1. Undeclared Variables
int main()
{
cin>>x;
cout<<x;
}
"Huh? Why do I get an error?" Your compiler doesn't know what x means. You need to declare it as a variable.
int main()
{
int x;
cin>>x;
cout<<x;
}
2. Uninitialized variables
int count;
while(count<100)
{
cout<<count;
}
"Why doesn't my program enter the while loop?" In C++ variables are not initialized to zero. In the above snippet of code, count could be any value in the range of int. It might, for example, be 586, and in that situation the while loop's condition would never be true. Perhaps the output of the program would be to print the numbers from -1000 to 99. In that case, once again, the variable was assigned a memory location with garbage data that happened to evaluate to -1000.
Remember to initialize your variables.
3. Setting a variable to an uninitialized value
int a, b; int sum=a+b; cout<<"Enter two numbers to add: "; cin>>a; cin>>b; cout<<"The sum is: "<<sum; When Run: Enter two numbers to add: 1 3 The sum is: -1393"What's wrong with my program?"
Often beginning programmers believe that variables work like equations - if you assign a variable to equal the result of an operation on several other variables that whenever those variables change (a and b in this example), the value of the variable will change. In C++ assignment does not work this way: it's a one shot deal. Once you assign a value to a variable, it's that value until you reassign the values. In the example program, because a and b are not initialized, sum will equal an unknown random number, no matter what the user inputs.
To fix this error, move the addition step after the input line.
int a, b; int sum; cout<<"Enter two numbers to add: "; cin>>b; cin>>a; sum=a+b; cout<<"The sum is: "<<sum;
4. Using a single equal sign to check equality
char x='Y';
while(x='Y')
{
//...
cout<<"Continue? (Y/N)";
cin>>x;
}
"Why doesn't my loop ever end?" If you use a single equal sign to check equality, your program will instead assign the value on the right side of the expression to the variable on the left hand side, and the result of this statement is the value assigned. In this case, the value is 'Y', which is treated as true. Therefore, the loop will never end. Use == to check for equality; furthermore, to avoid accidental assignment, put variables on the right hand side of the expression and you'll get a compiler error if you accidentally use a single equal sign as you can't assign a value to something that isn't a variable.
char x='Y';
while('Y'==x)
{
//...
cout<<"Continue? (Y/N)";
cin>>x;
}
5. Undeclared Functions
int main()
{
menu();
}
void menu()
{
//...
}
"Why do I get an error about menu being unknown?" The compiler doesn't know what menu() stands for until you've told it, and if you wait until after using it to tell it that there's a function named menu, it will get confused. Always remember to put either a prototype for the function or the entire definition of the function above the first time you use the function.
void menu();
int main()
{
menu();
}
void menu()
{
...
}
6. Extra Semicolons
int x; for(x=0; x<100; x++); cout<<x;"Why does it output 100?"
You put in an extra semicolon. Remember, semicolons don't go after if statements, loops, or function definitions. If you put one in any of those places, your program will function improperly.
int x; for(x=0; x<100; x++) cout<<x;
7. Overstepping array boundaries
int array[10]; //... for(int x=1; x<=10; x++) cout<<array[x];"Why doesn't it output the correct values?"
Arrays begin indexing at 0; they end indexing at length-1. For example, if you have a ten element array, the first element is at position zero and the last element is at position 9.
int array[10]; //... for(int x=0; x<10; x++) cout<<array[x];
8. Misusing the && and || operators
int value;
do
{
//...
value=10;
}while(!(value==10) || !(value==20))
"Huh? Even though value is 10 the program loops. Why?"
Consider the only time the while loop condition could be false: both value==10 and value==20 would have to be true so that the negation of each would be false in order to make the || operation return false. In fact, the statement given above is a tautology; it is always true that value is not equal to 10 or not equal to 20 as it can't be both values at once. Yet, if the intention is for the program only to loop if value has neither the value of ten nor the value of 20, it is necessary to use && : !(value==10) && !(value==20), which reads much more nicely: "if value is not equal to 10 and value is not equal to 20", which means if value is some number other than ten or twenty (and therein is the mistake the programmer makes - he reads that it is when it is "this" or "that", when he forgets that the "other than" applies to the entire statement "ten or twenty" and not to the two terms - "ten", "twenty" - individually). A quick bit of boolean algebra will help you immensely: !(A || B) is the equivalent of !A && !B (Try it and see; you can read more about this rule on Wikipedia: DeMorgan's Law). The sentence "value is other than [ten or twenty]" (brackets added to show grouping) is translatable to !(value==10 || value==20), and when you distribute the !, it becomes !(value==10) && !(value==20).
The proper way to rewrite the program:
int value;
do
{
//...
value=10;
}while(!(value==10) && !(value==20))
Next
the MFC application wizard will appear. If you do not choose the name
SameGame, then the names of your classes will be slightly different than
those that appear in this article. This allows you to select quite a
few options that the resulting generated code will include. For our
simple game we can disable quite a few of these options. The following
graphics show which options to select in order to get the project just
the way we want it.
Selecting
"Single document" allows the application to use the document/view
architecture when multiple documents aren't necessary. The last setting
of interest on this page is "Use of MFC". The two options are for a
shared DLL or as a static library. Using a DLL means that your users
must have the MFC DLLs installed on their computer, which most computers
do. The static library option links the MFC library right into your
application. The executable that is produced will be larger in size but
will work on any Windows machine.
(If
you are using Visual 2010, this screen does not have a "None" option
for Toolbars. Just choose "Use a Classic Menu" without checking either
toolbar.) A thick frame allows the user to resize the window. Since our
game is a static size, un-check this option. A maximize box isn't
needed, nor is a status bar or a toolbar. Advancing to the next page
will bring you to the "Advanced Features" page.
Turn
off printing, ActiveX controls and set the number of recent files to
zero. Since we won't actually be loading any files, this option won't
be necessary. The last page of the MFC Application Wizard presents you
with a list of generated classes.
Four
classes that will be generated for you are the basis for the game. The
first on the list is the view class, here it is called CSameGameView. I
will come back to this class in a minute. The next class in the list
is the application class. This class is a wrapper for the entire
application and a main function is provided for your application by this
class. The base class isn't selectable and must be CWinApp.
Notice
it has a default menu (File, Edit and Help) and an empty client area.
Before we get to actual coding I'd like to explain a little about the
document/view architecture that is used in MFC applications and how we
are going to apply it to our game.