Thread: newb loop question

Hi

I'm fairly new to c++ programming and have a simple question regarding a piece of code I have written.

I set up a do while loop as part of a simple menu that will repeat the menu if the user doesn't either enter a one or a two (the only choices available).

Below is the syntax for this code.

Code:

 do{
             cout<<"Please choose conversion type"<<endl;
             cout<<"1.) Ra to RMS\n";
             cout<<"2.) RMS to Ra\n";
             cin>>switchn;
             cin.ignore();
    
             system("cls");
          }while (!((switchn == 1)||(switchn == 2)));//end of do-while

It works great if the user enters in a wrong number, however if he enters any sort of other character like a letter then it just goes into a neverending loop that I have to force quit.

Is there a way to set it up so that if someone enters a letter by accident it will just go throught the loop once and not continuously uninteruupted?

Thanks

Originally Posted by StainedBlue

See De Morgan's laws

Your condition is actually saying, "Anything but the two I've specified."

Which results in an infinite loop.

OK I guess my question is this.
When the user enters in a wrong number such as 5 the program will loop through but stop at the cin command until it gets another input.

however if the user enters a letter like "y" the program will loop again but will bypass the cin command and just continue the loop forever by itself.

Why is this and how do I fix it?

Originally Posted by filio623

OK I guess my question is this.
When the user enters in a wrong number such as 5 the program will loop through but stop at the cin command until it gets another input.

however if the user enters a letter like "y" the program will loop again but will bypass the cin command and just continue the loop forever by itself.

Why is this and how do I fix it?

You attempt to read a character into an integer which sends cin into a fail state:

std::cin.fail() is equal to true.

You must first call cin.clear() to remove the error flag before you can read in any more data.

Consider using a switch control statement within your loop.

>>Consider using a switch control statement within your loop.

or learn about De Morgan's laws and fix this the way it should be.

and yes, if you're expecting a response from the user, make sure you're looking for a "char".

Don't i feel silly.

1) use a char for "switchn"

2) look at specifically:

Code:

switchn == 1

should be:

Code:

switchn == '1'

You're logical condition is actually correct.

Originally Posted by StainedBlue

>>Consider using a switch control statement within your loop.

or learn about De Morgan's laws and fix this the way it should be.

and yes, if you're expecting a response from the user, make sure you're looking for a "char".

De Morgan's laws have nothing to do with the infinite loop, the loop condition is correct. As I stated above, the reason for the infinite loop is the cin statement is being skipped while it remains in the error state. Therefore the value in switchn remains as a value other than 1 or 2.