if

syntax: "form": 
if (expression)         parens required. No "then".
    stmt		the "body" of the if.  
			If more than 1 statement
                        in body enclose in {  }
semantics: "meaning"/behaviour:
Evaluate the expression, if it's true (i.e. nonzero) execute the statements of the body (otherwise skip the body, i.e. when the expression is false,[zero]).

use:
execute a set of code (the body) if some condition is met (is true). Don't execute the set of code if the condition is false.

The expression is a condition of some sort. A question that is true or false. 

  
Relational operators in C:
<		
>
<=
>=
==  //equality   Single = is assignment operator.  Don't confuse them!
!=     //not equals
An expression with a relational operator evaluates to true or false: 
 a  <  b	this expression's value is true or false.
Every expression will have a single value of a particular type when evaluated. 

if (gpa > 3.5)
  cout << "Dean's list";

-------------------
if (y != 0)		//divide by y only if not zero
  x = 1 / y;

-------------------
if (y != 0) { 		//compound statement
  x = 1 / y;   	//indent the statements of the body
  cout << "Reciprocal is: " << x;
}
No semicolon after }

if else

if (expression)
  stmt
else		//if the expression is not true, 
                  execute the statements of the else part.
  stmt		// in an if else, either the if body or 
                // the else body will be executed, one
		//or the other but not both
Example program with bad_ifs.cpp 
//minimum of two numbers
if (x < y)
  min = x;
else
  min = y;                     //what if x equals y?

-------------------------------
if (y != 0) {
  x = 1 / y;
  cout << "Reciprocal is: " << x;
}
else          
  cout << "No division by zero";

-------------------------------------------------------------
// counting asterisks and all other chars
if (c == '*') {
  stars++;              //increment operator.  add one to an int var
  cout << "Another star";
}
else {
  non_stars++;
  cout << "Another non star";
}


-------------------------------------------------------------
if (c == '*') {
  stars++;
  if (stars > 100)
    cout << "More than 100 stars";
}
else              //matches the outer if
  cout << "A non star";    //not counting other chars

-------------------------------------------------------------

if (c == '*') {
  stars++;
  if (stars > 100) {
    again = 'y';
    cout << "More than 100 stars";
  }
  else
    cout << "Less than 100 stars";
}
else {
  non_stars++;
  cout << "Another non star";
}
What happens if any of the }'s are missing?

A program to calculate julian dates julian.cpp (i.e. what day of the year a particular day is).

Next (while)

©David Wills