Common Programming Mistakes

1.Undeclared Variables

int main()

{

  cin>>x;

  cout<<x;

}

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.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;

}

3.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()

{

  ...

}