CIS 234: Order of Operations, Shortcut & Other Operators

CIS 234: Order of Operations, Shortcut & Other Operators Dr. Ralph D. Westfall February, 2004

Some Things to Think About • order of operations • what gets calculated 1st? • type casting • changing data from one type of variable to a different type • shortcut operators • doing the same calculations with less typing

Order of Operations • order in which calculations and other operations are performed by language • 2 + 4 / 2 // is result 3 or 4? • operations that are done first have "higher precedence" • pp. 45-46, 188 • "Please excuse my dear aunt Sally"

Order of Operations - 2 • operations within parentheses are always done first • inner before outer parentheses e.g., (5 * (2 + 3)) ("please") • method calls next (lower precedence) Math.pow(2, 3) // Java's exponentiation // == 2*2*2 ("excuse")

Order of Operations - 3 • following all have the same precedence: • "unary" minus • unary minus doesn't subtract from anything value = -5 ; value = - x; // reverses sign • increment or decrement: ++ and -- • type casting (changes data type) value = (double) 5 / 2 ;

Type Casting • can change data type of a variable by putting a different data type in front • in parentheses e.g., (int), (char), (float) • can also put parentheses around an expression, and a cast before that, to change the data type of the result double a = (int) (3.0 / 2); // a? // how many conversions in above line?

Order of Operations - 4 • then multiply ("my"), regular division ("dear"), and modulus operations *, /, % • then addition ("aunt"), and subtraction ("Sally") +, -

Order of Operations - 5 • then comparison operations (in order) • <, <=, >, >= // highest among these • ==, != • && (AND) • || (OR) // lowest among these • then assignment (equals) = e.g., height = 60; or shortcut assignment e.g., x += y; • p. 188

Practice • evaluate the following: • x = 3 * 5 + 4 / 2 – Math.min(3, 8) • y = (9 + 2) == (7 + 4) • z = (double) 3 / 2 ; //unifying type? • a = (float) 5.0 / 2 ; //a decimal type • b = (int) (7 / 2.0) ;

Shortcut Operators • way to write code with less typing • result is exactly same • code may actually run faster • examples ++a; // exactly same as a = a + 1; • ++a; 4 keystrokes (including semicolon) • a=a+1; 6 keystrokes • a = a + 1; 10 keystrokes (4 spaces)

Addition and Subtraction • ++ is the "increment operator" • adds 1 • -- is the "decrement operator" • subtracts 1 • both are "unary" operators, because they only work on 1 variable • contra: a + b, c = d, e == f, etc.

Prefix versus Postfix • prefix means put the operator before the variable e.g., ++a, --b • increments or decrements a variable before it is used in an expression a = 2; x = ++a; • now x is 3, a is 3

Prefix versus Postfix - 2 • postfix means put the operator after the variable e.g., a--, b++ • decrements or increments a variable after it is used in an expression a = 2; x = a++; • now x is 2, a is 3

Practice a=4; b=3; x=++a; // x? a? y=a--; // y? a? x= ++a + b--; // x? a? b? y= a-- - (++b); // y? a? b?

Other Shortcut Operators • can add, subtract, multiply, divide, or get modulus • not limited to add or subtract just 1 a += 2; // same as a = a + 2; a -= 2; // same as a = a - 2; a *= 2; // same as a = a * 2; a /= 2.0; // same as a = a / 2.0;

Other Shortcut Operators - 2 • modulus = remainder after integer division ("clock arithmetic") • use % symbol • 5 % 2 == 1 • if num % 2 == 0, num is an even number • shortcut operator for modulus a %= 2; // same as a = a % 2;

Practice a=4; b=5; // a? a += 5; // a? b -= 4; // b? b *= 4; // b? a %= ++b; // a? b?

Shortcuts in Totals total = 0.00; sale = 3.00; total += sale; /* would work well in a loop: read a line from a file into sale variable, add it onto total, read another line into sale, etc. */

CIS 234: Order of Operations, Shortcut & Other Operators