//IntVariable.java
//demo an int variable storing a value and varying its value.
//to "understand the quintessence of programming."

public class IntVariable {
    public static void main (String [] args) {

	int students;
	//"declare"/create an int "variable" (a variable of "type" int).
	//think of it as a little box in the computer but it's actually
	//  a named location in memory.
	//its name is students.  
	//it can store/hold one int value (one integer).
	//names of variables can consist of letters, digits and _ but
	//  cannot start with a digit.  name should be meaningful,
	//  indicating what the variable is used for, what its purpose is.

	//now the variable exists but has no value.  it would be an error
	//to try to use the value of the students variable.  it must be
	//assigned a value first.

	students = 12;
	//assign 12 to be its value.  = is the "assignment" "operator".
	//it is an action, not an equation.  leftside must be a variable,
	//rightside an expression.  rightside is evaluated, then that (single) 
	//value is assigned to be the value of the leftside variable.
	//RHS value must be compatible type.

	System.out.println( "variable is now: " + students );
	//+ here is string "concatenation" (joining together two strings into one)

	students = students + 1;       //add one to the students variable.
	//assign its current's value plus one as its new value.
	//rightside is students+1     students is 12, so it's 12+1 which is 13,
	//so 13 is assigned to leftside variable
	System.out.println( "and now it's: " + students );

	//syntax errors: use undeclared variable, use before assigned a value,
	//assign incompatible type.


	//arithmetic operators
	int num1=12, num2=5;  //declare and initialize variables
	int sum, difference, product, quotient, remainder;

	sum = num1 + num2;           //addition
	difference = num1 - num2;    //subtraction
	product = num1 * num2;       //multiplication
	quotient = num1 / num2;      //division
	remainder = num1 % num2;     //remainder (modulus, "mod")

	System.out.println( "sum: " + sum +
			    "\ndifference: " + difference +
			    "\nproduct: " + product +
			    "\nquotient: " + quotient +
			    "\nremainder: " + remainder);
    }
}
// -5   -num
//try making the num2 0 and see the error.
//more complex expression:2*num1 + 4*num2
//change the types of the variables to double