CMIS 102
logical operators

Logical operators are very important.  && is the logical AND
operator. || is logical OR.  ! is logical NOT.  Warning: C++ also has
& and | as operators but we won't be using them in this course, don't
use them because it'll be an error in your program if you do.
The result of and-ing two values is True only if both values are true,
it's False otherwise (i.e. if either or both are False).  The result
of or-ing two values is True if either or both of the values are True,
it's False only if both values are False.  The "truth tables", completely
defining these operators by showing the resulting value for all
possible combinations of two truth values, are in the book.  They are
straight out of classical logic, which is based on human thought and
language. 
In real-life and human language:

if sunny OR weekend   then go outside
if sunny AND weekend  then go outside
if NOT rainy   then go outside


In logic and programming language:

 two truth values OR'ed together are True if either or both are True.
  Ex.      booleanExpr1 OR booleanExpr2
   Is True if booleanExpr1 is True or if booleanExpr2 is True (or if
   both are True).  
   Is False only if both are False.

 two truth values AND'ed together are True only if both are True.
  Ex.      booleanExpr1 AND booleanExpr2
   Is True if booleanExpr1 is True and booleanExpr2 is True. 
   Is False if either or both are False.

 a truth value NOT'ed reverses its truth value.
  Ex.  NOT booleanExpr
   Is True if booleanExpr is False
   Is False if booleanExpr is True

 
Here's some natural examples used as
conditions for if statements: notice the spacing style

char midExam;
cin >> midExam;
if (midExam=='A' || midExam=='B')
  cout << "congratulations!";
else if (midExam == 'C')
  cout << "could use some improvement";
else if (midExam == 'F')
  cout << "you're failing";


cin >> score;
if (score<0 || score>100)
  cout << "Error.  score must be between 0 and 100";


char ans;
cin >> ans;
if (ans=='y' || ans=='Y')    //allows user to enter either case y/Y
....


if (score>=90 && score<=100)  //between 90 and 100
  cout << "A";


cin >> ans;
if (ans!='y' && ans!='Y')  //anything but a y/Y
  cout << "whatever it is, it isn't a y or Y";


if (month==1 || month==3 || month==5 || month==7 ||
    month==8 || month==10 || month==12)
  cout << "31 day month";
else if (month==4 || month==6 || month==9 || month==11)
  cout << "30 day month";
else
  cout << "February: 28 or 29 days";


The fact that the logical operators "short-circuit" isn't important to
know now. 

Operator precedence is important. In an expression like
 a<b && b<c
the relational operators are done first because they have higher
precedence than the logical operator.
In an expression like
 a+2<b && b*2<c/3
the arithmetic operators are done first, followed by the relational,
and then the And.
And in
 a*c>b+2||a<c&&b==a%2
which is how the compiler "sees" this:
 a*c > b+2 || a<c && b==a%2
because the compiler throws out any spaces,
the evaluation is as if this full parenthesization existed:
 (((a*c) > (b+2)) || ((a<c) && (b==(a%2))))
Note that the || is done last because it has the lowest precedence.

You can use parentheses just like in math to force a certain order of
evaluation, or just to make the expression's intention clearer.
 (a*c > b+2) || (a<c && b==a%2)    makes structure clearer

 (a*c > b+2 || a<c) && b==a%2    changes the structure.  Now && is
the last operator that will be evaluated.


Errors in logical expressions. 
if (mid || final == 'A')   Not a syntax error.  but WRONG!

if (i == 3 || 4)      Not a syntax error.  but WRONG!  it's Always True.

if (i==3 || ==4)      Syntax error.  

if (12 < y < 20)      Not a syntax error.  but WRONG! It's as if the
full parenthesization:  ((12<y) < 20)  Value of 12<y, which is 1 or 0
is less than 20, which is Always True.
Expressions that are Always True or Always False are in error.


Minor point about == and != with float values: don't.

Programming takes practice in order to learn it.