if ( (num >= 1) && (num <= 10) ) 
    				{
    					cout << "Valid Number\n";
    					break;
    				}
    				cout << "Invalid Number" << endl;
    				continue;
    		   }
        }
        system("pause"); // not portable
        return 0; 
    }
    My solution is...
    Code:
    #include <iostream>
    
    int main() 
    {
        using namespace std;
        
        int num;
        cout<<"Enter integer: ";
        cout.flush();
        while (!(cin>>num))
        {
             
                  cout<<"Wrong input!"<<endl;
                  cin.clear();
                  cin.ignore ( numeric_limits<streamsize>::max(), '\n' );
                  cout<<"Enter again: ";
                  cout.flush();
        }
        if(cin.get() !='\n')
        {
    			//something is left in an input buffer, dicard it...
    		cin.ignore ( numeric_limits<streamsize>::max(), '\n' );
        }
    
        cout<<"You entered"<<num; 
    }
    I think this would be a better solution, if someone has better please post so we can all benefit...
    Last edited by Micko; 04-02-2005 at 11:20 AM.
    Gotta love the "please fix this for me, but I'm not going to tell you which functions we're allowed to use" posts.
    It's like teaching people to walk by first breaking their legs - muppet teachers! - Salem

  14. #14
    C++ Witch laserlight's Avatar
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    Could you replace:
    cin.ignore ( numeric_limits<streamsize>::max(), '\n' );
    with:
    cin.sync();
    ?
    C + C++ Compiler: MinGW port of GCC
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    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  15. #15
    Registered User Micko's Avatar
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    I must admit I'm not familiar with that function. I know it is used to synchronize stream but how exactly works when to use it I don't know. Can someone explain with more details, please?
    Gotta love the "please fix this for me, but I'm not going to tell you which functions we're allowed to use" posts.
    It's like teaching people to walk by first breaking their legs - muppet teachers! - Salem

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